MAX8867EUK50+T

Product Overview of the MAX8867/MAX8868 Series

The MAX8867/MAX8868 represent a class of low-noise, low-dropout linear voltage regulators designed specifically for battery-powered applications where power efficiency and signal integrity are paramount. Manufactured by Analog Devices Inc./Maxim Integrated, these devices deliver regulated output currents up to 150mA while maintaining exceptionally low quiescent current consumption. The regulators operate across an input voltage range of 2.5V to 6.5V, making them suitable for single-cell lithium-ion batteries, multi-cell alkaline configurations, and regulated supply sources.

The fundamental distinction between the MAX8867 and MAX8868 lies in the auto-discharge functionality present in the MAX8868, which actively discharges the output voltage to ground during shutdown mode. This feature proves valuable in applications requiring rapid power-down sequences or where residual charge on output capacitors could interfere with downstream circuitry. Both variants come in compact 5-pin SOT23 packages, available in both regular and thin profile configurations, enabling integration into space-constrained designs.

Core Architecture and Operating Principles of the MAX8867/MAX8868

The MAX8867/MAX8868 employ a precision voltage regulation architecture centered on a 1.25V bandgap reference and an error amplifier that continuously monitors output voltage accuracy. The regulator incorporates an internal P-channel MOSFET pass transistor with a typical on-resistance of 1.1Ω, which serves as the primary current delivery element. This architecture differs fundamentally from traditional PNP-based regulators in its approach to quiescent current management.

The operational principle functions as follows: the bandgap reference voltage connects to the inverting input of the error amplifier, while the output voltage feeds back through an internal resistor divider network to the non-inverting input. When the feedback voltage drops below the reference level, the error amplifier pulls the pass transistor gate to a lower potential, allowing increased current flow to the output and raising the output voltage. Conversely, when feedback voltage exceeds the reference, the gate voltage rises, restricting current flow and reducing output voltage. This continuous feedback loop maintains output voltage regulation within specified tolerances.

The P-channel MOSFET architecture provides distinct advantages over PNP-based designs. Unlike PNP transistors that require continuous base drive current proportional to load current, the MOSFET requires only gate capacitive charging. This eliminates the base-drive current losses that plague PNP regulators, particularly during dropout conditions where PNP transistors saturate and waste substantial current. The MAX8867/MAX8868 maintain a constant 100μA quiescent current regardless of load condition or dropout state, a performance characteristic that directly extends battery life in portable applications.

Input Voltage Range and Output Voltage Configuration in the MAX8867/MAX8868

The MAX8867/MAX8868 accept input voltages spanning 2.5V to 6.5V, providing flexibility across diverse power source configurations. This range accommodates single lithium-ion cells (nominal 3.7V), dual alkaline batteries (3.0V), triple alkaline cells (4.5V), and regulated supply rails up to 6.5V. The wide input range enables designers to employ these regulators in multi-chemistry battery systems without requiring separate regulator variants.

Output voltage configuration represents a key design parameter. The MAX8867/MAX8868 are supplied with factory-preset output voltages ranging from 2.5V to 5.0V in 100mV increments. This granular voltage selection accommodates the diverse requirements of modern semiconductor devices. For example, the MAX8867EUK33 variant delivers a fixed 3.3V output, suitable for standard digital logic and microcontroller applications. The MAX8867EUK50 variant provides 5.0V output for legacy TTL-compatible circuits and analog subsystems. Two specialized variants, the MAX8867EUK29 and MAX8867EUK32, offer preset voltages of 2.84V and 3.15V respectively, addressing specific application requirements.

The output voltage accuracy specification of ±1.4% ensures that regulated outputs remain within acceptable tolerance bands across the operating temperature range of -40°C to +85°C. This accuracy level proves sufficient for most digital logic families and analog circuits, though applications requiring tighter regulation should account for this tolerance in their design margins.

Low-Dropout Performance Characteristics of the MAX8867/MAX8868

Dropout voltage represents the minimum voltage differential between input and output at which the regulator maintains specified output voltage regulation. The MAX8867/MAX8868 exhibit exceptionally low dropout characteristics, with typical values of 55mV at 50mA output current and 165mV at maximum 150mA output current. This low-dropout performance directly translates to extended battery runtime in portable applications.

The dropout voltage relationship follows the fundamental equation: Dropout = I_OUT × R_DS(ON), where R_DS(ON) represents the on-resistance of the internal P-channel MOSFET pass transistor. At 150mA output current, the 165mV dropout corresponds to an effective on-resistance of approximately 1.1Ω, consistent with the device's specified pass transistor characteristics. This relationship means that at lower output currents, dropout voltage decreases proportionally, enabling operation from lower supply voltages when load current is reduced.

In battery-powered applications, dropout voltage directly determines the usable end-of-life battery voltage. For a system requiring 3.3V output from a lithium-ion battery, the minimum input voltage would be approximately 3.465V (3.3V + 0.165V) at full 150mA load. As the battery voltage decays below this threshold, the regulator enters dropout and can no longer maintain regulation. The low-dropout characteristic of the MAX8867/MAX8868 extends the usable battery capacity compared to regulators with higher dropout specifications, effectively increasing device runtime by 5-10% in typical battery-powered applications.

Noise Reduction Capabilities of the MAX8867/MAX8868

Output voltage noise represents a critical parameter in applications involving sensitive analog circuits, RF systems, and precision measurement instrumentation. The MAX8867/MAX8868 achieve exceptionally low output noise of 30μV_RMS, measured across the audio frequency band with a 10μF output capacitor and 0.01μF bypass capacitor at the BP pin.

The noise reduction architecture employs a passive RC filter formed by an internal 200kΩ resistor and an external 0.01μF bypass capacitor connected to the BP pin. This configuration creates an 80Hz lowpass filter that attenuates high-frequency noise components while preserving DC regulation accuracy. The bypass capacitor value represents a critical design parameter: the recommended 0.01μF value provides optimal noise performance without excessive startup delay. Increasing the bypass capacitance to 0.1μF or higher provides minimal additional noise reduction while significantly increasing the time required for the output to stabilize during power-on sequences.

The noise spectral density characteristics reveal that the majority of noise energy concentrates at low frequencies below 10kHz, with noise floor dropping progressively at higher frequencies. This frequency distribution reflects the regulator's internal architecture and the effectiveness of the RC filter network. For applications requiring noise measurements, the 30μV_RMS specification assumes proper PCB layout with the bypass capacitor positioned within 5mm of the BP pin to minimize parasitic inductance.

Current Management and Protection Features in the MAX8867/MAX8868

The MAX8867/MAX8868 incorporate a current limiter that monitors the pass transistor gate voltage and restricts output current to a nominal 390mA. For design purposes, the current limit should be considered as ranging from 160mA minimum to 500mA maximum, reflecting manufacturing process variations. This current limiting function protects the device from damage during output short-circuit conditions.

A distinctive feature of the MAX8867/MAX8868 is their ability to withstand indefinite output short-circuit duration without device damage. When the output is shorted to ground, the current limiter immediately restricts current flow to the maximum limit value, preventing excessive power dissipation and junction temperature rise. This robust short-circuit protection eliminates the need for external current-limiting resistors or fuses in many applications, simplifying circuit design and reducing component count.

The current limiter operates through a feedback mechanism that senses the pass transistor gate voltage. As output current increases, the gate voltage rises to maintain constant voltage drop across the pass transistor. When gate voltage reaches the current-limit threshold, further increases in load current produce no additional gate voltage change, effectively clamping output current. This architecture ensures that current limiting engages smoothly without abrupt transitions that could generate noise or transient overshoot.

Thermal Management and Power Dissipation in the MAX8867/MAX8868

Power dissipation in linear regulators represents the primary thermal challenge, calculated as P = I_OUT × (V_IN - V_OUT). For example, a regulator delivering 150mA from a 5V input to a 3.3V output dissipates 255mW of heat. The MAX8867/MAX8868 address thermal stress through both passive thermal management and active thermal protection mechanisms.

The device incorporates a thermal sensor that monitors junction temperature and triggers shutdown logic when temperature exceeds 170°C. Upon reaching this threshold, the pass transistor turns off, allowing the IC to cool. Once junction temperature drops by 20°C to 150°C, the pass transistor reactivates, creating a pulsed output condition during sustained thermal overload. This thermal cycling protects the device from permanent damage but should not be relied upon for normal operation. The absolute maximum junction temperature rating of 150°C represents the upper limit for continuous operation; sustained operation above this temperature degrades device reliability.

The thermal resistance specifications vary between package options. The regular SOT23-5 package exhibits a junction-to-board thermal resistance (θ_JB) of 140°C/W, while the thin SOT23-5 variant provides improved thermal performance at 110°C/W. These thermal resistance values enable calculation of maximum allowable power dissipation using the formula: P_MAX = (T_J - T_A) / (θ_JB + θ_BA), where T_J represents junction temperature, T_A represents ambient temperature, and θ_BA represents the thermal resistance from the package to ambient air through the PCB and surrounding environment.

Practical thermal management requires attention to PCB layout. The GND pin performs dual functions: providing electrical connection to ground and channeling heat away from the die. Connecting the GND pin to a large ground pad or ground plane significantly reduces θ_BA and improves overall thermal performance. A ground pad of at least 100mm² directly beneath the device can reduce θ_BA to approximately 50-80°C/W, enabling higher power dissipation before thermal limits are reached.

The continuous power dissipation rating at 70°C ambient temperature is 571mW for the regular package and 727mW for the thin package. Above 70°C ambient, power dissipation must be derated at 7.1mW/°C for the regular package and 9.1mW/°C for the thin package. For example, at 85°C ambient temperature, the regular package maximum power dissipation reduces to 571mW - (15°C × 7.1mW/°C) = 464.5mW.

Reverse Battery Protection in the MAX8867/MAX8868

Battery-powered applications frequently encounter reverse polarity conditions due to user error or manufacturing defects. The MAX8867/MAX8868 incorporate a unique reverse battery protection scheme that prevents device damage when either the VIN or VSHDN pins fall below ground potential. This protection mechanism monitors the polarity of these pins and disconnects internal circuitry and parasitic diodes when reverse polarity is detected.

The protection scheme limits reverse supply current to 1mA, preventing excessive current flow that could damage downstream circuitry or deplete the battery rapidly. This current limitation is achieved through internal circuitry that effectively disconnects the regulator's internal structures from the supply pins during reverse polarity conditions. The protection operates automatically without requiring external components or user intervention.

The reverse battery protection extends to both the main supply input (VIN) and the shutdown control input (VSHDN). This dual protection ensures that accidental reverse connection of either signal line does not compromise device integrity. The protection scheme proves particularly valuable in applications where users may connect batteries incorrectly or where manufacturing processes might inadvertently reverse polarity during assembly.

Capacitor Selection and Stability Considerations for the MAX8867/MAX8868

Capacitor selection profoundly influences regulator stability, noise performance, and transient response. The MAX8867/MAX8868 require careful attention to both input and output capacitor specifications to ensure stable operation across the full operating temperature range and load current range.

Input capacitor selection begins with a minimum 1μF capacitor connected between VIN and GND. Larger input capacitor values and lower equivalent series resistance (ESR) provide superior supply-noise rejection and improved line-transient response. When the input supply contains significant ripple or noise, increasing the input capacitor to 2.2μF or 4.7μF provides additional filtering. The input capacitor should be positioned as close as possible to the VIN pin to minimize parasitic inductance.

Output capacitor selection requires more nuanced consideration. The minimum recommended output capacitance is 1μF for stable operation across the full temperature range and load current range up to 150mA. However, larger output capacitors in the range of 1μF to 10μF provide improved load-transient response and reduced output voltage overshoot during rapid load current changes. The output capacitor's ESR significantly influences stability; the device exhibits stable operation with output capacitor ESR values ranging from approximately 0.1Ω to 2Ω at 150mA load current.

Ceramic capacitor dielectric selection proves critical for temperature stability. Capacitors with X7R or X5R dielectrics maintain relatively constant capacitance across the operating temperature range and exhibit low ESR variation with temperature. A single 1μF X7R ceramic capacitor provides stable operation at all temperatures from -40°C to +85°C. Conversely, capacitors with Z5U or Y5V dielectrics exhibit large capacitance and ESR variations with temperature, particularly below -10°C. When using these dielectrics, increasing the output capacitance to 2.2μF or higher ensures stability at low temperatures.

Tantalum capacitors present a different stability consideration. High-ESR tantalum capacitors may require 2.2μF or larger values to maintain ESR within the stable operating region. The typical operating characteristics include a graph showing the region of stable output capacitor ESR versus load current, enabling designers to verify that their selected capacitor falls within the stable region.

The bypass capacitor at the BP pin requires a 0.01μF capacitor for optimal noise performance. This value represents a compromise between noise reduction and startup time. Increasing the bypass capacitance above 0.1μF provides negligible noise improvement while significantly extending the time required for the output voltage to reach 95% of its final value during power-on sequences. The startup time graph in the typical operating characteristics demonstrates this relationship, showing that startup time increases from approximately 1ms with a 0.01μF bypass capacitor to over 10ms with a 0.1μF capacitor.

Load and Line Transient Response of the MAX8867/MAX8868

Transient response characteristics describe how the output voltage responds to rapid changes in either load current or input voltage. Understanding these responses enables designers to select appropriate output capacitor values and predict output voltage excursions during dynamic operating conditions.

Load-transient response occurs when the load current changes rapidly, such as when a microprocessor transitions from idle to active mode. The MAX8867/MAX8868 exhibit a typical transient response of 12mV for a step change in load current from 0mA to 50mA. This transient comprises two components: a DC shift resulting from the output impedance multiplied by the load current change, and a dynamic transient response reflecting the regulator's feedback loop response time.

The DC shift component is determined by the output capacitor's ESR. For example, with a 1μF output capacitor having 0.5Ω ESR, a 50mA load step produces a DC shift of 50mA × 0.5Ω = 25mV. The dynamic transient response, typically 5-10mV, reflects the regulator's ability to adjust the pass transistor gate voltage in response to the feedback error signal. Increasing the output capacitor value and decreasing its ESR both reduce the transient overshoot.

Line-transient response describes the output voltage change when the input voltage changes rapidly. The MAX8867/MAX8868 exhibit power-supply rejection ratio (PSRR) of 63dB at low frequencies, meaning that a 1V input voltage ripple produces approximately 1mV of output voltage ripple at DC and low frequencies. Above 10kHz, the PSRR rolls off at approximately 20dB per decade, reflecting the frequency-dependent behavior of the feedback loop.

Practical line-transient response can be improved through increased input and output bypass capacitance and passive filtering techniques. When the input supply contains significant high-frequency noise, adding a 10μF input capacitor with low ESR provides additional filtering. Some applications employ a series resistor (10-100Ω) between the input supply and the regulator input, creating an RC filter network that attenuates high-frequency noise before it reaches the regulator.

Power Supply Rejection and Supply Noise Immunity in the MAX8867/MAX8868

Power-supply rejection ratio (PSRR) quantifies the regulator's ability to reject noise and ripple present on the input supply. The MAX8867/MAX8868 achieve 63dB PSRR at low frequencies, corresponding to a rejection ratio of approximately 1000:1. This means that 1V of input ripple produces only 1mV of output ripple at frequencies below 10kHz.

The PSRR frequency response exhibits a characteristic shape: relatively flat at low frequencies with 63dB rejection, then rolling off above 10kHz at approximately 20dB per decade. This frequency-dependent behavior reflects the regulator's feedback loop characteristics. At very low frequencies (below 1Hz), the feedback loop has sufficient time to respond to input voltage changes, providing excellent rejection. At higher frequencies, the feedback loop cannot respond quickly enough, and input noise begins to appear at the output.

The practical implication of PSRR characteristics is that the MAX8867/MAX8868 perform well when powered from battery supplies or well-regulated DC sources. When powered from switching power supplies or other sources with significant high-frequency noise, additional input filtering becomes necessary. A simple RC filter formed by a 10Ω series resistor and a 10μF input capacitor can attenuate high-frequency noise by 20-40dB before it reaches the regulator, effectively extending the regulator's noise rejection capability to higher frequencies.

The regulator's low quiescent current of 85μA no-load and 100μA operating current contributes to its suitability for battery-powered applications. This low quiescent current means that the regulator itself consumes minimal power, allowing the majority of battery capacity to be delivered to the load. In contrast, older regulator designs consuming 1-5mA of quiescent current would waste 10-50% of battery capacity in low-power standby modes.

Package Specifications and Physical Characteristics of the MAX8867/MAX8868

The MAX8867/MAX8868 are available in two 5-pin SOT23 package variants: regular and thin profile. Both packages provide identical electrical performance but differ in physical dimensions and thermal characteristics.

The regular SOT23-5 package measures 2.9mm × 2.8mm × 1.3mm (length × width × height) and provides a junction-to-board thermal resistance of 140°C/W. This package suits applications where space is not severely constrained and standard PCB assembly equipment is employed.

The thin SOT23-5 package measures 2.9mm × 2.8mm × 0.9mm (length × width × height), reducing height by approximately 30% compared to the regular package. The reduced height enables integration into ultra-thin portable devices such as credit-card-sized electronics or slim mobile devices. The thin package provides improved thermal performance with a junction-to-board thermal resistance of 110°C/W, approximately 21% better than the regular package. This thermal improvement results from the reduced package height, which decreases the thermal path length from the die to the PCB.

Both packages employ a 5-pin configuration with the following pin assignments: VIN (input supply), GND (ground), OUT (regulated output), BP (bypass capacitor connection), and SHDN (shutdown control). The pin spacing of 1.27mm (50 mils) follows standard SOT23 conventions, enabling compatibility with automated assembly equipment.

The package outline specifications include detailed dimensional tolerances and lead-forming requirements. The lead tips must remain planar with respect to one another within 0.10mm at the seating plane, ensuring reliable solder joint formation during reflow assembly. The package complies with JEDEC specification MO-193 for regular SOT23 packages and EIAJ specification SC-74 for thin SOT23 packages.

The GND pin incorporates a large thermal pad that extends beneath the package body, providing a direct thermal path from the die to the PCB. Proper PCB layout requires connecting this thermal pad to a large ground plane or ground pad of at least 100mm² to maximize heat dissipation. Failure to provide adequate ground connection significantly degrades thermal performance and may cause the device to reach thermal shutdown during high-power operation.

Conclusion

The MAX8867/MAX8868 linear regulators represent a mature, well-optimized solution for battery-powered applications requiring low noise, low dropout, and minimal quiescent current consumption. The combination of a P-channel MOSFET pass transistor, precision bandgap reference, and integrated protection features delivers reliable voltage regulation across a wide range of input voltages and load currents. The exceptionally low 30μV_RMS output noise enables integration into sensitive analog and RF systems, while the 165mV dropout at 150mA output current extends battery runtime compared to conventional regulators. The comprehensive protection features including thermal shutdown, reverse battery protection, and indefinite short-circuit tolerance eliminate the need for external protection components in most applications. Careful attention to capacitor selection and PCB layout ensures stable operation across the full operating temperature range and enables optimal transient response performance. The compact SOT23 package options, combined with factory-preset output voltages spanning 2.5V to 5.0V in 100mV increments, provide flexibility for diverse application requirements while maintaining a minimal component footprint.

Frequently Asked Questions (FAQ)

Q1. What is the difference between the MAX8867 and MAX8868 variants?

A1. The primary difference between the MAX8867 and MAX8868 lies in the auto-discharge function present in the MAX8868. When the MAX8868 is placed in shutdown mode, it actively discharges the output voltage to ground through an internal discharge transistor. This feature proves valuable in applications where residual charge on output capacitors could interfere with downstream circuitry during power-down sequences or where rapid output voltage discharge is required for safety reasons. The MAX8867 lacks this auto-discharge function and maintains the output voltage at its regulated level during shutdown. Both devices are otherwise electrically identical and pin-compatible.

Q2. Can the MAX8867/MAX8868 operate from a single lithium-ion battery cell?

A2. Yes, the MAX8867/MAX8868 can operate from a single lithium-ion battery cell. A fully charged lithium-ion cell provides approximately 4.2V, which falls within the 2.5V to 6.5V input voltage range. As the battery discharges, the voltage decreases to approximately 3.0V at end-of-life. The low 165mV dropout at 150mA output current enables the regulator to maintain regulation until the battery voltage drops to approximately 3.465V when delivering 150mA to a 3.3V output. This low dropout extends the usable battery capacity compared to regulators with higher dropout specifications, effectively increasing device runtime by 5-10% in typical applications.

Q3. What output capacitor value should be used for stable operation at low temperatures?

A3. For stable operation across the full operating temperature range including temperatures below -10°C, the output capacitor selection depends on the dielectric type. Capacitors with X7R or X5R dielectrics maintain relatively constant capacitance and ESR across the temperature range, and a single 1μF capacitor provides stable operation at all temperatures. Capacitors with Z5U or Y5V dielectrics exhibit large capacitance and ESR variations with temperature, particularly below -10°C. When using these dielectrics, increasing the output capacitance to 2.2μF or higher ensures stability at low temperatures. High-ESR tantalum capacitors may also require 2.2μF or larger values to maintain ESR within the stable operating region across the temperature range.

Q4. How does the current limiter protect the device during output short-circuit conditions?

A4. The MAX8867/MAX8868 incorporate a current limiter that monitors the pass transistor gate voltage and restricts output current to a nominal 390mA (with a design range of 160mA minimum to 500mA maximum). When the output is shorted to ground, the current limiter immediately engages and prevents current from exceeding the limit value. This current limiting function protects the device from excessive power dissipation and junction temperature rise. A distinctive feature is that the device can withstand indefinite output short-circuit duration without damage, eliminating the need for external current-limiting resistors or fuses in many applications.

Q5. What is the significance of the bypass capacitor at the BP pin?

A5. The bypass capacitor at the BP pin, typically 0.01μF, works in conjunction with an internal 200kΩ resistor to create an 80Hz lowpass filter for noise reduction. This filter attenuates high-frequency noise components while preserving DC regulation accuracy. The 0.01μF value represents an optimal compromise between noise reduction and startup time. Increasing the bypass capacitance to 0.1μF or higher provides minimal additional noise reduction (typically less than 5μV_RMS improvement) while significantly increasing the time required for the output to stabilize during power-on sequences. Values above 0.1μF are not recommended as they provide no performance advantage and unnecessarily extend startup delay.

Q6. How does the P-channel MOSFET pass transistor improve battery life compared to PNP-based regulators?

A6. The P-channel MOSFET pass transistor provides superior battery life through elimination of base-drive current losses inherent in PNP-based designs. PNP transistors require continuous base drive current proportional to load current, wasting power particularly during dropout conditions where the transistor saturates. The MOSFET requires only gate capacitive charging, eliminating base-drive current losses. The MAX8867/MAX8868 maintain a constant 100μA quiescent current regardless of load condition or dropout state, compared to 1-5mA typical for older PNP-based regulators. In battery-powered applications with extended standby periods, this low quiescent current directly extends battery runtime by 10-50% depending on the duty cycle.

Q7. What thermal management considerations are necessary for high-power applications?

A7. Thermal management in high-power applications requires attention to both device thermal resistance and PCB thermal design. The GND pin performs dual functions: providing electrical connection to ground and channeling heat away from the die. Connecting the GND pin to a large ground pad or ground plane significantly reduces the thermal resistance from the package to ambient air (θ_BA). A ground pad of at least 100mm² directly beneath the device can reduce θ_BA to approximately 50-80°C/W. The maximum power dissipation is calculated as P_MAX = (T_J - T_A) / (θ_JB + θ_BA), where T_J is the maximum allowable junction temperature (150°C for continuous operation), T_A is ambient temperature, θ_JB is the package thermal resistance (140°C/W for regular SOT23-5, 110°C/W for thin SOT23-5), and θ_BA is the PCB thermal resistance. Above 70°C ambient temperature, power dissipation must be derated at 7.1mW/°C for the regular package and 9.1mW/°C for the thin package.

Q8. How does the reverse battery protection scheme function?

A8. The MAX8867/MAX8868 incorporate a unique reverse battery protection scheme that monitors the polarity of the VIN and VSHDN pins. When either pin falls below ground potential (indicating reverse polarity), internal circuitry automatically disconnects the regulator's internal structures and parasitic diodes from the supply pins. This protection limits reverse supply current to 1mA, preventing excessive current flow that could damage downstream circuitry or rapidly deplete the battery. The protection operates automatically without requiring external components or user intervention, making it particularly valuable in applications where users may connect batteries incorrectly or where manufacturing processes might inadvertently reverse polarity during assembly.

Q9. What is the relationship between dropout voltage and load current?

A9. Dropout voltage follows the fundamental relationship: Dropout = I_OUT × R_DS(ON), where I_OUT is the output current and R_DS(ON) is the on-resistance of the internal P-channel MOSFET pass transistor (approximately 1.1Ω). At 50mA output current, the typical dropout is 55mV, corresponding to 50mA × 1.1Ω = 55mV. At 150mA output current, the typical dropout is 165mV, corresponding to 150mA × 1.1Ω = 165mV. This linear relationship means that at lower output currents, dropout voltage decreases proportionally, enabling operation from lower supply voltages when load current is reduced. In battery-powered applications, this characteristic extends the usable battery voltage range and increases effective battery capacity.

Q10. How should the input and output capacitors be selected for optimal transient response?

A10. Optimal transient response requires careful selection of both input and output capacitor values and characteristics. The input capacitor should be a minimum 1μF, with larger values (2.2μF to 4.7μF) providing improved supply-noise rejection and line-transient response when the input supply contains significant ripple or noise. The output capacitor should be a minimum 1μF for stable operation, with larger values (1μF to 10μF) providing improved load-transient response and reduced output voltage overshoot. The output capacitor's ESR significantly influences transient response; lower ESR values reduce the DC shift component of the transient. For optimal performance, select output capacitors with ESR values between 0.1Ω and 2Ω at 150mA load current. Ceramic capacitors with X7R or X5R dielectrics provide superior temperature stability compared to Z5U or Y5V dielectrics. Positioning capacitors as close as possible to the device pins minimizes parasitic inductance and improves transient response.

Q11. What is the practical significance of the 63dB power-supply rejection ratio?

A11. The 63dB power-supply rejection ratio (PSRR) at low frequencies corresponds to a rejection ratio of approximately 1000:1, meaning that 1V of input ripple produces only 1mV of output ripple at frequencies below 10kHz. This excellent low-frequency rejection enables the MAX8867/MAX8868 to perform well when powered from battery supplies or well-regulated DC sources. However, above 10kHz, the PSRR rolls off at approximately 20dB per decade, and input noise begins to appear at the output. When powered from switching power supplies or other sources with significant high-frequency noise, additional input filtering becomes necessary. A simple RC filter formed by a 10Ω series resistor and a 10μF input capacitor can attenuate high-frequency noise by 20-40dB before it reaches the regulator, effectively extending the regulator's noise rejection capability to higher frequencies.

Q12. What output voltage options are available, and how are they selected?

A12. The MAX8867/MAX8868 are supplied with factory-preset output voltages ranging from 2.5V to 5.0V in 100mV increments. The output voltage is selected through the part number suffix. For example, the MAX8867EUK33 variant delivers a fixed 3.3V output, suitable for standard digital logic and microcontroller applications. The MAX8867EUK50 variant provides 5.0V output for legacy TTL-compatible circuits and analog subsystems. Two specialized variants, the MAX8867EUK29 and MAX8867EUK32, offer preset voltages of 2.84V and 3.15V respectively, addressing specific application requirements. The output voltage accuracy specification of ±1.4% ensures that regulated outputs remain within acceptable tolerance bands across the operating temperature range of -40°C to +85°C. Custom output voltages outside the standard range may be available through the manufacturer; contact the manufacturer for availability and minimum order quantities.