MP20046 – FAST TRANSIENT RESPONSE, 2A, LOW DROPOUT VOLTAGE REGULATOR
MP20046 Rev. 1.1 www.MonolithicPower.com 12
10/13/2010 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited.
© 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:
()
OUTOUTIN
IVVP ×−=
The allowable power dissipation can be
calculated using the following equation:
()
JAAmbientJunction(MAX)
TTP θ−= /
Where (T
Junction
T
Ambient
) is the temperature
difference between the junction and the
surrounding environment,
JA
is the thermal
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
17uV
RMS
over the range of the full load.
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
1.5 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.
