MP3120-1.2A, 1.1MHz SYNCHRONOUS BOOST

MP3120 Rev. 0.93

www.MonolithicPower.com

7

9/20/2011

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

© 2011 MPS. All Rights Reserved.

Current Sensing

Lossless current sensor converts the N-MOS

switch current signal to a voltage signal and it is

summed with the internal slope compensation.

The summed signal is compared with the error

amplifier output to provide a series of PWM

signal. The current limit threshold is about 1.2A

which is independent of input and output voltage.

The switch current signal is blanked for 60ns to

enhance noise rejection.

Synchronous Rectifier

The P-MOS synchronous rectifier is only enabled

800mV.

Thermal Shutdown

An internal temperature monitor will start to

reduce the peak current limit if the die

temperature

exceeds

125ºC.

If

the

die

temperature continues to rise and reaches to

160ºC, the part will thermally shutdown, all

switches will be turned off. The part will be

enabled again when the die temperature drops

by about 20ºC.

Output Disconnect and Inrush Current

Limiting

The MP3120 is designed to allow true output

disconnect by eliminating body diode conduction

of the internal PMOS rectifier. This allows VOUT

to go to zero volts during shutdown, and drawing

zero current from the input source. It also allows

for inrush current limiting during start-up,

minimizing surge currents from the input. Note

that to obtain the advantage of output disconnect,

there must be no external schottky diode

connected between the SW pin and OUT pin.

Board layout is extremely critical to minimize

voltage spike on SW pin caused by stray

inductance. Keep the output filter capacitors as

close as possible to the OUT pin and use very

low ESR/ESL ceramic capacitors tied to a good

ground plane.

Short Circuit Protection

Unlike most boost converters, the MP3120 allows

the output to be short circuit due to the output

disconnect feature. The device incorporates

internal features such as current limit feedback,

thermal regulation and thermal shutdown to

prevent the chip from burning out caused by

excessive overload or short circuit. In the event

of a short circuit, the current limit reduces to a

smaller value. The thermal protection circuit also

functions in addition to the current limit in case of

the device temperature rise up to above 125ºC.

The MP3120 maintains the output voltage to be

regulated even if the input voltage is above the

output voltage. In this mode the P-MOS no

longer acts as a low impedance switch, there will

be more power dissipation within the IC. This will

cause a sharp drop in the efficiency. The

maximum output current should be limited in

order to maintain an acceptable junction

temperature.