L6725 datasheet(13/27 Pages) STMICROELECTRONICS

13 / 27 page

L6725

Device description

13/27

The output of the error amplifier is clamped with this voltage (VSS) until it reaches the

programmed value. No switching activity is observable if VSS is lower than 0.5V and both

MOSFETs are OFF. When VSS is between 0.5V and 1.1V the low-side MOSFET is turned ON.

As VSS reaches 1.1V (i.e. the oscillator triangular wave inferior limit) even the high-side

MOSFET begins to switch and the output voltage starts to increase. During the soft-start phase

the current can’t be reversed in order to allow pre-biased start-up (see Figure 6. and Figure 7.).

If an over current is detected during the soft-start phase, the device provides a constant-

current-protection. In this way, in case of short soft-start time and/or small inductor value and/or

high output capacitors value, the converter can start in any case, limiting the current

(Chapter 5.8: Monitoring and protections on page 14). The soft-start phase ends when Vss

reaches 3.5V. After that the over-current-protection triggers the HICCUP mode.

Figure 6.

Start-up without pre-bias

Figure 7.

Start-up with pre-bias

LGate

VOUT

VSS

VOUT

LGate

L6725 Datasheet(PDF) 13 Page - STMicroelectronics