MP6900- FAST TURN-OFF INTELLIGENT CONTROLLER

MP6900 Rev. 1.12

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7

9/19/2012 MPS Proprietary Information. Patent Protected. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2012 MPS. All Rights Reserved.

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, VDD higher than 15V and/or Cload

larger than 5nF, it is recommended to use the

thermally-enhanced SOIC-8.

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 Fig.2).

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

triggered, a blanking time (Minimum on-time:

~200ns) 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).

Turn-off Phase

When Vds rises to trigger the turn off threshold (-

30mV), the gate voltage is pulled to low after

about 20ns turn off delay (defined in Fig.2) by the

control circuitry. Similar with turn-on phase, a

200ns

blanking

time

is

added

after

the

synchronous MOSFET is turned off to avoid error

trigger.

Fig.3 shows synchronous rectification operation

at heavy load condition. Due to the high current,

the gate driver will be saturated at first. After Vds

goes to above -70mV, gate driver voltage

decreases to adjust the Vds to typical -70mV.

Fig 4 shows synchronous rectification operation

at light load condition. Due to the low current, the

gate driver voltage never saturates but begins to

decrease as soon as the synchronous MOSFET

is turned on and adjust the Vds.

-70mV

-30mV

Vds

Isd

Vgs

t0

t1

t2

Figure 3—Synchronous Rectification

Operation at heavy load