SOIC-8

Si9121

CS

BYPASS

GND

COMP

5

6

7

8

Top View

2

3

4

1

Si9121

Vishay Siliconix

www.vishay.com

6

Document Number: 71112

S-40708—Rev. C, 19-Apr-04

PIN CONFIGURATION AND ORDERING INFORMATION

ORDERING INFORMATION

Part Number

Temperature Range

Package

Si9121DY-5-T1

Si9121DY-5-T1—E3

Tape and Reel

Si9121DY-3-T1

40 to 85_C

Tape and Reel

Si9121DY-3-T1—E3

−40 to 85_C

Si9121DY-5

Bulk

Si9121DY-3

Bulk

Eval Kit

Temperature Range

Board Type

Si9121DB-5

10 to 70_C

Surface Mount and

Si9121DB-3

−10 to 70_C

Surface Mount and

Thru-Hole

PIN DESCRIPTION

Pin Number

Name

Function

1

CS

Current sense pin to detect the inductor current for current mode control and over current protection

2

Negative supply voltage (−10 V to −60 V)

3

BYPASS

+1.5-V bandgap reference. Decouple with 0.1 mF capacitor.

4

COMP

Compensation node to stabilize the converter

5

Output voltage feedback connected to the PWM summing comparator

6

GND

Low impedance system ground

7

Internally generated supply voltage for the internal circuit and MOSFET drive circuit. Decouple with an external

bypass capacitor.

8

Inductor connection node

DETAILED DESCRIPTION

Start-Up

The UVLO circuit prevents the internal circuits from turning on

if VCC is less than 7.6 V (typical) above the negative supply

drops below 7.0 V. This hysteresis prevents the converter

from oscillating during the start-up phase and unintentionally

threshold the internal reference, oscillator, and soft-start

circuits are enabled.

Soft-Start

The Si9121 has an on-chip soft-start circuit which utilizes the

error amplifier external compensation network to ramp the

output NMOS transistor current limit which, in turn, allows the

output voltage to rise gradually without excessive overshoot.

the UVLO threshold.

For the recommended frequency

compensation components (see Typical Application Circuit)

the soft-start time is approximately 10 ms.

Oscillator

The oscillator is designed to operate at a nominal frequency

of 95 kHz with no external components. The 95-kHz operating

frequency allows the converter to operate in PWM mode

during the full load condition even though the duty cycle is very

low. The 95kHz switching frequency also allows the converter

to operate at optimal efficiency without a large output inductor

and capacitor.

PWM Mode and Current Limit

The Si9121 is a current-mode converter designed to operate

in PWM mode. It features pulse-by-pulse peak current limiting

such that when the peak current sensed voltage on the CS pin

is greater than 0.67 V the switch is turned off for the remainder

of the clock cycle.