MP20046 – FAST TRANSIENT RESPONSE, 2A, LOW DROPOUT VOLTAGE REGULATOR

MP20046 Rev. 1.1

www.MonolithicPower.com

12

10/13/2010

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© 2010 MPS. All Rights Reserved.

APPLICATION INFORMATION

Power Dissipation

The power dissipation for any package depends

on the thermal resistance of the case and circuit

board, the temperature difference between the

junction and ambient air, and the rate of air flow.

The power dissipation across the device can be

represented by the equation:

()

OUT

OUT

IN

I

V

V

P

×

−

=

The allowable power dissipation can be

calculated using the following equation:

()

JA

Ambient

Junction

(MAX)

T

T

P

θ

−

=

/

difference between the junction and the

resistance from the junction to the ambient

environment. Connecting the GND pin of

MP20046 to ground using a large pad or

ground plane helps to channel heat away.

Output Noise

Output noise voltage is the RMS output noise

voltage over a given range of frequencies

(10Hz to 30kHz) under the conditions of a

constant output current and a ripple-free input

voltage. Most of noise is caused by the internal

voltage

reference.

A

precision

bandgap

reference of MP20046 is used to generate the

internal reference voltage. And a low pass filter

is also used to reduce the output noise. These

make the output noise of MP20046 low to

Power Supply Rejection Ratio (PSRR)

Power supply rejection ratio (PSRR), also

known as ripple rejection, measures the LDO

regulator’s ability to prevent the regulated

output voltage fluctuating caused by input

voltage variations.

The ripple rejection is defined by

ripple

,

V

ripple

,

V

PSRR

OUT

IN

=

at all frequencies

The control loop tends to be the dominant

contributor of supply rejection. The output

capacitor of large value and low ESR, and

adding by-pass capacitors will improve the

PSRR performance.

Input Capacitor Selection

For a typical application, an input bypass

capacitor is recommended for device stability.

This capacitor should be as close to the input

pins as possible. For fast transient conditions

where droop at the input of the LDO may occur

because

of

high

inrush

current,

it

is

recommended to place a larger capacitor at the

input as well. The size of this capacitor depends

on the output current and response time of the

main power supply, as well as the distance to

the load (LDO).

Output Capacitor Selection

The MP20046 is designed specifically to work

with very low ESR ceramic output capacitor in

space-saving and performance consideration. A

47µF ceramic capacitor with ESR lower than

1.5Ω is suitable for the MP20046 application

circuit. Output capacitor of larger values will

help to improve load transient response and

reduce noise with the drawback of increased

size.