SC4808C Ver la hoja de datos (PDF) - Semtech Corporation

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SC4808C

High Performance Dual Ended PWM Controller

Semtech Corporation 

SC4808C

POWER MANAGEMENT

Application Information (Cont.)

VCC UNDER VOLTAGE LOCK OUT

Depending on the application and the voltages available,

the SC4808C can be used to provide the VCC undervoltage

lock out function to ensure the converters controlled start

up.

Before the VCC UVLO has been reached, the internal refer-

ence, oscillator, OUTA/OUTB drivers, and logic are disabled.

The oscillator frequency is set by connecting a RC network

as shown below.

REF

C26

2.2u,16V

VCC

0

LINE UNDER VOLTAGE LOCK OUT

The SC4808C also provides a line undervoltage (LUVLO =

Vref) function. The LUVLO pin is programmed via an exter-

nal resistor divider connected as shown below. The actual

start-up voltage can be calculated by using the equation

below:

( ) V Startup

= V REF

×

R 23 + R 33

R 33

R24 10k

15

R25 18

R26

2.2k

C29

82p

REF

C26

2.2u,16V

VCC

R27

15k

C31

200p

U4

5 REF

GND 6

4 FB

OUTB 7

3 CS

OUTA 8

2 RC

VCC 9

1 SYNC

LUVLO 10

R28

10

C33

0.1u,25V

SC4808

SYNC

56.2k

10k

Vin

R23

R33

R27

15k

C31

200p

U4

5 REF

GND 6

4 FB

OUTB 7

3 CS

OUTA 8

2 RC

VCC 9

1 SYNC

LUVLO 10

R28

10

C33

0.1u,25V

SC4808

0

SYNC

56.2k

10k

Vin

R23

R33

The oscillator has a ramp voltage of about Vref/2. The os-

cillator frequency is twice the frequency of the OUTA and

OUTB gate drive controls.

The oscillator capacitor C31 is charged by a current sourced

from the Vref through R27. Once the RC pin reaches about

Vref/2, the capacitor is discharged internally by the

SC4808C. It should be noted that larger capacitor values

will result in a longer dead time during the down slope of

the ramp.

The following equation can be used as an approximation

of the oscillator frequency and the Dead time:

FOSC

≅

1

ROSCCTOT

× 0.8

where:

REFERENCE

A 3.125V(SC4808C) reference voltage is available that can

be used to source a typical current of 5mA to the external

circuitry. The Vref can be used to provide the oscillator RC

network with a regulated bias.

CTOT = COSC + CSC4808 + CCircuit

CSC4808 ≅ 22pF

Tdeadtime

≅

COSC × VREF × 0.5

3 ⋅10−3

OSCILLATOR

The recommended range of timing resistors is between 10

kOhm and 200kOhm, range of timing capacitors is between

100pF and 1000pF. Timing resistors less than 10 kOhm

should be avoided.

 2006 Semtech Corp.

7

www.semtech.com

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