MP2101 – 1.6MHz SYNCHRONOUS STEP-DOWN PLUS 200mA LDO

MP2101 Rev. 1.0

www.MonolithicPower.com

8

8/18/2006

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TM

800mA Step-Down Switcher

The switcher integrates both a main switch and

a synchronous rectifier, which provides high

efficiency and eliminates the need for an

external Schottky diode.

This switcher can achieve 100% duty cycle.

The duty cycle (D) of a step-down switcher is

defined as:

%

100

V

V

%

100

f

T

D

IN

OUT

OSC

ON

×

≈

×

×

=

is the oscillator frequency (1.6MHz).

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. 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 to ramp

the inductor current at each rising edge of the

internal oscillator, then switched off when the

peak inductor current is above the error voltage.

When the main switch is turned off, the

synchronous rectifier is immediately turned on

and stays on until the next cycle begins.

Dropout Operation

The MP2101 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 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, which 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 MP2101 can operate down to a 2.5V input

voltage. However the maximum load current

decreases at lower inputs 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 MP2101 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 the device enters

shutdown. There is a built-in deglitch timer to

avoid a false PWROK trigger during load

transients (50µs for the switcher and 150µs for

the LDO). When the output is disabled, Power

OK remains low.

200mA Linear Regulator

The 200mA low dropout (LDO) linear regulator

has separate input (IN2) and output (OUT2)

pins for the internal power device. The control

circuitry of the LDO takes power from the main

input supply (IN1). Both IN1 and IN2 input

supplies must be present for the LDO to work

properly. The LDO power device input (IN2) can

be connected to the switcher output or directly

to the main supply (Figure 2). If IN2 is tied to

IN1, an optional RC filter can be inserted

between IN1 and IN2. The RC filter reduces

switching noise coupling from IN1 to IN2 and

power dissipation inside the MP2101. The

switcher guarantees 800mA output current, but

output current to the switcher load will be

reduced if the LDO draws current from the

switcher output.