AN557 Datasheet, PDF (20/52 Pages) STMicroelectronics – EASY APPLICATION DESIGN WITH THE L4970A

AN557 APPLICATION NOTE
The 40KHz internal oscillator is synchronized with the principal one. If the system work with a oscillator
tracks the principal oscillator frequency. In this way the switching frequency will not increase in overload
situation.
A particular care has to be taken in the inductor value in order to avoid problems during overload or short
circuit conditions. A critical situation is present with high switching frequency, (more than 200KHz) where
a small inductor value is used and with high capacitive load.
In order to return in nominal condition after a short circuit the inductor ripple at 40KHz with the nominal
output voltage and current has to be lower than the current limitation value.
Example
Let us consider L4970A, 10A. (the same approach can be used for all the family).
The inductor ripple current is given by the following formula:
∆I_L
=
(_---V_{----i---–----}V_----o----)_---⋅_---V_----o-
V_i ⋅ fsw ⋅ L
where fsw = 40KHz ±10%
In order to get the maximum inductor ripple current, the previous formula becomes:
∆I_L
=
-⁽--^V----ⁱ-^m-----^a---^x----–----^V-----^o---^m----^a---^x---⁾----^â‹---^V----^o---^m-----^a---^x
V_imax ⋅ f_swmin ⋅ L
The current limitation for L4970A will start to work at 13A. therefore:
I_lim
min
>
I_onom
+
∆_----I_--L-
2
where I_onom = 10A for L4970A.
POWER FAIL-RESET CIRCUIT
The L4970A include a voltage sensing circuit that may be used to generate a power on power off reset
signal for a microprocessor system. The circuit senses the input supply voltage and the output generated
voltage and will generate the required reset signal only when both the sensed voltages have reached the
required value for correct system operation. The Reset signal is generated after a delay time programma-
ble by an external capacitor on the delay pin. Fig 25 shows the circuit implementation of Reset circuit. The
supply voltage is sensed on an external pin, for programmability of the threshold, by a first comparator.
The second comparator has the reference threshold set at slightly less the ref. voltage for the regulation
circuit and the other input connected internally at the feedback point on the regulated voltage. When both
the supply voltage and the regulated voltage are in the correct range, transistor Q1 turns off and allows
the current generator to charge the delay capacitor discharges completely before initialization of a new
Reset cycle. The output gate assures immediate take of reset signal without waiting for complete dis-
charge of delay capacitor. Reset output is an open collector transistor capable of sinking 20mA at 200mV
voltage. Fig 26 shows reset waveforms.
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