MP2493 – 2A, 36V, LOW EMI STEP-DOWN WITH PROGRAMMABLE OUTPUT CURRENT LIMIT

MP2493 Rev. 0.9

www.MonolithicPower.com

7

4/23/2010

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OPERATION

Main Control Loop

The MP2493 is a current mode buck regulator.

That is, the error amplifier (EA) output voltage is

proportional to the peak inductor current.

At the beginning of a cycle, the integrated high

side power switch M1 (Fig.1) is off; the EA

output voltage is higher than the current sense

amplifier output; and the current comparator’s

output is low. The rising edge of the 130kHz

clock signal sets the RS Flip-Flop. Its output

turns on M1 thus connecting the SW pin and

inductor to the input supply.

The increasing inductor current is sensed and

amplified by the Current Sense Amplifier. Ramp

compensation is added to Current Sense

Amplifier output and compared to the Error

Amplifier output by the PWM Comparator.

When the Current Sense Amplifier plus Slope

Compensation signal exceeds the EA output

voltage, the RS Flip-Flop is reset and the

MP2493 reverts to its initial M1 off state.

If the Current Sense Amplifier plus Slope

Compensation signal does not exceed the

COMP voltage, then the falling edge of the CLK

resets the Flip-Flop.

The output of the Error Amplifier integrates the

voltage difference between the feedback and

the 0.8V bandgap reference. The polarity is

such that a FB pin voltage lower than 0.8V

increases the EA output voltage. Since the EA

output voltage is proportional to the peak

inductor current, an increase in its voltage

increases current delivered to the output. An

external Schottky Diode (D1) carries the

inductor current when M1 is off.

Load Current Limiting Loop

The output current information is sensed via the

ISP and ISN pins. The regulation threshold is

of the power supply will be set by the FB pin. If

will pull down SS and regulate the output at a

constant current determined by the external

sense resistor. The external capacitor on SS

pin is the dominant compensation capacitor for

load current regulation loop. The capacitor has

normal value of 100nF, which will put the

bandwidth of load current regulation loop to be

100mV, SS will not drop down to the final

regulation level immediately. It will cause the

load current to be higher than the programmed

level for a short period. A fast comparator is

added to shut down power switch when the

average load current is higher than 120% of the

programmed current limit level.

An inductor DC resistance (DCR) or accurate

sense resistor can be used for load current

sensing.