AN937 Datasheet, PDF (6/29 Pages) STMicroelectronics – The L4971 is a 1.5 A monolithic dc-dc converter

Power supply, UVLO and voltage reference
Figure 7. Maximum duty cycle vs. Rosc and Cosc as parameter
Dmax
5.3nF
D97IN685
0.90
0.80
4.7nF
2.2nF
1.2nF
0.8nF
AN937
0.70
0.60
0 4 8 12 16 20 24 28 32 ROSC(KΩ)
In fact, the slope of the ramp is modulated by the input ripple voltage, generally present in
the order of some tents of Volt, for both off-line and dc-dc converters using mains
transformers.
The charge and discharge time is approximately to:
Tch = R_osc ⋅ C_osc ⋅ In_⎝⎛⁶⁵--_⎠⎞
2.2
6/29
Tdis = 100 ⋅ C_osc
The maximum duty cycle is a function of Tch, Tdis and an internal delay and is represented
by the equation:
R_osc ⋅ C_osc ⋅ In_⎝⎛⁶⁵--_⎠⎞ – 80 ⋅ 10^–9
Dmax = ---------------------------------------------------------------------------------
R_osc ⋅ C_osc ⋅ In_⎝⎛⁶⁵--_⎠⎞ + 100 ⋅ C_osc
and is represented in Figure 7:
Current protection
The L4971 has two current limit levels, pulse by pulse and hiccup modes.
Increasing the output current till the pulse by pulse limiting current threshold (Ith1 typ. value
of 2.5 A) the controller reduces the on-time till the value of T_B = 300 ns that is a blanking
time in which the current limit protection does not trigger. This minimum time is necessary to
avoid undesirable intervention of the protection due to the spike current generated during
the recovery time of the freewheeling diode.
Doc ID 5655 Rev 15