R2

R1

C1

+

VREF

15

14

MAXI

4

VOUT

UDG-00017

R3

Q1

R2

R1

C1

+

VREF

15

14

MAXI

4

VOUT

Q1

UDG-11278

(

)

æ

ö

-

= ç

÷

ç

÷

è

ø

IN

DD

DD

V

10 V

R

I

(

)

æ

ö

-

= ç

÷

ç

÷

è

ø

IN

SS

DD

V

5 V

R

I

=

SENSE

FAULT

50 mV

R

I

UCC2917

UCC3917

www.ti.com

SLUS203D – FEBRUARY 2000 – REVISED MAY 2013

SELECTING OTHER EXTERNAL COMPONENTS

Other external components are necessary for correct operation of the device. Referring to Figure 13, resistors

(15)

(16)

(17)

(R17 + R18 + R19) > 20 k

Ω (current limit out of VREF)

Use a value of 0.1 µF for the external charge pump capacitors.

SOFT-START OPERATION

The soft-start circuits in Figure 4, and Figure 5 gradually ramp up the load current on power-up, retry, or if the

SHTDWN pin is pulled high. Control circuitry (not shown) turns on Q1 to discharge C1 when FLTOUT or

SHTDWN are low (i.e., external power MOSFET is off) so the load current always ramps from zero. The circuit in

Figure 4 uses an inexpensive bipolar transistor for Q1 so the component cost is lower than the circuit in Figure 5.

Figure 4. Soft-Start Circuit Using A Higher-Cost Bi-

Figure 5. Soft-Start Circuit Using A Lower-Cost

Polar Transistor

MOSFET

Soft-start operation minimizes the voltage disturbance on the power bus when a circuit card is inserted into a live

back plane. This disturbance could reset a system, which is not desirable when high availability is required. A

server is an example of a high availability system.

Soft-start operation is initiated with the SHTDWN pin in as shown in Figure 6. The anode of D2 is grounded

when the card is in the back plane. R2 limits the SHTDWN pin current to between 60 µA and 500 µA (i.e., 60 µA

< 0.65 V / R2 < 500 µA).

Copyright © 2000–2013, Texas Instruments Incorporated

Submit Documentation Feedback

11

Product Folder Links: UCC2917 UCC3917

Obsolete Devices