SC4806MLTRT Просмотр технического описания (PDF) - Semtech Corporation

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SC4806MLTRT

Multiple Function Double Ended PWM Controller

Semtech Corporation 

SC4806

POWER MANAGEMENT

Application Information (Cont.)

ternal resistor divider connected as shown below. The

actual start-up voltage can be calculated by using the

equation below:

V Startup

=

V REF

× (1 +

R1

R2

)

+VIN

+

-VIN

R1

ON/OFF

R2

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

oscillator frequency is twice the frequency of the OUTA

and OUTB gate drive controls.

The oscillator capacitor CT is charged from the Vref

through RT. Once the RC pin reaches about Vref/2, the

capacitor is discharged internally by the SC4806. 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

≅ R OSC C TOT

REF

SYNC

4 RC

5 ILim

6 RAMP

SC4806 Vcc

VCC 12

OUTA 11

OUTB 10

where:

C TOT = C OSC + C SC4806 + C Circuit

CSC4806 ≅ 22pF

Tdeadtime

≅

C OSC × VREF × 0.5

3 ⋅10 −3

REFERENCE

REF

FB

The recommended range of timing resistors is between

I sense

10 kohm and 200kohm, range of timing capacitors is

between 100pF and 1000pF. Timing resistors less than

10 kohm should be avoided.

A 5V 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 net-

work with a regulated bias.

OSCILLATOR

The oscillator frequency is set by connecting a RC net-

work as shown below.

+VIN

SYNC/Bi-Phase operation

In noise sensitive applications where synchronization of

the oscillator frequency to a reference frequency may

be required, the SYNC pin can accept the external clock.

By connecting an external control signal to the SYNC pin,

the internal oscillator frequency will be synchronized to

the positive edge of the external control signal. SYNC is

a positive edge triggered input with a threshold set to

1.75V.

REF

SYNC

RT

CT

4 RC

5 ILIM

6 RAMP

SC4806

Vcc

VCC 12

OUTA 11

OUTB 10

REF

FB

I sense

In a single controller operation, SYNC should be grounded

or connected to an external synchronization clock within

the SYNC frequency range.

In the Bi-phase operation mode a very unique oscillator

is utilized to allow two SC4806’s to be synchronized

together and work out of phase. This feature is set up

by a simple connection of the SYNC input to the RC pin

of the other part. The master oscillator forces two

PWMs to operate out of phase. This feature minimizes

the input and output ripples, and may reduce input and

output capacitors.

 2006 Semtech Corp.

9

www.semtech.com

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