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Product(s)

- Obsolete

Product(s)

AN1645 - APPLICATION NOTE

8/22

Figure 10: DCM clamped

6. INHIBIT OPERATION

One of the differences between diode rectification and synchronous rectification is the possibility for

Mosfets to conduct the current in both directions while diodes conduct just in one direction. In

discontinuous mode with diode rectification, when the inductor current reaches zero it cannot reverse

because the diode does not allow current flowing from cathode to anode. Using mosfets as rectifiers,

when the inductor current reaches zero, it continues to decrease becoming negative and flowing from

drain to source of the freewheeling mosfet. In this condition the converter works always in continuous

mode.

If discontinuous mode operation is required, the freewheeling mosfet must be turned off when the current

is zero, consequently the body diode operates as a common rectifier avoiding reversal of the inductor

current.

The INHIBIT pin is able to turn off the freewheeling mosfet when its current is approaching zero allowing

discontinuous mode operation of the converter.

On the INHIBIT pin, there is an internal comparator with a -25mV threshold. This pin is connected

through a resistor to the drain of the freewheeling mosfet. At the beginning of the off time (when Ck goes

is kept high until the voltage reaches -25mV. This is because when the freewheeling mosfet is

conducting, the voltage on its drain is: Vds= -Rds(on)*Idrain. If Vds is higher than -25mV, it means that

operation of the body diode of the mosfet (see figure 11). When the converter is in continuous mode the

mosfet occurs when the current is not exactly zero, the remaining amount of current is dependent on

On

Time

Off Time

V

CC

Peak

Detector

input

Dead Time

Peak

Detector

output

On

Time

Off Time

V

CC

Peak

Detector

input

Dead Time

Peak

Detector

output