MP6902- FAST TURN-OFF INTELLIGENT CONTROLLER

MP6902 Rev. 1.13

www.MonolithicPower.com

7

1/11/2013

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OPERATION

The MP6902 supports operation in DCM and

Quasi-Resonant Flyback converters. The control

circuitry controls the gate in forward mode and

will turn the gate off when the MOSFET current is

fairly low.

Blanking

The control circuitry contains a blanking function.

When it pulls the MOSFET on/off, it makes sure

that the on/off state at least lasts for some time.

The turn on blanking time is ~1.6us, which

determines the minimum on-time. During the turn

on blanking period, the turn off threshold is not

totally blanked, but changes the threshold

voltage to ~+50mV (instead of -30mV). This

assures that the part can always be turned off

even during the turn on blanking period. (Albeit

slower)

VD Clamp

Voltage JFET is used at the input. To avoid

excessive currents when Vg goes below -0.7V, a

the drain of the external MOSFET.

Under-Voltage Lockout (UVLO)

When the VDD is below UVLO threshold, the part

is in sleep mode and the Vg pin is pulled low by a

10k

Ω resistor.

Enable pin

If EN is pulled low, the part is in sleep mode.

Thermal shutdown

If the junction temperature of the chip exceeds

stops switching. The part will return to normal

function after the junction temperature has

Thermal Design

If the dissipation of the chip is higher than

100mW due to switching frequencies above

100kHz.

Turn-on Phase

When the synchronous MOSFET is conducting,

current will flow through its body diode which

generates a negative Vds across it. Because this

body diode voltage drop (<-500mV) is much

smaller than the turn on threshold of the control

circuitry (-70mV), which will then pull the gate

driver voltage high to turn on the synchronous

MOSFET after about 150ns turn on delay

(Defined in Figure 2).

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

triggered, a blanking time (Minimum on-time:

~1.6us) will be added during which the turn off

threshold will be changed from -30mV to +50mV.

This blanking time can help to avoid error trigger

on turn off threshold caused by the turn on

ringing of the synchronous MOSFET.

-70 mV

-30 mV

2V

Total

t

5V

Figure 2—Turn on and Turn off delay

Conducting Phase

When the synchronous MOSFET is turned on,

Vds becomes to rise according to its on

resistance, as soon as Vds rises above the turn

on threshold (-70mV), the control circuitry stops

pulling up the gate driver which leads the gate

voltage is pulled down by the internal pull-down

resistance (10kΩ) to larger the on resistance of

synchronous MOSFET to ease the rise of Vds.

By doing that, Vds is adjusted to be around -

70mV even when the current through the MOS is

fairly small, this function can make the driver

voltage

fairly

low

when

the

synchronous

MOSFET is turned off to fast the turn off speed

(this function is still active during turn on blanking

time which means the gate driver could still be

turned off even with very small duty of the

synchronous MOSFET).