Datasheet

LMR24220
www.ti.com SNVS737E –OCTOBER 2011–REVISED APRIL 2013
Soft-Start
The soft-start feature allows the converter to gradually reach a steady state operating point, thereby reducing
startup stresses and current surges. Upon turn-on, after V
CC
reaches the under-voltage threshold, an 8 µA
internal current source charges up an external capacitor C
SS
connecting to the SS pin. The ramping voltage at
the SS pin (and the non-inverting input of the regulation comparator as well) ramps up the output voltage V
OUT
in
a controlled manner.
The soft start time duration to reach steady state operation is given by the formula:
t
SS
=V
REF
x C
SS
/ 8µA = 0.8V x C
SS
/ 8µA (7)
This equation can be rearranged as follows:
C
SS
= t
SS
x 8µA / 0.8V (8)
Use of a 4.7nF capacitor results in a 0.5ms soft-start duration. This is a recommended value. Note that high
values of C
SS
capacitance will cause more output voltage drop when a load transient goes across the DCM-CCM
boundary. If a fast load transient response is desired for steps between DCM and CCM mode the softstart
capacitor value should be less than 18nF (which corresponds to a soft-start time of 1.8ms).
An internal switch grounds the SS pin if any of the following three cases happens: (i) V
CC
is below the under-
voltage lock-out threshold; (ii) a thermal shutdown occurs; or (iii) the EN pin is grounded. Alternatively, the output
voltage can be shut off by connecting the SS pin to ground using an external switch. Releasing the switch allows
the SS pin to ramp up and the output voltage to return to normal. The shutdown configuration is shown in
Figure 21.
Figure 21. Alternate Shutdown Implementation
Thermal Protection
The junction temperature of the LMR24220 should not exceed the maximum limit. Thermal protection is
implemented by an internal Thermal Shutdown circuit, which activates (typically) at 165°C to make the controller
enter a low power reset state by disabling the main MOSFET, disabling the on-timer, and grounding the SS pin.
Thermal protection helps prevent catastrophic failures from accidental device overheating. When the junction
temperature falls back below 145°C (typical hysteresis = 20°C), the SS pin is released and normal operation
resumes.
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