SC508 datasheet(13/32 Pages) SEMTECH

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SC508/SC508A

Synchronous Buck Converter

The SC508 is a step down synchronous DC-DC buck con-

troller with an internal 5V LDO. It provides efficient opera-

tion in a space saving 3x3 (mm) 20-pin package. The

programmable operating frequency range up to 1MHz

enables optimizing the configuration for PCB area and

efficiency. For automotive applications, the SC508A is

qualified to AEC-Q100.

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 SC508 requires two input supplies for normal opera-

tion: V

IN and VDDA/VDDP. VIN operates over the wide

range of 5V to 46V. VDDA and VDDP require a 5V supply

which can be from an external source or from the internal

LDO. VDDA and VDDP are derived from the same source

voltage.

Psuedo-fixed Frequency Adaptive On-time Control

The PWM control method used by the SC508 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 600mV 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

comparator and a capacitor. One comparator input is con-

nected to V

OUT, the other input is connected to the capaci-

tor. When the on-time begins, the capacitor charges from

zero volts through a current which is proportional to V

IN.

When the capacitor voltage reaches V

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

REF

COUT

VIN

ESR

VOUT

VLX

RTON

Figure 2 — On-Time Generation

•

Applications Information

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