SI9123 datasheet(9/16 Pages) VISHAY

部件名	SI9123

功能描述	500-kHz Half-Bridge DC-DC Converter With Integrated Secondary Synchronous Rectification Control

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制造商	VISHAY [Vishay Siliconix]

网页	http://www.vishay.com

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9 / 16 page

Si9123

Vishay Siliconix

New Product

Document Number: 72098

S-03639—Rev. B, 20-Mar-03

www.vishay.com

9

The pre-regulator will remain on until VCC equals VREG but

between VUVLO and VREG, excessive current may result in

VCC falling below VUVLOand stopping soft-start operation. This

situation is avoided by the hysteresis between VREG and

VUVLO and correct sizing of the VCC capacitor, bootstrap

capacitor, the soft-start capacitor, the primary MOSFET gate

driving charge, and load on the SEC_SYNC output. The value

of the VCC capacitor should be chosen to be capable of

maintaining soft-start operation with VCC above VUVLO until the

VCC current can be supplied from the external circuit (e.g., via

an auxiliary winding on the secondary inductor).

CVCC

4.7

mF

GND

VCC

VREF

VIN

VINEXT

Figure 6.

High-Voltage Pre-Regulator Circuit

HVDMOS

14.5 V

REXT = 1.4 kW

Auxillary

VCC

The feedback voltage from the output of the auxiliary winding

must sustain VCC above VREG to fully disconnect the

pre-regulator, isolating VCC from VINEXT. VCC is then

maintained above VREG for the duration of operation. In the

event of an over voltage condition on VCC, an internal voltage

clamp turns on at 14.5 V to shunt excessive current to GND.

In systems where operation is directly from a 12 V supply,

VINEXT and VCC can be connected to the 12 V bus.

The soft-start circuit is designed for the dc-dc converter to start

up in an orderly manner and reduce component stress.

Soft-start is achieved by ramping the maximum attainable duty

cycle during the soft-start time. The duty cycle is increased

from zero to the final value at the rate set by an external

capacitor, CSS as shown in Figure 7. The hiccup time is set by

an internal 20

µA current source charging CSS from 0 V to

2 Vbe, at which point switching begins. Then a 100 µA charging

current is applied to CSS to charge from 2 Vbe to the final value

controlling the duty cycle as it rises. In the event of UVLO,

shutdown or over current, the SS pin will be held low (<1 V)

disabling driver switching. A longer soft-start time may be

needed for highly capacitive loads and high peak-output

current applications. In the event of an over current condition

being detected, the soft-start pin will be pulled low and the

cycle will start again performing a hiccup as shown in

Figure 4. The hiccup off-time, t1, is given by:

t

1 [ CSS

1.2 V

20

mA

The soft-start time t2 is can be estimated as:

t

2 [

C

SS

V

OUT

n

(K

100

mA)

where VOUT is the output of the converter, and n is the turns

ratio of the primary to each secondary winding, and K is the

ratio of the resistive divider from VINEXT to VINDET (typically

10/1).

-

+

GM

CSS

+

-

AV

150 mV

Peak Detect

Blank

CS1

CS2

SS Control

Figure 7.

Current-Sense and Soft-Start Circuit Block Diagram

AV

100 mV

I

4I

VCC

SS

SS Enable

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