The adaptive driver dead-time allows operation without

shoot-through with a wide range of MOSFETs, minimiz-

ing delays and maintaining efficiency. There must be a

low-resistance, low-inductance path from DL and DH to

the MOSFET gates for the adaptive dead-time circuits

to function properly. The stray impedance in the gate

discharge path can cause the sense circuitry to inter-

MOSFET is still high. To minimize stray impedance, use

very short, wide traces.

Synchronous rectification reduces conduction losses in

the rectifier by replacing the normal low-side Schottky

catch diode with a low-resistance MOSFET switch. The

MAX15026 features a robust internal pulldown transis-

tor with a typical 1

on-resistance prevents DL from being pulled up during

the fast rise time of the LX node, due to capacitive cou-

pling from the drain to the gate of the low-side synchro-

nous rectifier MOSFET.

High-Side Gate-Drive Supply (BST)

and Internal Boost Switch

An internal switch between BST and DH turns on to

sary gate-to-source voltage to turn on the high-side

MOSFET. The boost capacitor connected between BST

and LX holds up the voltage across the gate driver dur-

ing the high-side MOSFET on-time.

The charge lost by the boost capacitor for delivering the

gate charge is replenished when the high-side MOSFET

turns off and LX node goes to ground. When LX is low,

an internal high-voltage switch connected between

Boost Capacitor section in the Applications Information

to choose the right size of the boost capacitor.

Enable Input (EN), Soft-Start,

and Soft-Stop

Drive EN high to turn on the MAX15026. A soft-start

sequence starts to increase step-wise the reference

voltage of the error amplifier. The duration of the soft-

start ramp is 2048 switching cycles and the resolution

is 1/64th of the steady-state regulation voltage allowing

a smooth increase of the output voltage. A logic-low on

EN initiates a soft-stop sequence by stepping down the

reference voltage of the error amplifier. After the soft-

stop sequence is completed, the MOSFET drivers are

both turned off. See Figure 1.

the accurate turn-on/-off thresholds, EN can be used as

UVLO adjustment input, and for power sequencing

together with the PGOOD output.

When the valley current limit is reached during soft-start

the MAX15026 regulates to the output impedance times

the limited inductor current and turns off after 4096

clock cycles. When starting up into a large capacitive

load (for example) the inrush current will not exceed the

current-limit value. If the soft-start is not completed

before 4096 clock cycles, the device will turn off. The

device remains off for 8192 clock cycles before trying

to soft-start again. This implementation allows the soft-

start time to be automatically adapted to the time nec-

essary to keep the inductor current below the limit while

charging the output capacitor.

Power-Good Output (PGOOD)

The MAX15026 includes a power-good comparator to

monitor the output voltage and detect the power-good

threshold, fixed at 94.5% of the nominal FB voltage. The

open-drain PGOOD output requires an external pullup

resistor. PGOOD sinks up to 2mA of current while low.

PGOOD goes high (high-impedance) when the regula-

tor output increases above 94.5% of the designed nom-

inal regulated voltage. PGOOD goes low when the

regulator output voltage drops to below 92% of the

nominal regulated voltage. PGOOD asserts low during

hiccup timeout period.

Startup into a Prebiased Output

When the MAX15026 starts into a prebiased output, DH

and DL are off so that the converter does not sink cur-

rent from the output. DH and DL do not start switching

until the PWM comparator commands the first PWM

pulse. The first PWM pulse occurs when the ramping

reference voltage increases above the FB voltage.

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

Range, DC-DC Synchronous Buck Controller

10

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