SC9301 datasheet(13/29 Pages) SEMTECH

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SC9301

Synchronous Buck Converter

The SC9301 is a step down synchronous DC-DC buck con-

verter with integrated power MOSFETs and a 200mA

capable 5V LDO. The device is capable of 10A 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 configuration for

PCB area and efficiency.

The buck controller uses a pseudo-fixed frequency adap-

tive on-time control. This allows fast transient response

which permits the use of smaller output capacitors.

Input Voltage Requirements

The SC9301 requires two input supplies for normal opera-

tion: V

and VDD. V

operates over the wide range of 3V

to 28V. VDD requires a 3V to 5.5V supply which can be

from an external source or from the internal fixed 5V

LDO.

Power Up Sequence

When the SC9301 uses an external power source at the

VDD pin, the switching regulator initiates the start up

when V

, VDD and EN are above their respective thresh-

olds. When EN is at a logic high, VDD can be applied after

rises. It is also recommended to use a10Ω resistor

between an external power source and the VDD pin. To

start up using the EN pin when both VDD and V

are above

their respective thresholds, apply EN to enable the start-

up process. For SC9301 in self-biased mode, refer to the

LDO section for a full description.

Shutdown

The SC9301 can be shutdown by pulling either VDD or EN

below its threshold. When VDD is active and EN 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 SC9301 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

. The period is proportional to output voltage and

inversely proportional to input voltage. With this adaptive

on-time configuration, the device automatically antici-

pates the on-time needed to regulate V

OUT

for the present

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.

One-Shot Timer and Operating Frequency

One-shot timer operation is shown in Figure 2. The FB com-

parator output goes high when V

is less than the internal

600mV reference. This feeds into the DH gate drive and

turns on the high-side MOSFET, and also starts the one-shot

•

Applications Information

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