SC401B datasheet(16/32 Pages) SEMTECH

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SC401B

Synchronous Buck Converter

The SC401B is a step down synchronous DC-DC buck

converter with integrated power MOSFETs and a 200mA

capable programmable LDO. The device is capable of 15A

operation at very high efficiency. A space saving 5x5 (mm)

32-pin package is used. The programmable operating

frequency of up to 1MHz enables optimizing the configu-

ration 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 SC401B online

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

user through the design process.

Input Voltage Requirements

The SC401B requires two input supplies for normal opera-

tion: V

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

to 17V. VDD requires a 3V to 5.5V supply input that can be

an external source or the internal LDO configured to

supply 3V to 5.5V from V

IN.

Power Up Sequence

When the SC401B uses an external power source at the

VDD pin, the switching regulator initiates the start up when

IN, VDD and EN/PSV are above their respective thresholds.

When EN/PSV is at a logic high, VDD needs to be applied

after V

IN rises. Also, it is recommended to use a 10Ω resistor

between an external power source and the VDD pin. To

start up by using the EN/PSV pin when both VDD and V

are above their respective thresholds, apply EN/PSV to

enable the start-up process. For SC401B in self-biased

mode, refer to the LDO section for a full description.

Shutdown

The SC401B can be shut down by pulling either VDD or

EN/PSV below its threshold. When using an external

power source, it is recommended that the VDD voltage

ramps down before the V

IN voltage. When VDD is active

and EN/PSV at low logic, the output voltage discharges

through an internal FET.

Psuedo-fixed Frequency Adaptive On-time Control

The PWM control method used by the SC401B 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 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 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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