MPQ5850

– 36V, SMART DIODE CONTROLLER W/ REVERSE PROTECTION, AEC-Q100

MPQ5850 Rev. 1.1

MonolithicPower.com

22

2/9/2022

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© 2022 MPS. All Rights Reserved.

OPERATION

The MPQ5850 is an 8-pin smart diode controller

that drives an external N-channel MOSFET to

minimize power loss, including DC forward loss

and AC rectification loss.

Smart Diode Operation

During normal operation, when VSOURCE > VDRAIN,

the MPQ5850 modulates the gate of the external

N-channel MOSFET to regulate the source-to-

drain voltage to about 20mV. This minimizes

power loss and allows small negative currents to

be easily detected.

Generally,

the

source-to-drain

voltage

is

regulated to about 20mV. When the load current

rises, the GATE voltage rises until the MOSFET

is fully on. If the load current drops, the GATE

amplifier drives the MOSFET

’s gate lower to

maintain a 20mV drop.

If the drain-to-source voltage exceeds 20mV, a

large current

modifies the GATE pin’s voltage to

regulate

the

drain-to-source

voltage.

If

a

negative current flows through the external N-

channel MOSFET, the device detects a source-

to-drain negative voltage drop. If the drop

exceeds -4mV, the MPQ5850 pulls down the

GATE pin to turn off the external N-channel

MOSFET quickly. If a forward current flows

through the external N-channel MOSFET (either

the body or channel), the device detects a

source-to-drain forward voltage drop. If the drop

exceeds 75mV, the MPQ5850 pulls up the GATE

pin to quickly turn on the external N-channel

MOSFET.

This

allows

high-frequency

AC

waveforms (up to 100kHz) to be rectified with

minimal system and N-channel MOSFET power

loss.

Boost Converter

The integrated boost converter uses fixed peak

current mode control. When the reservoir floating

supply is below 12V, the boost converter

requests another turn-on pulse. This pulse

terminates when the inductor current reaches

220mA.

The inductor current should reach 0mA before

the next on cycle begins. This sets a maximum

frequency (fMAX) depending on VDRAIN. fMAX can be

calculated with Equation (1):

MAX

ON

OFF

1

f

tt

=

+

(1)

Where tON is the on time and tOFF is the off time.

tON can be estimated with Equation (2):

PEAK

ON

PEAK

DRAIN

SW

IL

t

I

VR

2



=

−

(2)

tOFF can be calculated with Equation (3):

PEAK

OFF

RES

DIODE

IL

t

VV



=

+

(3)

Start-Up and Low-Voltage Operation

The MPQ5850 starts in the off state. In this state,

the gate is passively pulled to SOURCE, and the

boost

converter

is

disabled.

Once

VDRAIN

exceeds the under-voltage lockout (UVLO)

threshold, the device enters the disabled state.

The device is enabled when it enters a logic high

state. In the disabled state, the gate is passively

pulled to source, the boost converter is disabled,

and the PG pin is pulled low. Once the EN/UVLO

pin is pulled high, the device enters the start-up

state, which causes the boost converter to turn

on. When the reservoir capacitor exceeds the

reservoir UVLO threshold, normal operation

begins. During normal operation, the gate is

actively modulated based on VSOURCE and VDRAIN,

and the PG pin is high when the source-to-drain

voltage is regulated.

The DRAIN pin is the power supply of the

internal circuitry. DRAIN

also controls the

MOSFET’s gate voltage. If VDRAIN exceeds its

UVLO threshold, then the MPQ5850 operates

normally, even in cold-crank conditions down to

0V. A large electrolytic output capacitor is

required under these conditions.

If VDRAIN drops below the UVLO threshold, the

device pulls down the GATE pin to SOURCE

until the voltage on the reservoir capacitor

discharges below VRES_UVLO. This allows the

device to minimize forward voltage drops during

temporary low voltage transients, such as under

cold-crank conditions.