MP5921

– 16V, 50A, 1mΩ RDS(ON), HOT-SWAP INTELLI-FUSE SOLUTION

MP5921 Rev. 1.1

www.MonolithicPower.com

21

5/17/2022

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Input and Output Transient Protection

The hot-swap system experiences positive

transients on the input during a hot plug or rapid

turn-off with high current due to parasitic

inductance in the input circuit. For

input

transient

protection,

a

transient

voltage

suppressor (TVS) diode may be required on the

input to limit transient voltages below the

absolute maximum ratings.

The output may experience negative transients

during rapid turn off with high current due to

inductance in the output circuit. If a transient

makes OUT more negative, the power FET may

not turn off properly.

An output voltage clamp diode is required on

the output to limit negative transients. Select a

Schottky diode with a low forward voltage.

Current Sense Output (CS)

CS provides a current proportional to the output

current (the current through the power device).

The gain of the current sense is 10

μA/A when

the power FET is fully on. There is a resistor

(RCS) connected on CS to form an external

voltage. Determine a proper reference voltage

with Equation (5) and Equation (6):

ICS = IOUT*10µA/A

(5)

VCS = ICS*RCS

(6)

Once VCS reaches the CLREF current limit

threshold, the internal circuit regulates the gate

voltage to hold the current in the power FET

constant.

Current Monitor Output (IMON)

The gain of the current monitor is 10

μA/A.

There is a resistor (RIMON) connected from

IMON to ground. The IMON voltage range of

0V to 1.6V is required to keep IMON’s output

current

linearly

proportional

to

the

output

current. Determine a proper reference voltage

with Equation (7) and Equation (8):

IMON = IOUT*10µA/A

(7)

VIMON = IMON*RIMON

(8)

The MP5921 current monitor output can be

used by the controller to accurately monitor the

output current. Place a 100nF capacitor from

IMON to GND to smooth the indicator voltage.

Connect a 2

kΩ resistor (RIMON) to ground to set

the gain of the output, which is about 20mV per

ampere. For the best accuracy, use resistors

within 1%.

Current Balance for Parallel Operation

Multiple MP5921 devices can be used in

parallel for higher current applications. The

current balance loop in the MP5921 is used to

balance the start-up current per active channel.

All IMON pins must be connected together.

The sensed current from each active MP5921

IMON is summed together and divided by the

number

of

active

channels.

The

resulting

average load current provides a measure of the

total load current. The MP5921 current balance

is achieved by comparing the sensed current of

CS in each MP5921 to the average current to

make an appropriate adjustment to the power

FET gate voltage of each Intelli-Fuse during

start-up. The equivalent average IMON resistor

can be calculated by RCS/N, where N is the

number of active MP5921 devices.

Start-up

current

balance

is

essential

in

achieving the thermal advantage of paralleling

operation. With good current balance, the

power loss is dissipated equally over multiple

devices and a greater area.

Temperature Sense Output (VTEMP)

VTEMP reports the junction temperature when

there is no thermal gradient on the IC. VTEMP

is a voltage output proportional to the junction

temperature whenever VDD3 is higher than its

UVLO threshold and the MP5921 is in active

mode. The VTEMP output voltage is 8.7 mV/°C

with a 152.5mV offset and can be calculated

with Equation (9):

VTEMP = TJUNCTION*8.7mV+152.5mV

(9)

For example, if the junction temperature is

100°C, the VTEMP voltage is about 1.022V. If

VTEMP is 0V, the junction temperature is about

-18°C. The total temperature sense range is

-18°C to 140°C. When the junction temperature

is below -18°C, VTEMP remains at 0V.

In multi-fuse operation, the VTEMP pins of each

Intelli-Fuse

can

be

connected

to

the

temperature monitor pin of the controller (see

Figure 11).