MP5021 – 12V, 7

MP5021 Rev. 1.1

www.MonolithicPower.com

15

1/30/2013

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The SS capacitor value is given by

⋅ τ

=

SS

SS

SS

5

C

R

Where:

For example, a 100nF capacitor gives a soft-start

time of 20ms.

If the load capacitance is extremely large, the

current required to maintain the preset soft-start

time will exceed the current limit. Then the load

capacitor and the current limit control the rise

time.

Float the SS pin to generate a fast ramp-up

voltage. A 4μA 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 about 1ms, which is the

minimum soft-start time.

Enable Pin and EN Blanking Time

The EN pin enables the part when HIGH and

disables the part when low. Floating the EN pin

sets the part to auto-startup thanks to an internal

TIMER

t

1.23V

EN

ENTM

t1

EN blanking time

t2

Insertion delay

1.23V

1uA

Figure 5: EN Blanking Time

As shown in Figure 5, EN has a programmable

blanking time of up to 1s that prevents EN from

de-asserting during the blanking time. All fault

functionality continues to operative during the

start-up so that the power switch shuts down if a

fault was detected; however, the switch will not

turn off if EN goes LOW during this blanking time.

At the end of the blanking time, EN behaves

normally.

Set the blanking time with a capacitor connected

to the ENTM pin. The following estimates a value

for the blanking timer capacitor:

−

τ⋅

=

6

ENB

ENB

10

C

1.23

Where:

For example, a 1μF capacitor gives a blanking

time of 1.23s.

Floating the ENTM pin generates a fast ramp-up

voltage on the ENTM pin. The blanking time

during this period is negligible.

When EN enables the part, the insertion delay

timer starts. When the insertion delay time ends,

the internal 4μA current source charges the

power FET’s gate. Charging takes about 1.5ms

voltage rises following the SS-controlled slew

rate.

Damaged MOSFET Detection

The MP5021 can detect a shorted pass FET

during power-up by treating an output voltage

on the MOSFET. The FLTB pin goes low to

indicate a fault condition and the power switch

up normally.

Internal VCC Sub-Regulator

The MP5021 has an internal 5V linear sub-

regulator that steps down the input voltage to

generate a 5V power supply that powers low-

voltage circuitry. The regulator is enabled when

In EN shutdown mode, the internal VCC

regulator is disabled to reduce power dissipation.

PD Pin

When the PD pin connects to the output, the part

is in pull-down mode. In this mode, when the

enable pin is low, an integrated 500Ω pull-down