2A, 76V, High-Efficiency MAXPower Step-Down

DC-DC Converters

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Boost High-Side Gate Drive (BST)

Connect a flying bootstrap capacitor between LX and

BST to provide the gate-drive voltage to the high-side

n-channel DMOS switch. The capacitor is alternately

charged from the internally regulated output-voltage VD

and placed across the high-side DMOS driver. Use a

0.22µF, 16V ceramic capacitor located as close to the

device as possible.

On startup, an internal low-side switch connects LX to

Once the BST capacitor is charged, the internal low-side

switch is turned off and the BST capacitor voltage pro-

vides the necessary enhancement voltage to turn on the

high-side switch.

Synchronization (SYNC)

SYNC controls the oscillator frequency. Connect SYNC

to SGND to select 127kHz operation. Use the SYNC

input to synchronize to an external clock. SYNC has a

guaranteed frequency range of 119kHz to 200kHz

when using an external clock.

When SYNC is connected to SGND, the internal clock

is used to generate a ramp with the amplitude in pro-

internal clock frequency to modulate the duty cycle of

the high-side switch.

If an external clock (SYNC clock) is applied at SYNC for

four cycles, the MAX5090 selects the SYNC clock. The

MAX5090 generates a ramp (SYNC ramp) with the

sponding to the SYNC clock frequency. The MAX5090

initially blanks the SYNC ramp for 375µs (typ) to allow

the ramp to reach its target amplitude (proportion to the

ramp and the SYNC clock switch to the PWM controller

and replace the internal ramp and the internal clock,

respectively. If the SYNC clock is removed for three

internal clock cycles, the internal clock and the internal

ramp switch back to the PWM controller.

The minimum pulse-width requirement for the external

clock is 350ns, and if the requirement is not met, the

MAX5090 could ignore the clock as a noisy bounce.

Soft-Start (SS)

The MAX5090 provides the flexibility to externally pro-

gram a suitable soft-start time for a given application.

Connect an external capacitor from SS to SGND to use

the external soft-start. Soft-start gradually ramps up the

reference voltage seen by the error amplifier to control

the output’s rate of rise and reduce the input surge cur-

rent during startup. For soft-start time longer than 700µs,

use the following equation to calculate the soft-start

The MAX5090 also provides an internal soft-start

(700µs, typ) with a current source to charge an internal

capacitor to rise up to the bandgap reference voltage.

The internal soft-start voltage will eventually be pulled

up to 3.4V. The internal soft-start reference also feeds

to the error amplifier. The error amplifier takes the low-

est voltage among SS, the internal soft-start voltage,

and the bandgap reference voltage as the input refer-

Soft-start occurs when power is first applied and when

the device exits shutdown. The MAX5090 also goes

through soft-start when coming out of thermal-overload

is charged up to 1.46V in less than 700µs, the

MAX5090 takes its default internal soft-start (700µs) to

ramp up as its reference. After the SS and the internal

soft-start ramp up over the bandgap reference, the

error amplifier takes the bandgap reference.

Thermal-Overload Protection

The MAX5090 features integrated thermal-overload

protection. Thermal-overload protection limits power

dissipation in the device, and protects the device from

a thermal overstress. When the die temperature

exceeds +175°C, an internal thermal sensor signals the

shutdown logic, turning off the internal power MOSFET,

resetting the internal soft-start and allowing the IC to

cool. The thermal sensor turns the internal power

MOSFET back on after the IC’s die temperature cools

down to +155°C, resulting in a pulsed output under

continuous thermal-overload conditions.

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