Low-Cost, Small, 4.5V to 28V Wide Operating

Range, DC-DC Synchronous Buck Controller

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Detailed Description

The MAX15026 synchronous step-down controller oper-

ates from a 4.5V to 28V input voltage range and gener-

ates an adjustable output voltage from 85% of the input

voltage down to 0.6V while supporting loads up to 25A.

As long as the device supply voltage is within 5.0V to

The MAX15026 offers adjustable switching frequency

from 200kHz to 2MHz with an external resistor. The

adjustable switching frequency provides design flexi-

bility in selecting passive components. The MAX15026

adopts an adaptive synchronous rectification to elimi-

nate an external freewheeling Schottky diode and

improve efficiency. The device utilizes the on-resis-

tance of the external low-side MOSFET as a current-

sense element. The current-limit threshold voltage is

resistor-adjustable from 30mV to 300mV and is temper-

ature-compensated, so that the effects of the MOSFET

current-sensing scheme protects the external compo-

nents from damage during output overloaded condi-

tions or output short-circuit faults without requiring a

current-sense resistor. Hiccup-mode current limit

reduces power dissipation during short-circuit condi-

tions. The MAX15026 includes a power-good output

and an enable input with precise turn-on/-off threshold

to be used for monitoring and for power sequencing.

The MAX15026 features internal digital soft-start that

allows prebias startup without discharging the output.

The digital soft-start function employs sink current limit-

ing to prevent the regulator from sinking excessive cur-

rent when the prebias voltage exceeds the

programmed steady-state regulation level. The digital

soft-start feature prevents the synchronous rectifier

MOSFET and the body diode of the high-side MOSFET

from experiencing dangerous levels of current while the

regulator is sinking current from the output. The

MAX15026 shuts down at a junction temperature of

+150°C to prevent damage to the device.

DC-DC PWM Controller

The MAX15026 step-down controller uses a PWM volt-

age-mode control scheme (see the

Functional Diagram).

Control-loop compensation is external for providing max-

imum flexibility in choosing the operating frequency and

output LC filter components. An internal transconduc-

tance error amplifier produces an integrated error volt-

age at COMP that helps to provide higher DC accuracy.

The voltage at COMP sets the duty cycle using a PWM

comparator and a ramp generator. On the rising edge of

an internal clock, the high-side n-channel MOSFET turns

on and remains on until either the appropriate duty cycle

or the maximum duty cycle is reached. During the on-

time of the high-side MOSFET, the inductor current

ramps up. During the second-half of the switching cycle,

the high-side MOSFET turns off and the low-side n-chan-

nel MOSFET turns on. The inductor releases the stored

energy as the inductor current ramps down, providing

current to the output. Under overload conditions, when

the inductor current exceeds the selected valley current-

limit threshold (see the

Current-Limit Circuit (LIM) sec-

tion), the high-side MOSFET does not turn on at the

subsequent clock rising edge and the low-side MOSFET

remains on to let the inductor current ramp down.

Internal 5.25V Linear Regulator

nal supply to power the internal functions and to drive

when using an external 5V ±10% power supply. The

to GND with a 1µF ceramic capacitor. Bypass the output

MAX15026 also employs an undervoltage lockout circuit

below 3.6V (typ). The 400mV UVLO hysteresis prevents

chattering on power-up/power-down.

70mA to supply the IC, power the low-side gate driver,

recharge the external boost capacitor, and supply small

external loads. The current available for external loads

depends on the current consumed by the MOSFET

gate drivers.

For example, when switching at 600kHz, a MOSFET

(18nC x 600kHz) = 11mA. The internal control functions

consume 5mA maximum. The current available for

external loads is:

(70 – (2 x 11) – 5)mA

≅ 43mA

MOSFET Gate Drivers (DH, DL)

DH and DL are optimized for driving large-size n-chan-

nel power MOSFETs. Under normal operating condi-

tions and after startup, the DL low-side drive waveform

is always the complement of the DH high-side drive

waveform, with controlled dead-time to prevent cross-

conduction or shoot-through. An adaptive dead-time

circuit monitors the DH and DL outputs and prevents

the opposite-side MOSFET from turning on until the

other MOSFET is fully off. Thus, the circuit allows the

high-side driver to turn on only when the DL gate driver

has turned off, preventing the low-side (DL) from turn-

ing on until the DH gate driver has turned off.