MP5021 – 12V, 7

MP5021 Rev. 1.1

www.MonolithicPower.com

12

1/30/2013

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OPERATION

The MP5021 limits 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 dV/dt of the

voltage to the load. It provides an integrated

solution to monitor the input voltage, output

voltage, output current, and die temperature to

eliminate the need for an external current-sense

power resistor, power MOSFET, and thermal

sense device.

Current Limit

The MP5021 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. In order to limit the current, the gate to

source voltage needs to drop from 5V to around

1V. The typical response time is about 20µs and

the output current may have a small overshoot

during this time period.

When the current limit triggers, the fault timer

starts. If the output current falls below the current

limit threshold before the end of the fault timeout

period, the MP5021 resumes normal operation.

Otherwise, if the current limit duration exceeds

the fault timeout period, the power FET turns off.

The subsequent behavior relates to the AUTO

pin configuration. If the temperature reaches the

thermal protection threshold during the fault

timeout period, the power FET turns off.

When the AUTO pin is floating, the part functions

in hiccup mode for over-current protection (OCP).

The part enters latch-off mode when the AUTO

pin to pulls to ground once it detects an over-

current condition and the duration exceeds the

preset value.

When the device reaches either its current limit

or its over-temperature threshold, the FLTB pin is

driven low with a 10μs propagation delay to

indicate a fault. The desired current limit at

normal operation is a function of the external

current limit resistor.

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 a 25A secondary

current limit level, a fast turn-off circuit activates

to turn off the power FET using a 100mA pull-

down gate discharge current, as shown in Figure

2. This limits the peak current through the switch

to limit the input voltage drop. The total short

circuit response time is about 200ns. And the

FLTB switches low once it reaches a 25A current

limit, and asserts low until the circuit resume to

normal.

Fault Timer & Restart

When the current reaches its over current limit

threshold, a 200µA fault timer current source

pin. If the current limit state ceases before the

TIMER pin reaches 1.23V, the MP5021 returns to

normal operation mode and a low-value resistor

1.23V. If the current limit state continues after the

TIMER pin voltage reaches 1.23V, the power

FET switches off. The subsequent restart

procedure then depends on the selected retry

configuration.

If the AUTO pin connects to ground or low, the

MP5021 will latch off. Restart the input power or

cycle the EN signal to resume function.

Floating the AUTO pin causes the device to work

in hiccup mode, as shown in Figure 3. At the end

of the fault timeout period, the power switch turns

off, and a low current sink of 0.5µA discharges

When the TIMER voltage reaches the low

threshold 0.2V, the part restarts. If the fault

condition remains, the fault timeout period and

restart timer repeat.