MP2326 Rev. 1.1

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15

1/28/2016

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input voltage, output voltage, output inductor,

output capacitor, and frequency set. If the design

spec. is not the same as the specs. in Table 1,

equation 9 as a design guide.

In skip mode, the stability is determined mainly

in PWM operation is generally acceptable for skip

mode.

Soft-Start (SS)

MP2326 employs a soft-start (SS) mechanism to

ensure smooth output ramping during power-up.

When EN goes high, an internal current source

(8μA) charges up the SS capacitor. The SS

capacitor voltage transfers over to the REF

voltage to the PWM comparator. The output

voltage smoothly ramps up with the SS voltage.

ramp up until the REF voltage takes over. At this

point, the soft-start is complete and the device

enters steady-state operation.

The SS capacitor value can be determined as

follows:

ss

ss

ss

REF

T(ms) I (uA)

C(nF)

V(V)





(10)

If the output capacitance has a large value,

setting too short of a SS time is not

recommended. Otherwise, it’s easy to hit the

current limit during SS. A minimum value of

4.7nF is recommended if the output capacitance

is larger than 330μF.

Pre-Bias Start-Up

The MP2326 has been designed for a monotonic

start-up into pre-biased loads. If the output is pre-

biased to a certain voltage during start-up, the

BST voltage is refreshed and charged; the

voltage on the soft-start capacitor is charged as

well. If the BST voltage exceeds its rising

threshold voltage and the soft-start capacitor

voltage exceeds the sensed output voltage at the

FB, the part starts to operate.

Power Good (PG)

PG is an open-drain output. PG requires a pull-

up resistor (eg. 100k). PG is pulled to GND

before SS is ready. After FB voltage reaches

PG is pulled low.

Over-Current Protection (OCP) and Short-

Circuit Protection (SCP)

MP2326 has cycle-by-cycle over-current limit

control. During the HS-FET on state, the inductor

current is monitored. When the sensed inductor

current hits the peak current limit, the HS limit

comparator (see Fig. 1) is triggered, the device

enters over-current protection mode immediately,

and then turns off the HS-FET and turns on the

LS-FET. Meanwhile, the output voltage drops

threshold (50% below the reference, typically).

Once UV is triggered, the MP2326 enters hiccup

mode to re-start the part periodically.

During over-current protection, the device tries to

recover from the over-current fault with hiccup

mode. The chip disables the output power stage,

discharges

the

soft-start

cap,

and

then

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

current condition still holds after the soft-start

ends, the device repeats this operation cycle until

the over-current condition disappears. Then the

output rises back to regulation level. The OCP is

non-latch protection.

Enable Control (EN)

EN is a digital control pin that turns the regulator

on and off. Drive EN high to turn on the regulator;

drive EN low to turn off the regulator. An internal

1MΩ resistor from EN to GND allows EN to be

floated to shut down the chip.

EN is clamped internally using a 6.5V series-

Zener-diode. Connect the EN input through a

resistance needs to be large enough to limit the

EN current to less than100µA. For example, with

100µA = 55kΩ.

Connecting EN to a voltage source directly

without a pull-up resistor requires limiting the

amplitude of the voltage to less than 6V to

prevent damage to the Zener diode.