AN937 Datasheet, PDF (7/29 Pages) STMicroelectronics – The L4971 is a 1.5 A monolithic dc-dc converter

AN937
Power supply, UVLO and voltage reference
In this condition, because of this fixed blanking time, the output current is given by:
I_max
=
--------------------------^[--^V----^C----^C-----^â‹---^T----^B----^â‹---^F----^S----^W------–----^V----^f---^â‹---⁽---¹-----–----^T----^B-----^â‹---^F---^S----^W-----⁾--^]--------------------------
[R_O + (R_D + R_L)(1 – T_B ⋅ F_SW) + (R_dson + R_L)T_B ⋅ F_SW]
In Figure 8, the pulse by pulse protection is sufficient to limit the current.
In Figure 9 the pulse by pulse protection is no more effective to limit the current due to the
minimum Ton fixed by the blanking time TB, and the hiccup protection intervenes because
the output peak current reaches the relative threshold. At the pulse by pulse intervention
(point A) the output voltage drops because of the Ton reduction, and the current is almost
constant. Going versus the short circuit condition, the current is only limited by the series
resistances RD and RL (see relation above) and could reach the hiccup threshold (point B),
set 20% higher than the pulse by pulse. Once the hiccup limiting current is operating, in
output short circuit condition, the delivered average output current decreases dramatically at
very low values (point C).
Figure 8. Output characteristics
V_O
A
B
C
D98IN908A
2.5A 3A
I_O
Figure 9. Output characteristics
V_O
A
D97IN809
2.5A 3A
I_O
Doc ID 5655 Rev 15
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