AN1215 Datasheet, PDF (1/7 Pages) STMicroelectronics – HOW TO HANDLE SHORT CIRCUIT CONDITIONS

AN1215
APPLICATION NOTE
HOW TO HANDLE SHORT CIRCUIT CONDITIONS
WITH ST’s ADVANCED PWM CONTROLLERS
by Claudio Adragna & Giuseppe Gattavari
Purpose of this note is to provide some advice on how to manage a short circuit condition for a switching
converter controlled by ST's advanced PWM controllers L5991 and L5993.
After a quick review of merits and limits of the current limitation functionality embedded in these IC's, a
simple circuit will be given that allows to fulfill designer's typical requirements on what the converter is
supposed to do in case of overload or short circuit.
Current limitation schemes: merits and limits
ST's advanced PWM controllers L5991 and L5993 [1] [2] include a two-level overcurrent protection: the "pulse-
by-pulse" (or "cycle-by-cycle") current limitation and the so-called "hiccup mode" operation.
Pulse-by-pulse current limitation ideally limits the peak primary current at 1/R_s (see figure 1) when the converter
is overloaded or there is a load short circuit. Even neglecting non-idealities, which worsen the scenario and must
be accounted for in a safe design, pulse-by-pulse current limitation just prevents the system from losing control
of the peak primary current. Nothing more.
Figure 1. PWM control and current limiting in L5991 and L5993.
Vac
Vin
Is
2.5 V
ERROR
AMPLIFIER
+
2R
-
R
ADVANCED PWM
CONTROLLER
(L5991, L5993)
PWM
COMPARATOR
-
1V
+
CLOCK
S
R
Q
LATCH
Lp
DRIVER
+
-
1.2V
HICCUP
SOFT-START
Vdrain
Ip
Vcs
Rs
Vout
ISOLATED
FEEDBACK
Css
December 1999
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