LP38869 datasheet(14/23 Pages) TI

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CIN

1 PF

ON-

OFF-

VOUT

COUT

1 PF

LP38869

OUT

SET

GND

SHDN

RST

DAP

RESET

VIN

CSS

RSS

LP38869

SNVS877B – AUGUST 2012 – REVISED APRIL 2013

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where CSS is in nF, and the calculated tSS result is in milli-seconds.

Typical Soft-Start capacitor values are between 10nF to 100nF. The typical tSS with a CSS of 100 nF is:

tSS = (100 nF / 6.7 uA ) x 1.25V = 18.6 ms

(5)

Use of a low leakage capacitor for CSS is required, and voltage rating of 5V for CSS is adequate.

Because the CSS ramp is applied to the output current-limit comparator, the actual time for the output voltage to

ramp-up depends on the actual load current demand and output capacitor value. Leaving the SS pin open will

disable the Soft-Start behavior by setting the tSS time to zero.

Current Limiting

The LP38869 features a 2A current limit when the output voltage is in regulation. When the output voltage drops

by 8% below its nominal value, the current limit folds back to 1.7A. While the LP38869 output can be shorted to

ground for an indefinite period of time without damaging the device, power dissipation due to continuous output

currents of more than 1A may stress, or damage, adjacent components. The nominal current limit can be

reduced by holding the voltage at the Soft-Start (SS) pin below 1.25V. For VSS rising from approximately 300 mV

to 1,25V the current limits scale proportionately with the VSS by:

ILIM = 1A x (VSS / 1.25V)

(6)

Since the SS pin sources a typical 6.7 µA current (ISS), the current limit can be reduced below 1A by connecting

a resistor (RSS) between the SS pin and GND, so that:

ILIM = 1A x (( ISS x RSS) / 1.25V))

(7)

where the typical ISS = 6.7 µA.

The useful range of RSS values is approximately 75 kΩ to 200 kΩ. RSS values less than 75 kΩ may not be able to

reliably raise VSS above the minimum needed (typically 300 mV) to activate the current limit. RSS values greater

than 200 k

Ω will only have a marginal impact on the nominal current limit.

With RSS in place, Soft-Start can still be achieved by placing a capacitor (CSS) in parallel with RSS. The output

current now ramps up asymptotically to the reduced current limit rather than the nominal value, increasing the

soft-start time. The time required for the current limit to reach 90% of its steady-state value with RSS in place is

estimated by:

tSS = 2.5 x RSS x CSS

(8)

Figure 34. Typical Operating Circuit Soft-Start and Current Limit Reduction

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