Obsolete

Product(s)

- Obsolete

Product(s)

AN1645 - APPLICATION NOTE

9/22

During transition in which the primary mosfet is turned off, the INHIBIT voltage must fall from high value

to -25mV in less than 250ns. The resistor value R26 must be chosen in order to fit this specification.

When the converter operates in parallel with other power supplies, the INHIBIT pin, detecting the voltage

across the freewheeling mosfet, also avoids the converter to sink current from the output.

Although the INHBIT pin allows operation in Discontinuous Mode, the -25mV threshold could be sensitive

to the ringing present at the SR Mosfet drain (see figure 13), causing incomplete turn on of OUTGate2.

This inconvenience can be avoided using the clock signal to provide a negative voltage to the INHIBIT

pin acting as a blanking time. This negative voltage can be easily generated with some discrete

components as shown in figure 12.

Figure 11: INHIBIT Voltage and Current

Figure 12: Circuit to provide negative voltage to INHIBIT

On Time

Off Time

Dead Time

Freewheeling

source-drain

voltage

-25mV

Inductor

Current

INHIBIT

Voltage

Freewheeling

Gate Voltage

Mosfet Drain

CK

INHIBIT

R24

D15

R25

R26

C11