SC21152 datasheet(12/18 Pages) SEMTECH

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SC21152

Synchronous Buck Converter

The SC21152 is a synchronous buck regulator that employs

an adaptive on-time architecture to provide fast transient

response and very small circuit area. The SC21152 oper-

ates over an input voltage range of 2.3 to 5.5V.

Operating Modes

The SC21152 operates in one of four different modes

depending on the status of VIN, VOUT, and load.

PWM Adaptive On-time operation (AOT)

PWM Minimum Off-time operation (Min Off)

PFM (Pulse Frequency Modulation for Power Saving)

Drop-Out operation (LDO mode)

PWM AOT Mode Operation

During PWM AOT operation the controller uses adaptive

on-time control. The FB ripple signal is routed to a gain

stage and then to the internal comparator and reference

which trigger the high-side pulses. The reference includes

a compensation ramp which emulates the inductor

current to provide stable switching.

AOT operation is used when the low-side pulse-width (off-

time) is greater than the minimum 60nsec.

A switching cycle in AOT mode is as follows:

On-time: the cycle starts with the high-side

mosfet turn-on. The pulse width is set by the

following formula where f

sw is 4MHz:

Off-time: after the high-side on-time is com-

pleted, the low-side mosfet turns on to begin

the off-time. The off-time pulse width is set by

the internal compensation ramp and under

static conditions results in typically 4MHz switch-

ing frequency.

Note that the SC21152 enters AOT operation if VIN is less

than VOUT/0.88.

PWM Min Off Operation

The device will enter Minimum Off-time operation (Min

Off) when VIN, VOUT, and load conditions force the off-

time (time between high-side pulses) to the minimum

•

value, typically 60nsec. This indicates that the controller

has reached maximum duty cycle for AOT operation and

longer duty cycles are needed.

In Min Off operation the SC21152 uses constant off-time

control. As in AOT operation, the FB ripple signal is routed

to a gain stage and then to the internal AOT comparator

and reference. In Min Off mode, the FB ripple triggers the

low-side turn-on. Min Off switching cycle operation is as

follows:

Off-time: The low-side mosfet is on for a fixed

Minimum Off time of 60nsec. This draws current

from the output capacitor through the inductor

via the LX pin and causes the output to fall.

On-time: after the Min Off time of 60nsec has

elapsed, the high-side mosfet turns on. This

helps recharge the output capacitor and bring

the output voltage up. The on-time pulse width

is determined by the internal compensation

ramp.

Drop-out Operation

When operating in Min Off mode, the off-time is constant

and the high-side on-time varies in order to regulate the

output. For cases where VIN is near VOUT, the high-side

will remain on continually with 100% duty cycle. The

output voltage will then be limited by the input voltage,

high-side RDS

ON and inductor DCR as shown in the fol-

lowing equation:

Note that RDS

ON is a function of input voltage and tem-

perature. Inductor DC resistance also increases with

temperature.

Power Save (PFM) Operation

In PFM operation, the FB ripple signal is routed to a gain

stage and compared to an internal reference. A switch-

ing cycle is as follows.

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Applications Information

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