LP38869_15 Datasheet, PDF (14/23 Pages) Texas Instruments – 1A FlexCap, Low-Dropout Linear Regulator with 0.75% Accuracy

LP38869
SNVS877B – AUGUST 2012 – REVISED APRIL 2013
www.ti.com
where C_SS is in nF, and the calculated t_SS result is in milli-seconds.
Typical Soft-Start capacitor values are between 10nF to 100nF. The typical t_SS with a C_SS of 100 nF is:
t_SS = (100 nF / 6.7 uA ) x 1.25V = 18.6 ms
(5)
Use of a low leakage capacitor for C_SS is required, and voltage rating of 5V for C_SS is adequate.
Because the C_SS 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 t_SS 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 V_SS rising from approximately 300 mV
to 1,25V the current limits scale proportionately with the V_SS by:
I_LIM = 1A x (V_SS / 1.25V)
(6)
Since the SS pin sources a typical 6.7 µA current (I_SS), the current limit can be reduced below 1A by connecting
a resistor (R_SS) between the SS pin and GND, so that:
I_LIM = 1A x (( I_SS x R_SS) / 1.25V))
(7)
where the typical I_SS = 6.7 µA.
The useful range of R_SS values is approximately 75 kΩ to 200 kΩ. R_SS values less than 75 kΩ may not be able to
reliably raise V_SS above the minimum needed (typically 300 mV) to activate the current limit. R_SS values greater
than 200 kΩ will only have a marginal impact on the nominal current limit.
With R_SS in place, Soft-Start can still be achieved by placing a capacitor (C_SS) in parallel with R_SS. 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 R_SS in place is
estimated by:
t_SS = 2.5 x R_SS x C_SS
(8)
V_IN
C_IN
1 PF
ON-
OFF-
R_SS C_SS
IN
OUT
IN
OUT
IN
OUT
IN
OUT
LP38869
RST
SHDN ^SET
SS
GND
DAP
V_OUT
C_OUT
1 PF
R1
RESET
R2
Figure 34. Typical Operating Circuit Soft-Start and Current Limit Reduction
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