MP5981

– 16V, 50A, 1.1mΩ RDS(ON), HOT-SWAP INTELLI-FUSE SOLUTION

MP5981 Rev. 1.3

MonolithicPower.com

17

5/16/2022

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t

VIN

VGS

VOUT

3.3V

VDD3

MP5981 UVLO

GOK

ON/PD

CLREF

VCLREF_SS

e.g. 500 mV

VCLREF_NORM

e.g. 1.4V

Inse rtio n Dela y

On Delay

Determined

by Controlle r

>1.4V

VCS

Figure 4: Start-Up when the MP5981 is

Controlled by a Hot-Swap Controller

If the MP5981 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 exceeds 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

MP5981

VIN

VOUT

CS

IMON

ON/PD

VDD3

CLREF

VTEMP

SS

GOK

GND

D_OC

RIMON

RCS

RCLREF

CON

CSS

TVS

Figure 5: MP5981 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 turns 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 below the

clamp voltage, the actual current limit reference

is determined by the CLREF voltage.

t

VIN

VGS

VOUT

3.3V

VDD3

MP5981 UVLO

GOK

ON/PD

CLREF

Insertion Delay

1.4V

VBE

80% of VIN

1.6V

VCS

VCLREF_NORM

Figure 6: Start-Up when the MP5981 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 with

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 8.89ms. 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.

The approximate soft-start time is then 1.5ms,

which is the minimum output voltage soft-start

time.

ON/PD Control

ON/PD can be used for on/off control of the

power FET and the pull-down mode of the

output voltage. There are three types of

connection on ON/PD: