Datasheet

1
T
ON-MIN
x
V
OUT
V
PVIN-max
=
f
s-max
1
T
ON-MIN
x
V
OUT
=
V
PVIN-max
f
S
LM21305
SNVS639F –DECEMBER 2009–REVISED MARCH 2013 www.ti.com
(5)
Similarly, if the PVIN input rail is fixed, the maximum switching frequency without imposing minimum on-time can
be found by:
(6)
In rare cases where steady-state operation at minimum duty cycle is unavoidable, the regulator will automatically
skip cycles to keep V
OUT
regulated, similar to light-load DCM operation.
THERMAL PROTECTION
Internal thermal shutdown circuitry is provided to protect the integrated circuit in the event that the maximum
junction temperature is exceeded. When activated, typically at 160°C, the LM21305 tri-states the power
MOSFETs and resets soft-start. After the junction temperature cools to approximately 150°C, the LM21305 starts
up using the normal startup routine. This feature is provided to prevent catastrophic failures from due to device
overheating.
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