AN937 Datasheet, PDF (8/29 Pages) STMicroelectronics – The L4971 is a 1.5 A monolithic dc-dc converter

Power supply, UVLO and voltage reference
Figure 10. Current limiting internal schematic circuit
OSC
SQ
R
V_Th1
+
-
V_Th2
+
-
OSC
VFB
PWM
+-
HICCUP
THERMAL
UNDERVOLTAGE
VREF
D97IN658
SOFT START
LATCH
SQ
R
+
0.4
-
Figure 11. Output current and soft start voltage
AN937
V_CC
OUT
12V
C_SS
5.4A
4.5A
Figure 10 shows the internal current limiting circuitry. Vth1 is the pulse by pulse while Vth2 is
the hiccup threshold.
The sense resistor is in series with a small MOS realized as a partition of the main DMOS.
The Vth2 comparator (20% higher than Vth1) sets the soft start latch, initializing the
discharge of the soft start capacitor with a constant current (about 22 mA). Reaching about
0.4 V, the valley comparator resets the soft start latch, restarting a new recharge cycle.
Figure 11 shows the typical waveforms of the current in the output inductor and the soft start
voltage (pin 2).
During the recharging of the soft start capacitor, the Ton increases gradually and, if the short
circuit is still present, when Ton>TB and the output peak current reaches the threshold, the
hiccup protection intervenes again. So, the value of the soft start capacitor must not be too
high (in this case the Ton increases slowly thus taking much time to reach the TB value) to
avoid that during the soft start slope, the current exceeds the limit before the protection
activation.
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Doc ID 5655 Rev 15