SPDC12L00010

Application information

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5.4

Soft-start

The soft-start phase begins when both VCC and VIN raise above their turn-on thresholds,

otherwise the SS_INL pin is internally shorted to SGND.

A ramp is generated at SS_INL pin during start-up, charging the external capacitor CSS with

an internal current generator.

The initial value for this current is 30

μA and it charges the capacitor up to 0.5 V, after that, it

is reduced to 10

μA until the final charge value approximately 4 V.

In the meanwhile, the controller internal voltage reference raises to its final value, following

the SS_INL pin voltage slope.

During soft-start, the module provides a constant current protection, limiting the output

current at the maximum value, without entering in HICCUP mode.

If there is not current limitation, the output voltage slope follows the SS_INL pin slope.

The output voltage rise time, can be set choosing proper CSS value.

The soft-start phase ends when the SS_INL pin voltage reaches 3.5 V.

A capacitor CSS = 3 3 nF is present on the module, to perform a minimum soft-start time,

suitable for Co = 10000

μF max. output capacitor;

in this condition and with 10 A output current resistor load, the output voltage rise time is

around 5ms, but the complete soft-start time is around 10ms.

Using the minimum output capacitor Co = 660

μF and with 10 A resistor load, the output

voltage rise time is around 2 ms.

5.5

Multiple units synchronization

Using more than one unit on the same circuit, it is possible to synchronize the switching

frequency oscillators, connecting all SYNC (pin L6) together.

The device with the higher switching frequency will be the Master, while the other will be the

Slaves.

The best way to synchronize two or more devices is to set same switching frequency, in any

case, the switching frequencies can differ for a maximum of 50% of the lowest one.

Using and external clock signal, to synchronize one or more devices working at a different

switching frequency, it is recommended to follow the below formula:

The phase shift between master and slaves is approximately 180°.