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

MP2101 Rev. 1.0

www.MonolithicPower.com

10

8/18/2006

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TM

Switcher Input Capacitor Selection

current drawn from the input and switching

noise from the device. The input capacitor

impedance at the switching frequency should

be less than the input source impedance to

prevent high frequency switching current

passing to the input. Ceramic capacitors with

X5R

or

X7R

dielectrics

are

highly

recommended because of their low ESR and

small

temperature

coefficients.

For

most

applications, a 4.7µF capacitor is sufficient.

Switcher Output Capacitor Selection

voltage ripple small and ensures regulation loop

stability. The output capacitor impedance

should be low at the switching frequency.

Ceramic capacitors with X5R or X7R dielectrics

approximately:

()

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

×

×

+

×

×

×

−

×

≤

∆

1

O

OSC

OSC

1

IN

1

OUT

1

IN

1

OUT

1

OUT

C

f

8

1

ESR

L

f

V

V

V

V

V

Thermal Dissipation

Power dissipation should be considered when

both channels of the MP2101 provide maximum

output current at high ambient temperatures. If

the junction temperature rises above 150

°C, the

two channels will shut down.

The junction-to-ambient thermal resistance of

The maximum power dissipation is about 1.6W

when the MP2101 is operating in a 70

°C

ambient temperature environment.

W

6

.

1

W

/

C

50

C

70

C

150

PD

o

o

o

MAX

=

−

=

Start-Up Consideration

To ensure a smooth start-up of OUT1 and

OUT2, it is recommended that the enable

signals (EN1 and EN2) be asserted only after

the input power rails have been stabilized. If

EN1 and EN2 are tied to input rails directly, the

UVLO of the MP2101 will dictate when the part

starts switching. Since for certain systems, the

input

supply

may

have

relatively

high

impedance

during

ramp

up,

therefore

depending solely on UVLO to start the part may

cause input rail dip and output bounce. If the

system designer can not provide the enable

signal after input power rail is fully established,

it is recommended that EN1 and EN2 are

connected to the input power rail through a RC

delay network (as shown in Figure 2). The RC

time constant needs to be significantly large

compare to the ramp-up time of the input power

rail, which is usually of a few ms.

PC Board Layout

The high current paths (GND, IN1/IN2 and

SW1) should be placed very close to the device

with short, direct and wide traces. Input

capacitors should be placed as close as

possible to the respective IN and GND pins.

The external feedback resistors should be

placed next to the FB pins. Keep the switching

nodes SW1 short and away from the feedback

network.