MP2497 – 3A, 50V, 100kHz STEP-DOWN CONVERTER

MP2497 Rev. 1.12

www.MonolithicPower.com

8

12/22/2011

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© 2011 MPS. All Rights Reserved.

OPERATION

Main Control Loop

The MP2497 is a current mode buck regulator.

That is, the EA output voltage is proportional to

the peak inductor current.

At the beginning of a cycle SW is off; the Error

Amplifier output voltage is higher than the

Current Sense Amplifier output. The rising edge

of the 100kHz CLK signal sets the RS Flip-Flop.

Its output turns on SW 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 summed 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

Error

Amplifier output voltage, the RS Flip-Flop is

reset and the MP2497 reverts to its initial SW

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 reference. The polarity is such that an FB

pin voltage is 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

SW is off.

Hiccup Mode Current Limit Protection

The output current information for current limit

protection is sensed via the ISP and ISN pins.

The sense voltage limit threshold is set at

100mV. MP2497 has hiccup over current limit

the current limit loop will turn off high side

switch immediately. Meanwhile, internal soft

start circuit will be reset after FB is lower than

0.3V, and then the high side switch turns on

and MP2497 restarts with a full soft start. This

hiccup process is repeated until the fault is

removed. And, current limit value can be

programmed to be lower by internal current

source and external resistors connected to ISN

and ISP pins, when output voltage is lower than

200mV. Then, the average short circuit current

can be greatly reduced.

Output Over Voltage Protection

The MP2497 has output over voltage protection.

The OVP reference 1.23V is on the positive

input of the OVP comparator. The output

voltage is fed to OVP pin through an external

resistor divider. If the voltage on OVP pin is

higher than 1.23V, the high side switch will be

turned off immediately and part will be lathed off

after a timer delay.

Output Line Drop Compensation

If the trace from MP2497 output terminator to

the load is too long, there will be a voltage drop

on the long trace which is variable with load

current. MP2497 is capable of compensating

the output voltage drop to keep a constant

voltage at load, whatever the load current is.

The output voltage is compensated by feeding

a current to the top feedback resistance R1.

The load line compensation gain can be

(Figure 2) values.