MP5087

– 5.5V, 7A LOW RDSON LOAD SWITCH WITH CURRENT MONITORING

MP5087 Rev. 1.2

www.MonolithicPower.com

11

8/16/2017

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© 2017 MPS. All Rights Reserved.

OPERATION

The MP5087 is designed to limit the in-rush

current to the load when a circuit card is

inserted into a live backplane power source,

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, output voltage and output current

to eliminate the need for an external current

power MOSFET, and current sense device.

Enable

When input voltage is greater than the under-

voltage lockout threshold (UVLO), typically 0.5V,

MP5087 can be enabled by pulling EN pin to

higher than 1.5V. Pulling down to ground will

disable MP5087.

Current Limit

The MP5087 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:

(1)

S is the current sense ratio of MP5087, and this

almost a constant value when Vin is changing

from 1.2V to 5.5V, and when Vin is smaller than

1.2V, a step change will come to S value, the

value will change from 82000 to about 80000.

Meanwhile, when Vcc is changing, there is also

a little shift on S value, for more information,

please refer the curves in typical performance

characteristics.

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

over

temperature threshold

is

155°C and

hysteresis is 30°C.

Power-Good Function

The PG pin is the push pull of a MOSFET that

on with the application of an input voltage so

that the PG pin is pulled to GND. After the

than 150mV, the PG pin is pulled high after a

90µs delay. When the voltage gap is higher

than 200mV, the PG pin will be pulled low. The

PG pin has a nominal pull down resistance of

200

Ω and a pull-up resistance of 250KΩ.The

maximum sink current when the pin is pulled to

GND through the internal pull-down resistor

should be <10mA

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, if the part is still in short-circuit

condition. MP5087 will reduce the current limit

to 2/3 of pre-set value, and hold it until the part

is so hot and thermal shutdown. After the short-

circuit condition removed, the current limit will

recover to the pre-set value automatically.

Output Discharge

MP5087 has output discharge function. This

down resistance when IC disabled 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: