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

MP2308 Rev. 1.0

www.MonolithicPower.com

9

7/9/2012

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© 2012 MPS. All Rights Reserved.

OPERATION

The MP2308 is a high frequency synchronous

rectified step-down switch mode converter with

built in internal power MOSFETs. It offers a

very compact solution to achieve more than 4A

continuous output current over a wide input

supply range with excellent load and line

regulation.

The MP2308 operates in a fixed frequency,

peak current control mode to regulate the

output voltage. A PWM cycle is initiated by the

internal clock. The integrated high-side power

MOSFET is turned on and remains on until its

current reaches the value set by the COMP

voltage. When the power switch is off, it

remains off until the next clock cycle starts. If, in

90% of one PWM period, the current in the

power MOSFET does not reach the COMP set

current value, the power MOSFET will be

forced to turn off.

Error Amplifier

The error amplifier compares the FB pin voltage

with the internal 0.8V reference (REF) and

outputs a current proportional to the difference

between the two. This output current is then

used to charge or discharge the internal

compensation network to form the COMP

voltage, which is used to control the power

MOSFET current. The optimized internal

compensation network minimizes the external

component counts and simplifies the control

loop design.

Internal Regulator

Most of the internal circuitries are powered from

the 5V internal regulator. This regulator takes

the VIN input and operates in the full VIN range.

When VIN is greater than 5.0V, the output of

the regulator is in full regulation. When VIN is

lower than 5.0V, the output decreases, a 1μF

ceramic capacitor for decoupling purpose is

required.

Enable Control

The MP2308 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 in Figure 2.

Connect the EN input pin 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.

(12V – 6.7V)/100µA = 53kΩ.

Connecting the EN pin is directly to a voltage

source without any pullup resistor requires

limiting the amplitude of the voltage source to

below 6V to prevent damage to the Zener

diode.

EN

EN LOGIC

Zener

6.7V typ.

GND

Figure 2: Zener Diode between EN and GND

The EN pin also features an internal 2.3μA

current source. Connect a capacitor to the EN

pin for delayed startup. When VIN exceeds the

input UVLO, an internal 2.3μA current source

charges the external capacitor. The external

capacitor connects to the non-inverting input of

a comparator. The part is enabled once the

capacitor voltage exceeds the 1.5V internal

reference voltage.

External Soft-Start

The soft start time can be adjusted by

connecting a capacitor from this pin to ground.

When the soft-start period starts, an internal

8μA current source begins charging the

external capacitor. During soft-start, the voltage

on the soft-start capacitor is connected to the

non-inverting input of the error amplifier. The

soft-start period lasts until the voltage on the

soft-start capacitor exceeds the reference

voltage of 0.815V. At this point the reference

voltage takes over at the no inverting error

amplifier input. The soft-start time can be

calculated as follows:

SS

SS

0.815V C (nF)

t(ms)

8A





