MP2905- 3V to 28V INPUT, HYSTERETIC SYNCHRONOUS STEP-DOWN CONTROLLER

MP2905 Rev. 0.91

www.monolithicpower.com

9

4/18/2011

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© 2011. All Rights Reserved.

OPERATION

MP2905 uses a hysteretic control loop to

regulate output voltage. It senses the voltage at

FB pin, which is compared with SS voltage with

20mV hysteresis. When FB is lower than SS

20mV, high side switch turns on and FB voltage

rises up. After FB voltage reaches SS voltage,

high side switch turns off and low side switch

turns on, which cause FB voltage drop until FB is

lower than SS 20mV. So Vout is regulated at a

stable voltage because FB voltage is regulated at

the voltage (SS-10mV). See the setting output

voltage for detailed information. Hysteretic

voltage-mode control provides fast transient

response without additional loop compensation.

Soft-start

An external cap is connected at SS pin to realize

soft-start function. When SS pin pull down

transistor turns off, internal reference begins to

charge SS external cap through a resistor-divider.

So FB rises slowly following SS voltage and

inrush current is avoided. Soft-start time is

determined by external cap and internal resistor-

divider.

If SS has pre-bias voltage at startup, both HG

and LG keep low, SS cap begins to discharge

until lower than 50mV. Then SS will rise slowly

and FB tracks SS.

Startup Sequence

In MP2905, at startup, if FB>SS, which means

output has pre-bias voltage, HG and LG don’t

toggle until SS greater than FB.

Current Limit Function

A resistor is connected from the Drain of the high

side MOSFET to ILIM pin to set current limit

value. Internal 50uA current sink from ILIM to

GND limits the maximum VDS cross high side

switch drain and source. When VFB<300mV, if

high side switch current hits the current limit, high

side switch turns off immediately. If VFB>300mV,

over current event occurs in four sequential

cycles, high side switch turns off. Once high side

MOSFET turns off, SS cap will be discharged by

250nA current. After SS voltage is lower than

50mV, SS cap is stopped discharging and high

side switch tries to turn on again.

Output Over-voltage protection

Output over-voltage is monitored by FB voltage.

If FB voltage higher than 750mV, HG is set low

and LG is set high. This status will be latched

until restart EN or VIN.

Enable

MP2905 turns off by pulling down SS pin to lower

than 50mV. Releasing SS will start the run cycle.

High Side Gate Driver

MP2905 doesn’t need Schottky, but still needs

0.1uF BST cap between BST pin and SW pin.

Internal Regulator

Most of the internal circuitries are powered from

the 5V internal regulator (REF). This regulator

lower than 5.0V, the output decreases. Bypass

REF pin to GND with a 4.7uF or greater capacitor.

Under Voltage Lockout (UVLO)

Under-voltage lockout (UVLO) is implemented to

protect the chip from operating at insufficient

supply voltage.

The MP2905 UVLO comparator monitors the

output voltage of the internal regulator (REF).

Thermal protection

The purpose of thermal protection is to prevent

damage in the IC by allowing exceptive current to

flow

and

heating

the

junction.

The

die

temperature is internally monitored until the

thermal limit is reached. Once this temperature is

reached, the part will be shut down and allow the

chip to cool. When the IC is cool enough, the part

will be turned on again. There is a built-in

hysteresis.