SC419 datasheet(11/24 Pages) SEMTECH

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SC419

General Description

The SC419 is a step down synchronous DC-DC buck con-

troller. It provides high efficiency operation in a space

saving 3 x 3 (mm) 20-pin package. The programmable

operating frequency range from 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.

Input Voltage Requirements

The SC419 requires three input supplies for normal opera-

tion: V

IN, VDDA, and VDDP. VIN can operate over the wide

range of 3V to 28V. VDDA and VDDP require an external

5V supply. VDDA and VDDP should be connected to the

same source voltage.

Psuedo-fixed Frequency Adaptive On-time Control

The PWM control method used by the SC419 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 gate drive

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

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

parator and a capacitor. One comparator input is con-

nected to V

OUT, the other input is connected to the

capacitor. When the on-time begins, the internal capaci-

tor charges from zero volts through a current which is

proportional to V

IN. When the capacitor voltage reaches

OUT, the on-time is completed and the high-side MOSFET

turns off.

Gate

Drives

FB Comparator

One-Shot

Timer

On-time = K x RTON x (VOUT/VIN)

VOUT

VIN

500mV

COUT

VIN

ESR

VOUT

VLX

RTON

Figure 2 — On-Time Generation

•

Applications Information

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