SC3303 datasheet(12/26 Pages) SEMTECH

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SC3303

Synchronous Buck Converter

The SC3303 is a step down synchronous buck DC-DC regu-

lator. The device is capable of 3A operation at very high

efficiency in a tiny 3 x 3-10 pin package. The programma-

ble operating frequency range of 200kHz – 1MHz enables

the user to optimize the design for minimum board space

and optimum efficiency.

The buck regulator employs pseudo-fixed frequency

adaptive on-time control. This control method allows fast

transient response thereby lowering the size of the power

components used in the system.

Input Voltage Range

The SC3303 can operate with a wide input voltage ranging

from 5.5V to 28V. The internal LDO generates a fixed 5V

output that provides power for the bias of the SC3303. The

LDO can also provide additional power to an external load.

Psuedo-fixed Frequency Adaptive On-time Control

The PWM control method used by the SC3303 is pseudo-

fixed frequency, adaptive on-time, as shown in Figure 1.

The ripple voltage generated at the output capacitor ESR

is divided down by the feedback resistor network and

used as a PWM ramp signal. The ripple seen at the FB pin

is used to trigger the on-time of the controller.

COUT

ESR

CIN

VOUT

FB Threshold

VFB

VLX

TON

VIN

Figure 1 — PWM Control Method, V

OUT Ripple

The adaptive on-time is determined by an internal one-

shot timer. When the one-shot is triggered by the feed-

back ripple, the device sends a single on-time pulse to the

high-side MOSFET. The pulse period is determined by the

output voltage value and V

. The period is proportional

to output voltage and inversely proportional to input

voltage. The value of the output voltage is obtained by

filtering the voltage seen on the LX pin.

With this adaptive on-time design, the device automati-

cally anticipates the on-time needed to regulate V

OUT

for

the present V

condition and at the selected frequency.

The advantages of adaptive on-time control are:

Predictable operating frequency during CCM

compared to other variable frequenc y

methods.

Reduced component count by eliminating the

err or

amplifier

and

c ompensation

components.

Reduced component count by removing the

need to sense and control inductor current.

Fast transient response — the response time is

controlled by a fast comparator instead of a

typically slow error amplifier.

Reduced output capacitance due to fast tran-

sient response

One-Shot Timer and Operating Frequency

The one-shot timer operates as shown in Figure 2. The

feedback comparator output goes high when V

is less

than the internal 750mV reference. This feeds into the

gate drive and turns on the high-side MOSFET, and also

starts the one-shot timer. The one-shot timer uses an

internal comparator, timing capacitor, and a low pass

filter (LPF) which regenerates V

OUT

from LX. One compara-

tor input is connected to the filtered LX voltage, the other

input is connected to the capacitor. When the on-time

begins, the internal capacitor charges from zero volts

through a current which is proportional to V

. When the

capacitor voltage reaches V

OUT

, the on-time is completed

and the high-side MOSFET turns off.

•

Applications Information

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