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ductor value 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 arrives before, increasing the switching frequency. The PFM waveforms may appear more noisy and

asynchronous than normal operation, but this is normal behaviour mainly due to the very low load. If the PFM

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 and the low side MOSFET off (high impedance mode). After the SHDN pin is turned on the SS pin

voltage begins to increase and the system starts to switch. The softstart is realized by gradually increasing the

output current limit increase linearly) starts from 0.6V to 1V. 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 1V and the max-

imum current limit is active. Output protections OVP & UVP are disabled until the SS pin voltage reaches 1V

(see figure 5).

Once the SS pin voltage reaches the 1V value, the voltage on SS pin doesn't impact the system operation any-

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

Figure 5. Soft -Start Diagram

Because the system implements the soft start controlling the inductor current, the soft start capacitor selection

is function of the output capacitance, the current limit and the soft start active range (

In order to select the softstart capacitor it must be imposed that the output voltage reaches the final value before

the soft start voltage reaches the under voltage value (1V). In other words the output voltage charging time has

to be lower than the uvp time.

The UVP time is given by:

Eq 7

In order to calculate the output volatge chargin time it should be calculated, before, the output volatrge function

versus time. This function can be calculated from the inductor current function; the inductor current function can

Time

Time

0.6V

Maximum current limit

Soft-start active range

5

µA

4.1V

1V

Ilim current

Vss

T

()

V

uv p

Iss

------------ C

SS

⋅

=