MP9181 – 3A, 4.5V-20V INPUT, FAST TRANSIENT SYNCHRONOUS STEP-DOWN CONVERTER IN QFN12 (2X3mm)

MP9181 Rev 1.01

www.MonolithicPower.com

12

12/10/2012

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

© 2012 MPS. All Rights Reserved.

Enable Control

The MP9181 has a dedicated enable control pin

(EN): pulling it high enables the IC, pulling it low

disables it. Tie EN to VIN through a resistor for

automatic start up. EN must be pulled low to

disable the part.

The EN pin is clamped internally using a 6.7V

series-Zener-diode as shown below.

The EN

input pin can be connected through a pullup

resistor to any voltage connected to the VIN pin

such that the pullup resistor limits the EN input

current to less than 100µA.

For example, with 12V connected to Vin, Rpullup

≥ (12V – 6.7V) ÷ 100µA = 53kΩ.

If the EN pin is directly connected to a voltage

source without any pullup resistor, then the

amplitude of the voltage source should be limited

below 6 to prevent from causing damage to the

Zener diode.

Soft Start/Stop

MP9181

employs

a

soft

start/stop

(SS)

mechanism to ensure smooth output during

power up and power shut-down. The soft start

circuit includes an internal soft start capacitor.

When the EN pin goes high, the internal SS

voltage slowly ramps up. The output voltage

smoothly ramps up with the SS voltage. Once SS

voltage rises above the VREF, it continues to

ramp up while the PWM comparator only

compares the VREF and the FB voltage. At this

point, the soft start finishes and it enters steady

state operation. The SS time is set about 1ms

internally. When the EN pin goes low, an internal

current source discharges the internal SS voltage.

Once the SS voltage falls below the VREF, the

PWM comparator will only compare the VREF to

the SS voltage. The output voltage will decrease

smoothly with the SS voltage until the voltage

level zeros out.

Power-Good (PG)

The PG pin is the open drain of a MOSFET that

through a resistor (ex. 100k). The MOSFET turns

on with the application of an input voltage so that

the PG pin is pulled to GND before SS is ready.

is pulled high after a 0.5ms delay.

PG pin will be pulled low.

Over-Current Protection (OCP) and Short-

Circuit Protection (SCP)

The MP9181 has cycle by cycle over-current limit

control. The inductor current is monitored during

the ON state to achieve OCP or SCP.

For MP9181 at OCP or SCP, it enters hiccup

mode, that periodically restarts the part when the

inductor current peak value exceeds the current

threshold. Typically, the UV threshold is 50%

below the REF voltage. In OCP/SCP, the

MP9181 will disable the output voltage power,

discharge internal soft-start cap, and then

automatically try to soft-start again. If the over-

current circuit condition still holds after soft-start

ends, it repeats this operation cycle until the

over-current circuit condition disappears, and

output rises back to regulation level.

Under-voltage Protection (UVP)

The MP9181 monitors the output voltage through

a resistor-divided feedback (FB) voltage to detect

under voltage on the output. The UVP triggers

(0.815V) Usually UVP accompanies a drop in the

current limit and this results in SCP.