SC418 datasheet(13/30 Pages) SEMTECH

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SC418

Synchronous Buck Converter

The SC418 is a step down synchronous DC-DC buck con-

troller with a programmable LDO. It provides high effi-

ciency operation in a space saving 3x3 (mm) 20-pin

package. The programmable operating frequency range

of 200kHz to 1MHz enables optimizing the configuration

for PCB area and efficiency.

The controller uses a pseudo-fixed frequency adaptive

on-time control. This allows fast transient 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 SC418 online

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

user through the design process.

Input Voltage Requirements

The SC418 requires three input supplies for normal opera-

tion: V

IN, VDDA, and VDDP. VIN operates over the wide

range of 3V to 28V. VDDA and VDDP require a 3.3V or 5V

supply which can be from an external source or from the

internal LDO configured to 3.3V or 5V. VDDA and VDDP

should derive from the same source voltage. Power-save

operation and Ultrasonic Power-save are not supported

for VDDA/VDDP below 4.5V.

Psuedo-fixed Frequency Adaptive On-time Control

The PWM control method used by the SC418 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 configuration, 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.

One-Shot Timer and Operating Frequency

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

comparator output goes high when V

FB is less than the

internal 500mV reference. This feeds into the DH gate

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

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

•

Applications Information

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