E-L5973AD Datasheet, PDF (9/22 Pages) STMicroelectronics – 2 A switch step down switching regulator

L5973AD
Functional description
In particular, connecting together two devices, the one with the lower switching frequency
works as Slave and the other one works as Master.
To synchronize the device, the SYNC pin has to pass from a low level to a level higher than
the synchronization threshold with a duty cycle that can vary approximately from 10% to
90%, depending also on the signal frequency and amplitude.
The frequency of the synchronization signal must be at least higher than the internal
switching frequency of the device (500 kHz).
Figure 5. Oscillator circuit
FREQUENCY
SHIFTER
Ibias_osc
CLOCK
GENERATOR
CLOCK
t
RAMP
GENERATOR
RAMP
D00IN1131
SYNCHRONIZATOR
SYNC
4.4
Current protection
The L5973AD has two current limit protections, pulse by pulse and frequency fold back.
The schematic of the current limitation circuitry for the pulse by pulse protection is shown in
Figure 6.
The output power PDMOS transistor is split in two parallel PDMOS. The smallest one has a
resistor in series, R_SENSE. The current is sensed through Rsense and if reaches the
threshold, the mirror is unbalanced and the PDMOS is switched off until the next falling edge
of the internal clock pulse.
Due to this reduction of the ON time, the output voltage decreases.
Since the minimum switch ON time (necessary to avoid false overcurrent signal) is not
enough to obtain a sufficiently low duty cycle at 500 kHz, the output current, in strong
overcurrent or short circuit conditions, could increase again. For this reason the switching
frequency is also reduced, so keeping the inductor current under its maximum threshold.
The Frequency Shifter (see Figure 5) depends on the feedback voltage. As the feedback
voltage decreases (due to the reduced duty cycle), the switching frequency decreases too.
Doc ID 9552 Rev 9
9/22