MP2321 – 19V, 2A, LOW IQ, STEP-DOWN CONVERTER

MP2321 Rev. 1.1

www.MonolithicPower.com

15

5/24/2019

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Pre-Bias Start-Up

The MP2321 is designed for 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, and 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

FB, the MP2321 begins working.

Power Good (PG)

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

up resistor (e.g.: 100kΩ). PG is pulled to GND

before SS is ready. After the FB voltage

reaches 90% of VREF, PG is pulled high after a

140μs delay. When the FB voltage drops below

85% of VREF, PG is pulled low.

Note: If PG is pulled up to an external voltage,

PG will not de-assert (Logic low) if EN is low or

if Vin < 0.8V (typ). If PG is pulled up to the VCC

pin, PG will de-assert (Logic low) if either EN is

Low or if Vin < 0.8V (typ). If connecting two or

more PG together, please refer to Application

section.

Over-Current Protection (OCP) and Short-

Circuit Protection (SCP)

The MP2321 has a cycle-by-cycle over-current

limit control. During HS-FET on, the inductor

current is monitored. When the sensed inductor

current reaches the peak current limit, the HS

limit comparator is triggered. The MP2321

enters over-current protection (OCP) mode

immediately, turns the HS-FET off, and turns

the LS-FET on. Meanwhile, the output voltage

drops until VFB is below the under-voltage (UV)

threshold, typically 50% below the reference.

Once UV is triggered, the MP2321 enters

hiccup mode to restart the part periodically.

During OCP, the device attempts to recover

from the over-current fault with hiccup mode. In

hiccup mode, the chip disables the output

power stage, discharges the soft-start capacitor,

and attempts to soft start again automatically. If

the over-current condition still remains after the

soft start ends, the device repeats this

operation cycle until the over-current condition

is removed and the output rises back to the

regulation level. OCP is a non-latch protection.

Enable (EN) Control

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

pull-up resistor to the voltage on VIN. The pull-

up resistance must be large enough to limit the

EN current below 100µA. For example, with

12V connected to VIN, RPULLUP ≥ (12V - 6.5V) ÷

100µA = 55kΩ.

Connecting EN to a voltage source directly

without a pull-up resistor requires limiting the

amplitude of the voltage below 6V to prevent

damage to the Zener diode.

Under-Voltage Lockout (UVLO) Protection

The

MP2321

has

under-voltage

lockout

protection (UVLO). When the input voltage is

higher than the UVLO rising threshold voltage,

the MP2321 powers up. The MP2321 shuts off

when the input voltage is lower than the UVLO

falling threshold voltage. This is a non-latch

protection.

Thermal Shutdown

The MP2321 employs thermal shutdown by

monitoring the junction temperature of the IC

internally. If the junction temperature exceeds

the threshold value (typically 150°C), the

converter shuts off. This is a non-latch

protection. There is a hysteresis of about 20°C.

Once the junction temperature drops below

130°C, the MP2321 starts up.