MP6903- FAST TURN-OFF INTELLIGENT CONTROLLER

MP6903 Rev. 1.04

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8

1/31/2013

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Thermal shutdown

If the junction temperature of the chip exceeds

180°C, the VG will be pulled low and the part

stops switching. The part will resume normal

function after the junction temperature has

dropped to 150°C.

Turn-On Phase

When the synchronous MOSFET is on, current

flows through its body diode and generates a

500mV) is much smaller than the turn-on

threshold of the control circuitry (-70mV), which

then pulls the gate driver voltage high to turn on

the synchronous MOSFET after about 250ns

turn-on delay (shown in Figure 2).

When the turn-on delay ends, turn-on starts with

a blanking time (minimum on-time: ~0.8µs), and

the turn-off threshold changes from +30mV to

+100mV. This blanking time helps to avoid errors

around the turn-off threshold caused by turn-on

ringing of the synchronous MOSFET.

Figure 2: Turn On/Off Timing Diagram

Conducting Phase

rises according to the MOSFET’s ON resistance.

70mV), the control circuitry stops pulling up the

gate driver, so the gate voltage is pulled down by

the internal pull-down resistance (10kΩ) and

leakage to increase the ON resistance of the

when the current through the MOSFET is fairly

small. This function limits the driver voltage when

the synchronous MOSFET is turned off (this

function is still active during turn-on blanking,

which means the gate driver could still be turned-

off even with very small duty cycles of the

synchronous MOSFET).

Turn-Off Phase

the gate voltage is pulled to low after a 20ns turn-

off delay (shown in Figure 2) by the control

circuitry.

Figure

3

shows

synchronous

rectification

operation at heavy load. The gate driver initially

above -70mV, the gate driver voltage decreases

Figure

4

shows

synchronous

rectification

operation at light load. The gate driver voltage

never saturates due to the low current, but

decreases as soon as the synchronous MOSFET

Figure 3: Synchronous Rectification

Operation at Heavy Load

Figure 4: Synchronous Rectification

Operation at Light Load