MAXREFDES125#

MAXREFDES125# Datasheet

MAXREFDES125# Non-isolated 24V to 5.1V, 20W Power Supply

Part	Datasheet
MAXREFDES125#	MAXREFDES125# (pdf)

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System Board 6303 MAXREFDES125# Non-isolated 24V to 5.1V, 20W Power Supply MAXREFDES125# is a high efficiency, 20W power supply for industrial and broad power supply applications. This breakthrough design achieves over 96% efficiency and features a wide input-voltage range of 17V to 36V. The system provides a 5.1V output rail at 4A to meet the needs of many power hungry systems. The MAX17536 synchronous step-down converter, lies at the heart of the system. Capable of operating in multiple modes, the MAX17536 features synchronous operation without a Schottky diode, as well as internal compensation and built-in soft-start. This translates into a small, ultra-efficient power supply that works in any system. The MAXREFDES125# circuit fits on a 20mm x 35mm board. All design documents are online and boards are available for purchase. • High efficiency • Compact and flexible • Low power dissipation • Minimal external components • Robust operation in adverse industrial environments • PLCs • Industrial process control and sensors • Telecom and datacom power supplies Introduction The MAXREFDES125# reference design Figure 1 demonstrates the application of the MAX17536 high-efficiency, high-voltage, synchronous step-down controller. The MAX17536 can be operated in the pulse-width modulation PWM , pulse-frequency modulation PFM , or discontinuous-conduction mode DCM control schemes. The reference design operates over a 17V to 36V input-voltage range, and provides up to 4A at 5.1V output. The reference design features high-efficiency, multiple mode, synchronous buck topology for step-down converters. In PWM mode, the inductor current is allowed to go negative, PWM operation provides constant frequency opera-tion at all loads, and is useful in applications sensitive to switching frequency. PFM mode of operation disables negative inductor current and additionally skips pulses at light loads for high efficiency. DCM mode of operation features constant-frequency operation down to lighter loads than PFM mode by not skipping pulses and only disabling negative inductor current at light loads. DCM operation offers efficiency performance that lies between PWM and PFM modes. The reference design features pins beneath the board to enable the board to be designed into a larger system, as a module. Pins are 15.24mm 0.6in from power to ground, and 27.94mm 1.1in from the input side to the output side. The input undervoltage lockout EN/UVLO is provided for programming the input-supply start voltage set to 14.5V in the design and to ensure proper operation during brownout conditions. The EN/UVLO input is also used to turn on/off the IC. To control inrush current, the device incorporates a soft-start SS pin to set the soft-start time for the regulator. Power dissipation under fault conditions is minimized by hiccup-overcurrent protection hiccup mode . The reference design delivers a peak efficiency of with the supplied components when the input is 24V. This general-purpose power solution can be used in many different types of power applications, such as industrial PLCs, actuators and sensors. System Diagram Figure The MAXREFDES125# reference design block diagram. Detailed Circuit Description Hardware Description The MAX17536 high-efficiency, high-voltage, synchronous step-down DC-DC converter with integrated high-side MOSFET operates over a 4.5V to 60V input. The converter delivers up to 4A and generates output voltages from 0.9V up to x VIN. The MAX17536 uses peak current-mode control. The device may be operated in the pulse-width modulation PWM , pulse-frequency modulation PFM , and discontinuous-conduction mode DCM control schemes. Mode Selection The logic state of the MODE/SYNC pin is latched when VCC and EN/UVLO voltages exceed the respective UVLO rising thresholds and all internal voltages are ready to allow LX switching. If the MODE/SYNC pin is open at power-up, the device operates in PFM mode at light loads. If the MODE/SYNC pin is grounded at power-up, the device operates in constant-frequency PWM mode at all loads. Finally, if the MODE/SYNC pin is connected to VCC at power-up, the device operates in constant-frequency DCM mode at light loads. State changes on the MODE/SYNC pin are ignored during normal operation. In PWM mode, the inductor current is allowed to go negative. PWM operation provides constant frequency operation at all loads, and is useful in applications sensitive to switching frequency. However, the PWM mode of operation gives lower efficiency at light loads compared to PFM and DCM modes of operation. PFM mode of operation disables negative inductor current and additionally skips pulses at light loads for high efficiency. In PFM mode, the inductor current is forced to a fixed peak of 2A every clock cycle until the output rises to of the nominal voltage. Once the output reaches of the nominal voltage, both the high-side and low-side FETs are turned off and the device enters hibernate operation until the load discharges the output to of the nominal voltage. Most of the internal blocks are turned off in hibernate operation to save quiescent current. After the output falls below of the nominal voltage, the device comes out of hibernation, turns on all internal blocks, and again commences the process of delivering pulses of energy to the output until it reaches of the nominal output voltage. The advantage of the PFM mode is higher efficiency at light loads because of lower quiescent current drawn from the supply. The disadvantage is that the output-voltage ripple is higher compared to PWM or DCM modes of operation and switching frequency is not constant at light loads. DCM mode of operation features constant-frequency operation down to lighter loads than PFM mode, by not skipping pulses but only disabling negative inductor current at light loads. DCM operation offers efficiency performance that lies between PWM and PFM modes. Table 1 shows Reference Design jumper JU1 settings that can be used to configure the desired mode of operation. Table MODE Description JU1 Shunt Position EN/UVLO Pin Connected to SGND Not installed Unconnected Connected to VCC MAX17536 Mode PWM mode of operation PWM mode of operation DCM mode of operation *Default position. Linear Regulator VCC and EXTVCC The device has two internal low-dropout LDO regulators which power VCC. One LDO is powered from VIN IN LDO and the other LDO is powered from EXTVCC LDO . Only one of the two LDOs is in operation at any given time, depending on the voltage levels present at EXTVCC. If the EXTVCC voltage is greater than 4.7V typ , VCC is powered from EXTVCC. If EXTVCC is lower than 4.7V typ , VCC is powered from VIN. Powering VCC from EXTVCC increases efficiency at higher input voltages. EXTVCC voltage should not exceed 24V. Loop Compensation The MAX17536 is internally loop compensated. However, if the switching frequency is less than 450kHz, connect a 0402 capacitor C10 between the CF pin and the FB pin to keep the compensation loop stable. Input Undervoltage-Lockout Level Setting EN/UVLO The MAX17536 offers an adjustable input undervoltage-lockout level. Set the voltage level with a resistive voltage-divider connected from VIN to SGND. Connect the center node of the divider to EN/UVLO. Choose R1 to be 3.32M and then calculate R2 as follows: where VINU is the voltage required to turn on the device. The VINU in this design is 14.5V. For normal operation, a shunt should be installed across pins 1-2 on JU2. To disable the output, install a shunt across pins 2-3 on JU2 and pull the EN/UVLO pin to GND. See Table 2 for JU2 settings. Table Regulator Enable EN/UVLO Description JU2 Shunt Position EN/UVLO Pin Connected to VIN Not installed Connected to the center node of divider R1 and R2 Connected to SGND MAX17536 Output Enabled, UVLO level set through the R1 and R2 resistors Disabled

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System Board 6303

MAXREFDES125# Non-isolated 24V to 5.1V, 20W Power Supply

MAXREFDES125# is a high efficiency, 20W power supply for industrial and broad power supply applications. This breakthrough design achieves over 96% efficiency and features a wide input-voltage range of 17V to 36V. The system provides a 5.1V output rail at 4A to meet the needs of many power hungry systems. The MAX17536 synchronous step-down converter, lies at the heart of the system. Capable of operating in multiple modes, the MAX17536 features synchronous operation without a Schottky diode, as well as internal compensation and built-in soft-start. This translates into a small, ultra-efficient power supply that works in any system. The MAXREFDES125# circuit fits on a 20mm x 35mm board. All design documents are online and boards are available for purchase.
• High efficiency
• Compact and flexible
• Low power dissipation
• Minimal external components
• Robust operation in adverse industrial environments
• PLCs
• Industrial process control and sensors
• Telecom and datacom power supplies

Introduction

The MAXREFDES125# reference design Figure 1 demonstrates the application of the MAX17536 high-efficiency, high-voltage, synchronous step-down controller. The MAX17536 can be operated in the pulse-width modulation PWM , pulse-frequency modulation PFM , or discontinuous-conduction mode DCM control schemes. The reference design operates over a 17V to 36V input-voltage range, and provides up to 4A at 5.1V output. The reference design features high-efficiency, multiple mode, synchronous buck topology for step-down converters. In PWM mode, the inductor current is allowed to go negative, PWM operation provides constant frequency opera-tion at all loads, and is useful in applications sensitive to switching frequency. PFM mode of operation disables negative inductor current and additionally skips pulses at light loads for high efficiency. DCM mode of operation features constant-frequency operation down to lighter loads than PFM mode by not skipping pulses and only disabling negative inductor current at light loads. DCM operation offers efficiency performance that lies between PWM and PFM modes. The reference design features pins beneath the board to enable the board to be designed into a larger system, as a module. Pins are 15.24mm 0.6in from power to ground, and 27.94mm 1.1in from the input side to the output side. The input undervoltage lockout EN/UVLO is provided for programming the input-supply start voltage set to 14.5V in the design and to ensure proper operation during brownout conditions. The EN/UVLO input is also used to turn on/off the IC. To control inrush current, the device incorporates a soft-start SS pin to set the soft-start time for the regulator. Power dissipation under fault conditions is minimized by hiccup-overcurrent protection hiccup mode . The reference design delivers a peak efficiency of with the supplied components when the input is 24V. This general-purpose power solution can be used in many different types of power applications, such as industrial PLCs, actuators and sensors.

System Diagram

Figure The MAXREFDES125# reference design block diagram.

Detailed Circuit Description

Hardware Description The MAX17536 high-efficiency, high-voltage, synchronous step-down DC-DC converter with integrated high-side MOSFET operates over a 4.5V to 60V input. The converter delivers up to 4A and generates output voltages from 0.9V up to x VIN.

The MAX17536 uses peak current-mode control. The device may be operated in the pulse-width modulation PWM , pulse-frequency modulation PFM , and discontinuous-conduction mode DCM control schemes.

Mode Selection The logic state of the MODE/SYNC pin is latched when VCC and EN/UVLO voltages exceed the respective UVLO rising thresholds and all internal voltages are ready to allow LX switching. If the MODE/SYNC pin is open at power-up, the device operates in PFM mode at light loads. If the MODE/SYNC pin is grounded at power-up, the device operates in constant-frequency PWM mode at all loads. Finally, if the MODE/SYNC pin is connected to VCC at power-up, the device operates in constant-frequency DCM mode at light loads. State changes on the MODE/SYNC pin are ignored during normal operation.

In PWM mode, the inductor current is allowed to go negative. PWM operation provides constant frequency operation at all loads, and is useful in applications sensitive to switching frequency. However, the PWM mode of operation gives lower efficiency at light loads compared to PFM and DCM modes of operation.

PFM mode of operation disables negative inductor current and additionally skips pulses at light loads for high efficiency. In PFM mode, the inductor current is forced to a fixed peak of 2A every clock cycle until the output rises to of the nominal voltage. Once the output reaches of the nominal voltage, both the high-side and low-side FETs are turned off and the device enters hibernate operation until the load discharges the output to of the nominal voltage. Most of the internal blocks are turned off in hibernate operation to save quiescent current. After the output falls below of the nominal voltage, the device comes out of hibernation, turns on all internal blocks, and again commences the process of delivering pulses of energy to the output until it reaches of the nominal output voltage. The advantage of the PFM mode is higher efficiency at light loads because of lower quiescent current drawn from the supply. The disadvantage is that the output-voltage ripple is higher compared to PWM or DCM modes of operation and switching frequency is not constant at light loads.

DCM mode of operation features constant-frequency operation down to lighter loads than PFM mode, by not skipping pulses but only disabling negative inductor current at light loads. DCM operation offers efficiency performance that lies between PWM and PFM modes.

Table 1 shows Reference Design jumper JU1 settings that can be used to configure the desired mode of operation.

Table MODE Description JU1

Shunt Position

EN/UVLO Pin

Connected to SGND

Not installed

Unconnected

Connected to VCC

MAX17536 Mode PWM mode of operation PWM mode of operation DCM mode of operation
*Default position.

Linear Regulator VCC and EXTVCC The device has two internal low-dropout LDO regulators which power VCC. One LDO is powered from VIN IN LDO and the other LDO is powered from EXTVCC LDO . Only one of the two LDOs is in operation at any given time, depending on the voltage levels present at EXTVCC. If the EXTVCC voltage is greater than 4.7V typ , VCC is powered from EXTVCC. If EXTVCC is lower than 4.7V typ , VCC is powered from VIN. Powering VCC from EXTVCC increases efficiency at higher input voltages. EXTVCC voltage should not exceed 24V.

Loop Compensation The MAX17536 is internally loop compensated. However, if the switching frequency is less than 450kHz, connect a 0402 capacitor C10 between the CF pin and the FB pin to keep the compensation loop stable.

Input Undervoltage-Lockout Level Setting EN/UVLO The MAX17536 offers an adjustable input undervoltage-lockout level. Set the voltage level with a resistive voltage-divider connected from VIN to SGND. Connect the center node of the divider to EN/UVLO.

Choose R1 to be 3.32M and then calculate R2 as follows:
where VINU is the voltage required to turn on the device. The VINU in this design is 14.5V.

For normal operation, a shunt should be installed across pins 1-2 on JU2. To disable the output, install a shunt across pins 2-3 on JU2 and pull the EN/UVLO pin to GND. See Table 2 for JU2 settings.

Table Regulator Enable EN/UVLO Description JU2

Shunt Position

EN/UVLO Pin

Connected to VIN

Not installed

Connected to the center node of divider R1 and R2

Connected to SGND

MAX17536 Output

Enabled, UVLO level set through the R1 and R2 resistors Disabled

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Datasheet ID: MAXREFDES125# 647471