LM3101 datasheet(11/20 Pages) NSC

11 / 20 page

Functional Description (Continued)

Soft-Start Section

Soft-Start is accomplished by gradually increasing the refer-

ence voltage during start-up The gradual increase is imple-

mented by charging the Soft-Start capacitor CSFST onpin

3 (the SFST pin) The charging current is set according to

the following equation

ISFST e 045RT

A X

TLH11436 – 12

FIGURE 6 Soft-Start Block Diagram

Typically ISFST starts to flow when the supply voltage is

raised above 3V

As shown in

Figure 6 at the beginning of start-up CSFST is

not charged up and the SFST pin pulls down the reference

voltage from its nominal value to

VSFSTO e ISFST R1

V A X

VSFSTO is designed to be 85% of the nominal reference

voltage The reference voltage rises smoothly from VSFSTO

to its nominal value as CSFST charges up When CSFST

charges up completely the reference voltage is at its nomi-

nal value start-up is over and steady-state operation be-

gins

The discharge time of CSFST is set by the RC network as

tDS e 5 CSFST R2

This is the time delay required to prepare the Soft-Start cy-

cle after the LM3101 and the entire regulator has been

turned off R2 must be large enough so that the final value

of VSFST is greater than the reference voltage

VSFST e ISFST (R1 a R2) t 124V

V A X

Power Supply Monitor Functions

The LM3101 provides two monitor functions a power-on

reset flag with programmable delay and a crowbar driver

output for overvoltage conditions

POWER-ON RESET

The power-on reset (POR) flag monitors the output voltage

via the feedback pin (FB - pin 13) The POR flag will go low

after the output voltage reaches 95% of its nominal value

and the subsequent programmed delay has passed The

POR flag pin (pin 11) is an open-collector pin which needs

an external resistor to pull it up This pin is valid with supply

voltages as low as 1V while sinking 16 mA

To program the reset delay connect an external capacitor

to the CRD pin (pin 12) The practical range of delay is from

10 ms to 5 ms and follows the equation

TRD e CRD 60 103

s F

For a power-on reset delay of 120 ms the reset delay ca-

pacitor must be 0002 mF

CROWBAR DRIVER OUTPUT

The second monitor function is a crowbar driver output

(CD – pin 10) If the output voltage gets higher than 120% of

its nominal value the CD pin can supply more than 200 mA

to an external SCR trigger input The SCR will fire shorting

the regulator output and saving the load circuitry from ex-

cessive supply voltage

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