SC4808CMSTRT datasheet(7/24 Pages) SEMTECH

7 / 24 page

 2006 Semtech Corp.

www.semtech.com

SC4808C

POWER MANAGEMENT

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.

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:

()

REF

Startup

R27

15k

R24 10k

200p

C31

VCC

R26

2.2k

R28

2.2u,16V

C26

82p

C29

REF

0.1u,25V

C33

R25 18

R33

10k

SYNC

SC4808

REF

LUVLO

SYNC

GND

OUTB

OUTA

VCC

R23

56.2k

Vin

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.

OSCILLATOR

Application Information (Cont.)

The oscillator frequency is set by connecting a RC network

as shown below.

R27

15k

200p

C31

VCC

R28

2.2u,16V

C26

REF

0.1u,25V

C33

R33

10k

SYNC

SC4808

REF

LUVLO

SYNC

GND

OUTB

OUTA

VCC

R23

56.2k

Vin

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:

TOT

OSC

≅

where:

Circuit

4808

OSC

TOT

C 4808

≅

REF

OSC

deadtime

−

⋅

≅

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.

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