SC3203 datasheet(12/20 Pages) SEMTECH

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SC3203

Synchronous Buck Converter

The SC3203 is a step down synchronous buck DC-DC regu-

lator. The device supports 3A operation at high efficiency

in an SOIC-8 package.The buck regulator employs pseudo-

fixed frequency adaptive on-time control. The 500kHz

operating frequency enables the user to optimize the

design for minimal board space and optimum efficiency.

The adaptive on-time control provides fast transient

response and allows reduced size for the power filter.

Input Voltage Range

The SC3203 operates over the input range of 9V to 16V. The

internal LDO generates a fixed 5V output that provides bias

power for the device.

Adaptive On-time Control

The pseudo-fixed frequency, adaptive on-time control is

shown in Figure 1. The ripple voltage generated at the

output capacitor is divided down by the feedback resistor

network and used as a PWM ramp signal. When the FB pin

falls to the FB threshold a single on-time pulse for the

high-side mosfet Q1 is triggered.

COUT

CIN

VOUT

FB Threshold

VFB

VLX

TO N

VIN

SC3203

VIN

PGND

RTOP

C TOP

RBOT

Figure 1 — Adaptive On-time Control

One-Shot Timer and Operating Frequency

When the FB pin falls to the FB Threshold (750mV), the

device sends a single on-time pulse to the high-side

mosfet.

The on-time pulse width is determined by the DC voltage

of LX and by V

IN. The pulse width is proportional to the

DC voltage of LX and inversely proportional to the input

voltage. With this adaptive on-time design, the device

automatically anticipates the on-time needed to regulate

OUT for the present VIN condition. The on-time is approxi-

mated by the following equation:

When the on-time completes, the low-side mosfet Q2

(Figure 1) is turned on. Q2 must stay on for the minimum

off-time of 260nsec, and remains on until one of the fol-

lowing occurs:

FB falls below the 750mV reference. If this

occurs, Q2 turns off and Q1 turns on for another

high-side on-time.

If operating in PSAVE mode, Q2 turns off if the

inductor current falls to zero. If the FB pin is

above the 750mV reference, both Q2 and Q1

remain off, and the output current is supplied

by the output capacitor.

OUT Voltage Selection

The output voltage is regulated by comparing the voltage

at the FB pin to the internal 750mV reference voltage, see

Figure 2.

VOUT

To FB pin

Rbot

Rtop

CTOP

49.9Ω

Figure 2 — Output Voltage Selection

•

Applications Information

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