SC403B datasheet(15/32 Pages) SEMTECH

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SC403B

Synchronous Buck Converter

The SC403B is a step down synchronous DC-DC buck

converter with integrated power MOSFETs and a 200mA

programmable LDO. The device operates at a current up

to 6A at very high efficiency. A space saving 5x5 (mm) 32-

pin package is used. The programmable operating fre-

quency of up to 1MHz enables optimizing the

configuration for PCB area and efficiency.

The buck controller uses a pseudo-fixed frequency adap-

tive on-time control. This control method allows fast tran-

sient response which permits the use of smaller output

capacitors.

In addition to the following information, the user can

click on the applicable link to go to the SC403B online

C-SIM design and simulation tool, which will lead the

user through the design process.

Input Voltage Requirements

The SC403B requires two input supplies for normal opera-

tion: V

IN and VDD. VIN operates over a wide range from 3V to

28V. VDD requires a supply voltage between 3V to 5V that

can be an external source or the internal LDO from V

IN.

Power Up Sequence

The SC403B initiates a start up when VIN, VDD, and EN/

PSV pins are above the applicable thresholds. When using

an external bias supply for the V

DD voltage, it is recom-

mended that the V

DD is applied to the device only after

the V

IN voltage is present because VDD cannot exceed VIN

at any time. A 10Ω resistor must be placed between the

external V

DD supply and the VDD pin to avoid damage to

the device during power-up and or shutdown situations

where V

DD could exceed VIN unexpectedly.

Shutdown

The SC403B can be shutdown by pulling either VDD or

EN/PSV pin below its threshold. When using an external

supply voltage for V

DD, the VDD pin must be deactivated

while the V

IN voltage is still present. A 10Ω resistor must

be placed between the external V

DD supply and the VDD

pin to avoid damage to the device.

When the VDD pin is active and EN/PSV is at low logic level,

the output voltage discharges through an internal FET.

Psuedo-fixed Frequency Adaptive On-time Control

The PWM control method used by the SC403B is pseudo-

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

The ripple voltage generated at the output capacitor ESR

is used as a PWM ramp signal. This ripple is used to trigger

the controller on-time.

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 output

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

side MOSFET. The pulse period is determined by V

OUT and

IN. The period is proportional to output voltage and

inversely proportional to input voltage. With this adaptive

on-time arrangement, the device automatically antici-

pates the on-time needed to regulate V

OUT for the present

IN condition and at the selected frequency.

The advantages of adaptive on-time control are:

Predictable operating frequency compared to

other variable frequency methods.

Reduced component count by eliminating the

error amplifier and compensation 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 typi-

cally slow error amplifier.

Reduced output capacitance due to fast tran-

sient response

•

Applications Information

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