13/26

L6996

continuous mode at high load. The transition from PFM to PWM occurs when load current is around half the

ue is, the smaller the threshold is. On the other hand, the bigger the inductor value is, the slower the transient

response is. In PFM mode the frequency changes, with the output current changing, more than in PWM mode;

in fact if the output current increase, the output voltage decreases more quickly; so the successive turn-on ar-

rives before, increasing the switching frequency. The PFM waveforms may appear more noisy and asynchro-

nous than normal operation, but this is normal behaviour mainly due to the very low load. The NOSKIP feature

cannot be disabled.

1.4 Softstart

If the supply voltages are already applied, the SHDN pin gives the start-up. The system starts with the high side

MOSFET off and the low side MOSFET on. After the SHDN pin is turned on the SS pin voltage begins to in-

crease and the system starts to switch. The softstart is realized by gradually increasing the current limit thresh-

old to avoid output overvoltage. The active soft start range (where the output current limit increase linearly)

starts from 0.6V to 1.5V. In this range an internal current source (5

µA typ) charges the capacitor on the SS pin.

The reference current (for the current limit comparator) forced through ILIM pin is proportional to SS pin voltage

and it saturates at 5

µA (typ.) when SS voltage is close to 1.5V; so the maximum current limit is active. Output

protections like undervoltage is disabled until SS pin voltage reaches 1.5V, instead the overvoltage is always

present.

Once the SS pin voltage reaches the 1.5V value, the voltage on SS pin doesn't impact the system operation

anymore. If the SHDN pin is turned on before the supplies, the correct start-up sequence is the following: first

Figure 15. Soft-start diagram

1.5 Current limit

The current limit comparator senses inductor current through the sense resistor when the low side MOSFET is

on and compares this value with the ILIM pin voltage. While the current is above the prefixed value, the control

inhibits the one-shot start.

To properly set the current limit threshold, it should be noted that this is a valley current limit. Average current

Eq 10

Time

Time

0.6V

Maximum current limit

Soft-start active range

5

µA

4.1V

1.5V

Ilim current

Vss