Micrel, Inc.

MIC5331

February 2008

7

M9999-021408-A

Application Information

MIC5331 is a tiny dual low quiescent current 300mA

LDO. The MIC5331 regulator is fully protected from

damage due to fault conditions, offering linear current

limiting and thermal shutdown.

Input Capacitor

The MIC5331 is a high-performance, high bandwidth

device. Therefore, it requires a well-bypassed input

supply for optimal performance. A 1µF capacitor is

required from the input to ground to provide stability.

Low-ESR

ceramic

capacitors

provide

optimal

performance at a minimum of space. Additional high-

frequency capacitors, such as small-valued NPO

dielectric-type capacitors, help filter out high-frequency

noise and are good practice in any RF-based circuit.

X5R or X7R dielectrics are recommended for the input

capacitor. Y5V dielectrics lose most of their capacitance

over temperature and are therefore, not recommended.

Output Capacitor

The MIC5331 requires an output capacitor of 1µF or

greater to maintain stability. The design is optimized for

use with low-ESR ceramic chip capacitors. High ESR

capacitors may cause high frequency oscillation. The

output capacitor can be increased, but performance has

been optimized for a 1µF ceramic output capacitor and

does not improve significantly with larger capacitance.

X7R/X5R

dielectric-type

ceramic

capacitors

are

recommended

because

of

their

temperature

performance. X7R-type capacitors change capacitance

by 15% over their operating temperature range and are

the most stable type of ceramic capacitors. Z5U and

Y5V dielectric capacitors change value by as much as

50% and 60%, respectively, over their operating

temperature ranges. To use a ceramic chip capacitor

with Y5V dielectric, the value must be much higher than

an X7R ceramic capacitor to ensure the same minimum

capacitance over the equivalent operating temperature

range.

No-Load Stability

Unlike many other voltage regulators, the MIC5331 will

remain stable and in regulation with no load. This is

especially

important

in

CMOS

RAM

keep-alive

applications.

Enable/Shutdown

The MIC5331 comes with dual active-high enable pins

that allow each regulator to be disabled independently.

Forcing the enable pin low disables the regulator and

sends it into a “zero” off-mode-current state. In this state,

current consumed by the regulator goes nearly to zero.

Forcing the enable pin high enables the output voltage.

The active-high enable pin uses CMOS technology and

the enable pin cannot be left floating; a floating enable

pin may cause an indeterminate state on the output.

Thermal Considerations

The MIC5331 is designed to provide 300mA of

continuous current for both outputs in a very small

package. Maximum ambient operating temperature can

be calculated based on the output current and the

voltage drop across the part. For example if the input

power dissipation of the regulator circuit can be

determined using the equation:

Because this device is CMOS and the ground current is

typically <100µA over the load range, the power

dissipation contributed by the ground current is < 1%

and can be ignored for this calculation.

To

determine

the

maximum

ambient

operating

temperature of the package, use the junction-to-ambient

thermal resistance of the device and the following basic

equation:

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

−

=

JA

A

J(MAX)

D(MAX)

θ

T

T

P

operating temperature will give the maximum operating

conditions for the regulator circuit. The junction-to-

ambient thermal resistance for the minimum footprint is

90°C/W.

The maximum power dissipation must not be exceeded

for proper operation.

For example, when operating the MIC5331-PMYMT at

an input voltage of 3.6V and 300mA loads at each output

with a minimum footprint layout, the maximum ambient

Therefore, a 3.0V/2.8V application with 300mA at each

output current can accept an ambient operating

temperature of 87°C in a 2mm x 2mm MLF

For a full discussion of heat sinking and thermal effects

on voltage regulators, refer to the “Regulator Thermals”

Regulators

handbook. This information can be found on

Micrel's website at:

http://www.micrel.com/_PDF/other/LDOBk_ds.pdf