MP2116 –2A, 6V, 100% DUTY CYCLE SYNCHROOUS STEP-DOWN CONVERTER WITH 0.5A LDO

MP2116 Rev.1.0

www.MonolithicPower.com

10

1/17/2013

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Current Mode PWM Control

Slope compensated current mode PWM control

provides stable switching and cycle-by-cycle

current limiting for superior load and line

response in addition to protection of the internal

main switch and synchronous rectifier. The

MP2116 switches at a constant frequency

(1.25MHz) 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 MP2116 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

becomes the input voltage minus the voltage

drop across the main switch and the inductor.

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 Switcher Output Current

The MP2116 switcher can provide up to 2A

output current, and operate down to 3V input

voltage; however the maximum output current

decreases at lower input voltage due to a 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

Power OK

The MP2116 provides an open-drain PWROK

output that goes high after both channels reach

regulation during startup. PWROK goes low after

one of the output channels goes out of regulation

by ±10% or when device enters shutdown. There

are deglitch timers built in to avoid PWROK false

triggered during load transient: 50µS for the

switcher and 150µS for the LDO.

Low Input 0.5A Linear Regulator

The low input 0.5A low dropout (LDO) linear

regulator has separate input IN2 and output

OUT2 pins for the internal power PNP device.

The control circuitry of the LDO takes power from

the main input supply IN1. Both IN1 and IN2

input supplies must be presented for the LDO

working properly. The LDO power device input

IN2 can be connected to the switcher output

(Figure1) or directly to the main supply IN1

(Figure3). If the IN2 tied to the IN1, it is optional

to insert a RC filter between IN1 and IN2. The

RC filter will reduce switching noise coupling

from IN1 to IN2 and power dissipation inside the

MP2116