AN937 Datasheet, PDF (18/29 Pages) STMicroelectronics – The L4971 is a 1.5 A monolithic dc-dc converter

Typical application
AN937
I_rms = I_O ⋅
D – ²-----^â‹---^D-----²- + ^D-----²-
η
η²
where η is the expected system efficiency, D is the duty cycle and Io the output dc current.
This function reaches the maximum value at D = 0.5 and the equivalent rms current is equal
to Io/2. The following diagram Figure 23 is the graphical representation of the above
equation, with an estimated efficiency of 85%, at different output currents.
The maximum and minimum duty cycles are:
D_max
=
-----^V-----^O-----+-----^V----^f-----
V_ccmin + V_f
=
0.66
D_max
=
------^V----^O-----+-----^V-----^f-----
V_ccmax + V_f
=
0.1
where Vf is the freewheeling diode forward voltage. This formula is not taking into account
the power MOS Rdson, considering negligible the inherent voltage drop, respect input and
output voltages. At full load, 1.5 A, and D=0.5, the rms capacitor current to be sustained is of
0.75 A.
The selected 180 μF/63 V KY, guarantying a life time of 10000 hours at an ambient
temperature of 105°C and switching frequency of 100 kHz, can support 1.15 A RMS current.
Figure 22. Efficiency vs. output current
η
(%)
D97IN738
V_CC=8V
90
V_CC=12V
85
V_CC=24V
80
75
V_CC=48V
70
65
fsw=100KHz
V_O=5.1V
60
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 I_O(A)
18/29
Doc ID 5655 Rev 15