MP2308 – 18V, 4A SYNCHRONOUS STEP-DOWN CONVERTER

MP2308 Rev. 1.0

www.MonolithicPower.com

10

7/9/2012

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© 2012 MPS. All Rights Reserved.

If the output of the MP2308 is pre-biased to a

certain voltage during startup, the IC will disable

the switching of both high-side and low-side

switches until the voltage on the internal soft-

start capacitor exceeds the sensed output

voltage at the FB pin.

Over-Current-Protection

The MP2308 has a hiccup over current limit

when the inductor current peak value exceeds

the set current limit threshold.

When output voltage drops below 70% of the

reference, and the inductor current exceeds the

current limit at the meantime, MP2308 will

hiccup. This is especially useful to ensure

system safety under fault conditions.

Light Load Mode

The MP2308 can operate in discontinuous

mode (DCM) and Pulse skip mode (PSM) under

light load to improve efficiency. To enable Light

Load Mode, connect AAM pin to VCC pin or to

a voltage divider from VCC. To disable Light

Load Mode, connect AAM pin to ground and the

converter will always operate in CCM.

When the AAM pin connects to VCC, the

inductor peak current in set internally which is

about 240mA for VIN=12V, VOUT=3.3V and

L=4.7μH. Peak inductor current also can be set

by connecting AAM pin to a voltage divider from

VCC with 2 external resistors. By program AAM

voltage, efficiency and output ripple can be

traded off and optimize. Higher AAM gives

higher peak inductor current thus more efficient

but more output ripple. To help select the AAM

voltage, the curve of inductor peak current vs.

low power mode voltage is shown in Page 5,

Typical Performance Characteristics section.

Under-Voltage Lockout (UVLO)

Under-voltage lockout (UVLO) protects the chip

from operating at insufficient supply voltage.

The MP2308’s UVLO comparator monitors the

output voltage of the internal regulator, VCC.

The UVLO rising threshold is 4.25V while its

falling threshold is 3.38V.

Thermal Shutdown

Thermal shutdown is implemented to prevent

the chip from operating at exceedingly high

temperatures. When the silicon die temperature

is higher than 150

C, it shuts down the whole

chip. When the temperature is lower than its

lower threshold, typically 140

C, the chip is

enabled again.

Floating Driver and Bootstrap Charging

The floating power MOSFET driver is powered

by an external bootstrap capacitor. This floating

driver has its own UVLO protection. This

UVLO’s rising threshold is 2.2V with a

hysteresis of 150mV. The bootstrap capacitor

voltage is regulated internally by VIN through

more than 5V, U1 will regulate M1 to maintain a

5V BST voltage across C4.

D1

M1

BST

C4

L1

SW

U1

5V

C2

Figure 3—Internal Bootstrap Charging

Circuit

Startup and Shutdown

appropriate thresholds, the chip starts. The

reference block starts first, generating stable

reference voltage and currents, and then the

internal regulator is enabled. The regulator

provides stable supply for the remaining

circuitries.

Three events can shut down the chip: EN low,

procedure, the signaling path is first blocked to

avoid any fault triggering. The COMP voltage

and the internal supply rail are then pulled down.

The floating driver is not subject to this shutdown

command.