(ILIM x RDS(ON)) ± (ISEN x RCS)

RCLF =

RCS x VOUT

VIN ± 9.5V

RCS

10 mA

t

PLIM x RDS(ON)

= RCS

ISEN

RCLF

VOUT

RCS

SW

+

-

40 PA

ILIM

ISEN

18

LM2743

SNVS276I – APRIL 2004 – REVISED FEBRUARY 2019

www.ti.com

Product Folder Links: LM2743

Submit Documentation Feedback

Copyright © 2004–2019, Texas Instruments Incorporated

Feature Description (continued)

In order to minimize the time period in which peak inductor current exceeds the current limit threshold, the IC

also discharges the soft-start capacitor through a fixed 90-µA sink. The output of the LM2743 internal error

amplifier is limited by the voltage on the soft-start capacitor. Hence, discharging the soft-start capacitor reduces

the maximum duty cycle D of the controller. During severe current limit this reduction in duty cycle will reduce the

output voltage if the current limit conditions last for an extended time. Output inductor current will be reduced in

turn to a flat level equal to the current limit threshold. The third benefit of the soft-start capacitor discharge is a

smooth, controlled ramp of output voltage when the current limit condition is cleared.

7.3.11

Foldback Current Limit

In the case where extra protection is used to help an output short condition, a current foldback resistor (RCLF)

should be considered, see Figure 29. First select the percentage of current limit foldback (PLIM):

PLIM = ILIM x P

(12)

where P is a ratio between 0 and 1.

Figure 29. Foldback Current Limit Circuit

Obtain the RCS with the following equation:

(13)

where ISEN = 40 μA. If the switch node goes above 9.5 V the following criterion must be satisfied:

(14)

The equation for calculating the foldback resistance value is:

(15)

7.4 Device Functional Modes

7.4.1 Shutdown

If the shutdown pin is pulled low, (below 0.8 V) the LM2743 enters shutdown mode, and discharges the soft-start

capacitor through a MOSFET switch. The high and low-side MOSFETs are turned off. The LM2743 remains in

this state as long as VSD sees a logic low (see the Electrical Characteristics table). To assure proper IC start-up

the shutdown pin should not be left floating. For normal operation this pin should be connected directly to VCC or

to another voltage between 1.3 V to VCC (see the Electrical Characteristics table).