MP5921

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

MP5921 Rev. 1.1

www.MonolithicPower.com

18

5/17/2022

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© 2022 MPS. All Rights Reserved.

t

VIN

VGS

VOUT

3.3 V

VDD3

MP5921 UVLO

GOK

ON/PD

CLREF

VCLREF_SS

eg, 500 mV

VCLREF_NORM

eg. 1.4V

Insertion delay

On delay

determined by

controller

>1.4V

VCS

Figure 4: Start-Up when MP5921 is Controlled by

Hot-Swap Controller

If the MP5921 works in standalone mode, an

external capacitor (CON) can be connected from

ON/PD to ground for an automatic start-up (see

Figure 5). The internal 4.2µA current source

charges the capacitor when VDD3 is higher

than the UVLO. ON/PD can also be pulled up

externally to the VDD3 voltage. A 10µA CLREF

current source determines the current limit level

through a resistor to ground.

VIN

VOUT

MP5921

VIN

VOUT

CS

IMON

ON/PD

VDD3

CLREF

VTEMP

SS

GOK

GND

D_OC

RIMON

RCS

RCLREF

CON

CSS

TVS

Figure 5: MP5921 Schematic when Operating in

Standalone Mode

Once the ON/PD voltage exceeds 1.4V and the

insertion delay time ends, the power FET is

charged up by the internal 12

μA charge pump.

The power FET is turned on when VGS reaches

VGSTH, and then the output voltage rises. The

current-limit

reference

voltage

is

clamped

internally to different voltage levels according to

VOUT. The external CLREF voltage is higher

than the internal VBE clamp (see Figure 6).

If the external CLREF voltage is lower than the

clamp voltage, the actual current limit reference

is determined by the CLREF voltage.

t

VIN

VGS

VOUT

3.3 V

VDD3

MP5921 UVLO

GOK

ON/PD

CLREF

Insertion delay

1.4 V

VBE

80%VIN

≤1.6V

VCS

VCLREF_NORM

Figure 6: Start-Up when MP5921 Operates in

Standalone Mode

Soft Start (SS)

A capacitor connected to SS determines the

soft-start time. When ON/PD is pulled high and

the insertion delay time ends, a constant

current source proportional to the input voltage

charges up the SS voltage. The output voltage

rises at a similar slew rate to the SS voltage.

The SS capacitor value can be calculated by

Equation (3):

SS

SS

SS

R

t

9

C



=

(3)

Where tSS is the soft start time (ms), CSS is the

soft-start capacitor (nF), and RSS is 0.8MΩ.

For example, a 100nF capacitor provides a soft-

start time of 8.9ms. If the load capacitance is

extremely

large,

the

current

required

to

maintain the preset soft-start time exceeds the

start-up current limit. In this case, the rise time

is controlled by the load capacitor and the start-

up current limit. Float SS to generate a fast

ramp-up voltage. A 12

μA current source pulls

up the gate of the power FET. The gate charge

current controls the output voltage rise time.