MAX15108 Datasheet, PDF(10/19 Page) Maxim Integrated Products – High-Efficiency, 8A, Current-Mode Synchronous Step-Down Switching Regulator

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High-Efficiency, 8A, Current-Mode

Synchronous Step-Down Switching Regulator

Detailed Description

The MAX15108 high-efficiency, current-mode switching

regulator delivers up to 8A of output current. The regu-

lator provides output voltages from 0.6V to (0.95 x VIN)

with 2.7V to 5.5V input supplies, making the device ideal

for on-board point-of-load applications.

The IC delivers current-mode control architecture using a

high gain transconductance error amplifier. The current-

mode control architecture facilitates easy compensation

design and ensures cycle-by-cycle current limit with fast

response to line and load transients.

The regulator features a 1MHz fixed switching fre-

quency, allowing for all-ceramic capacitor designs with

fast transient responses. The high operating frequency

minimizes the size of external components. The IC is

available in a 2.5mm x 2mm (4 x 5 array), 0.5mm pitch

WLP package.

The regulator offers a selectable skip-mode function to

reduce current consumption and achieve a high effi-

ciency at light output loads. The low RDS(ON) integrated

switches ensure high efficiency at heavy loads while

minimizing critical inductance, making the layout design

a much simpler task than that of discrete solutions. The

ICâs simple layout and footprint assure first-pass success

in new designs.

The IC features PWM current-mode control, allowing for

an all-ceramic capacitor solution. The regulator offers

capacitor-programmable soft-start to reduce input inrush

current. The device safely starts up into a prebiased

output. The IC includes an enable input and open-drain

PGOOD output for sequencing with other devices.

Controller FunctionâPWM Logic

The controller logic block determines the duty cycle of

the high-side MOSFET under different line, load, and

temperature conditions. Under normal operation, where

the current-limit and temperature protection are not trig-

gered, the controller logic block takes the output from

the PWM comparator to generate the driver signals for

both high-side and low-side MOSFETs. The control logic

block controls the break-before-make logic and all the

necessary timing.

The high-side MOSFET turns on at the beginning of the

oscillator cycle and turns off when the COMP voltage

crosses the internal current-mode ramp waveform. The

internal ramp is the sum of the compensation ramp and

the current-mode ramp derived from the inductor current

(current sense block). The high-side MOSFET also turns

off if the maximum duty cycle exceeds 95%, or when the

current limit is reached. The low-side MOSFET turns on

for the remainder of the switching cycle.

Starting into a Prebiased Output

The IC can soft-start into a prebiased output without dis-

charging the output capacitor. In safe prebiased startup,

both low-side and high-side MOSFETs remain off to

avoid discharging the prebiased output. PWM operation

starts when the voltage on SS crosses the voltage on FB.

The IC can start into a prebiased voltage higher than

the nominal set point without abruptly discharging the

output. Forced PWM operation starts when the SS volt-

age reaches 0.58V, forcing the converter to start. When

the low-side sink current-limit threshold of 1A is reached,

the low-side switch turns off before the end of the clock

period. The low-side sink current limit is 1A. The high-

side switch turns on until one of the following conditions

is satisfied:

â¢ High-side source current hits the reduced high-side

current limit (14A). The high-side switch turns off for

the remaining time of clock period.

â¢ The clock period ends.

Reduced high-side current limit is activated in order to

recirculate the current into the high-side power switch

rather than into the internal high-side body diode, which

can cause damage to the device. The high-side current

limit is set to 14A.

Low-side sink current limit protects the low-side switch

from excessive reverse current during prebiased

operation.

Enable Input

The IC features independent device enable control

and power-good signal that allow for flexible power

sequencing. Drive the enable input (EN) high to enable

the regulator, or connect EN to IN for always-on opera-

tion. Power-good (PGOOD) is an open-drain output that

deasserts when VFB is above 555mV, and asserts low if

VFB is below 530mV.

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