MP2128 – 3MHz, 1A SYNCHRONOUS STEP-DOWN CONVERTER

MP2128 Rev. 0.9

www.MonolithicPower.com

6

7/20/2009

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FUNCTIONAL DESCRIPTION

MP2128 is a 3MHz constant frequency, current

mode, PWM step-down converter. The device

integrates both a main switch and a synchronous

rectifier, which provides high efficiency and

eliminates an external Schottky diode. It is ideal

for powering portable equipment that runs from a

single Li-lon or Li-Polymer battery.

The MP2128 can achieve 100% duty cycle. The

duty cycle D of a step-down converter is defined

as:

OUT

ON

OSC

IN

V

DT

f

100%

100%

V

=×

×

≈

×

Where,

Ton: Main Switch ON time

Fosc: Switching frequency (3MHz)

Vout: Output voltage

Vin: Input voltage

Peak Current Mode PWM Control

Slope compensated current mode PWM control

provides stable switching and cycle-by-cycle

current limit for superior load and line response

and protection of the internal main switch and

synchronous rectifier. The MP2128 switches at a

constant frequency (3MHz) and regulates the

output voltage. During each cycle the PWM

comparator modulates the power transferred to

the load by changing the inductor peak current

based on the feedback error voltage.

During normal operation, the main switch is

turned on for a certain time to ramp the inductor

current at each rising edge of the internal

oscillator, and switched off when the peak

inductor current is above the error voltage.

When the main switch is off, the synchronous

rectifier will be turned on immediately and stay on

until the next cycle starts.

Dropout Operation

The MP2128 allows the main switch to remain on

for more than one switching cycle and increases

the duty cycle while the input voltage is dropping

close to the output voltage. When the duty cycle

reaches 100%, the main switch is held on

continuously to deliver current to the output up to

the PFET current limit. The output voltage then is

the input voltage minus the voltage drop across

the main switch and the inductor.

Short Circuit Protection

The MP2128 has short circuit protection. When

the output is shorted to ground, the oscillator

frequency is reduced to prevent the inductor

current from increasing beyond the PFET current

limit. The PFET current limit is also reduced to

lower the short-circuit current. The frequency and

current limit will return to the normal values once

the short circuit condition is removed and the

feedback voltage reaches 0.6V.

Maximum Load current

The MP2128 can operate down to 2.5V input

voltage, however the maximum load current

decreases at lower input due to large IR drop on

the main switch and synchronous rectifier. The

slope compensation signal reduces the peak

inductor current as a function of the duty cycle to

prevent sub-harmonic oscillations at duty cycles

greater than 50%. Conversely the current limit

increases as the duty cycle decreases.