MAX5090ASLASHBSLASHC datasheet(9/17 Pages) MAXIM

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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 as

possible to the device.

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

ground and charges the BST capacitor to (VD - VDIODE).

Once the BST capacitor is charged, the internal low-

side switch is turned off and the BST capacitor voltage

provides 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 guaran-

teed 119kHz to 200kHz frequency range when using an

external clock.

When SYNC is connected to SGND, the internal clock is

used to generate a ramp with the amplitude in proportion

to VIN and the period corresponding to the 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 amplitude in proportion to VIN and the period

corresponding 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 VIN

supply). After the SYNC blanking time, the SYNC ramps

and the SYNC clock switches to the PWM controller

and replaces 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

program 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 current

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

the following equation to calculate the soft-start capacitor

(CSS) required for the soft-start time (tSS):

10 10

1.46

−

where tSS > 700µs and CSS is in Farads.

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 ampli-

fier. The error amplifier takes the lowest voltage among

SS, the internal soft-start voltage, and the bandgap

reference voltage as the input reference for VOUT.

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

protection. During a soft-start, if the voltage at SS (VSS)

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 shut-

down 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.

MAX5090A/B/C

2A, 76V, High-Efficiency MAXPower

Step-Down DC-DC Converters

www.maximintegrated.com

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