MP6922DN datasheet(8/13 Pages) MPS

8 / 13 page

MP6922—DUAL FAST TURN-OFF INTELLIGENT CONTROLLER

MP6922 Rev. 1.23

www.MonolithicPower.com

9/19/2012

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

stops switching. The part will return to normal

operation after the junction temperature has

dropped to 120oC.

Turn-on Phase

When the switch current flows through the body

diode of the MOSFET, there will be a negative

VDS (VD-VSS) across it (<-500mV), the VDS is

much lower than the turn on threshold of the

control circuitry (-70mV), which then turns on the

MOSFET after 200ns turn-on delay (defined in

Fig.2).

As soon as the turn on threshold (-70mV) is

triggered, a blanking time (Minimum on-time:

~1us) will be added during which the turn off

threshold will be changed from 0mV to +50mV.

This blanking time can help to avoid error trigger

on turn off threshold caused by the turn on

ringing of the synchronous power switch.

VDS

VGATE

tDon

tDoff

-70mV

0mV

Figure 2—Turn on and Turn off delay

Conducting Phase

When the MOSFET is turned on, VDS (-ISD X

rDS(ON)) becomes to rise according to the drop of

the switch current (ISD) , as soon as VDS rises

above the turn on threshold (-70mV), the control

circuitry stops pulling up the gate driver and the

driver voltage of the MOSFET dropped, which

makes the on resistance rDS(ON) of the MOSFET

becomes larger. By doing that, VDS (-ISD x rDS(ON))

is adjusted to be around -70mV even when the

switch current ISD is fairly small, this function can

make the turn off threshold (0mV) of the internal

driver never triggered until the current through

the MOSFET has dropped to near zero.

Turn-off Phase

When VDS rises to trigger the turn off threshold

(0mV), the driver voltage of the switch is pulled to

low after about 20ns turn off delay (defined in

Fig.4) by the control circuitry. Similar with turn-on

phase, a 1.6us blanking time is added after the

switch is turned off, during which the MOSFET is

never turned on to avoid error trigger.

Fig.3 show the MP6922 operation at heavy load

condition. Due to the high current, the driver

voltage will be saturated at first. After VDS goes to

above -70mV, driver voltage decreases to adjust

the VDS to typical -70mV.

Fig.4 show the MP6922 operation at light load

condition. Due to the low current, the driver

voltage never saturates but begins to decrease

as soon as the synchronous power switch is

turned on and adjust the VDS.

-70mV

0mV

Vds

Vgs

Isd

Figure 3—Synchronous Rectification

Operation at heavy load

-70mV

0mV

Vds

Vgs

Isd

Figure 4

—Synchronous Rectification

Operation at light load

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