MP5087A Rev. 1.0

www.MonolithicPower.com

11

7/18/2014

MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.

© 2014 MPS. All Rights Reserved.

OPERATION

The MP5087A is designed to limit the in-rush

current to the load, thereby limiting the

backplane’s voltage drop and the slew rate of

the voltage to the load. It provides an integrated

solution to monitor the input voltage and output

current to eliminate the need for an external

current power MOSFET, and current switch

device.

Enable

When input voltage is greater than the under-

voltage lockout threshold (UVLO), typically 0.5V,

MP5087A can be enabled by pulling EN pin to

higher than 1.5V. Pulling down and floating to

ground will disable MP5087A.

The recommend start up sequence is power up

high.

Current Limit

The MP5087A provides a constant current limit

that can be programmed by an external resistor.

Once the device reaches its current limit

threshold, the internal circuit regulates the gate

voltage to hold the current in the power FET

constant. The typical response time is about

20µs and the output current may have a small

overshoot during this time period.

The pre-set current limit value can be

calculated by below equation:

LIM

LIM

82000

I(A)

R( )

=

Ω

(1)

ground in Figure 4.

If the current limit block starts to regulate the

output current, the power loss on power

MOSFET will cause the IC temperature rise. If

the junction temperature rose to high enough, it

will trigger thermal shutdown. After thermal

shutdown happened, it will disable the output

until the over temperature fault remove. The

Power-Good Function

The PG pin is the push pull output that can be

connected internally to monitored system

over 3V, PG is high. A threshold hysteresis will

prevent the chip from responding perturbation

Short-Circuit Protection

If the load current increases rapidly due to a

short circuit, the current may exceed the current

limit threshold by a lot before the control loop

can respond. If the current reaches an internal

secondary current limit level (about 13A), a fast

turn-off circuit activates to turn off the power

FET. This limits the peak current through the

switch to limit the input voltage drop. The total

short circuit response time is about 200ns. If

fast off works, it will keep off the power FET for

80µs. After that time period, it will re-turn on

power FET.

Output Discharge

MP5087A has output discharge function. This

and the load is very light.

Soft-Start

A capacitor connected to the SS pin determines

the soft-start time. There is an internal 9μA

constant current source charge SS cap and

ramps up the voltage on the SS pin. The output

voltage rises at 3 times the slew rate to the SS

voltage.

The soft-start time can be calculated by below

equation:

OUT

SS

SS

SS

V(V) C (nF)

1

T(ms)

3I (uA)

⋅

=×

(2)

The suggestion minimum SS cap should be

bigger than 4.7nF. If the SS pin is floated or SS

cap is too small, the Vout rising time will be just

limited by power MOS charge time.