Micrel, Inc.

MIC5264

April 2005

8

M9999-041505

(408) 955-1690

Application Information

Enable/Shutdown

The MIC5264 comes with two independent active-high

enable pins that allow the regulator in each output to be

disabled separately. 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. This part is CMOS and the enable pin

cannot be left floating; a floating enable pin may cause

an indeterminate state on the output.

Input Capacitor

The MIC5264 is a high performance, high bandwidth

device. Therefore, it requires well-bypassed input

supplies for optimal performance. A 1uF 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.

Output Capacitor

The MIC5264 requires capacitors at both outputs for

stability. The design requires 1uF or greater on each

output to maintain stability. The design is optimized for

use with low-ESR ceramic chip capacitors. High ESR

capacitors may cause high frequency oscillation. The

maximum recommended ESR is 300m

Ω. The output

capacitor can be increased, but performance has been

optimized for a 1uF 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.

Bypass Capacitor

A capacitor can be placed from the noise bypass pin to

ground to reduce output voltage noise. The capacitor

bypasses the internal reference. A 0.01uF capacitor is

recommended for applications that require low-noise

outputs. The bypass capacitor can be increased, further

reducing noise and improving PSRR. Turn-on time

increases slightly with respect to bypass capacitance. A

unique quick-start circuit allows the MIC5264 to drive a

large capacitor on the bypass pin without significantly

slowing turn-on time.

Active Shutdown

The MIC5264 also features an active shutdown clamp,

which is an N-channel MOSFET that turns on when the

device is disabled. This allows the output capacitor and

load to discharge, de-energizing the load.

No-Load Stability

The MIC5264 will remain stable and in regulation with no

load unlike many other voltage regulators. This is

especially important in CMOS RAM keep-alive

applications.

Thermal Considerations

The MIC5264 is designed to provide 150mA of

continuous current per output in a very small package.

Maximum ambient operating temperature can be

calculated based on the output current and the voltage

drop across the part. Given that the input voltage is

output voltage is 2.8V at 100mA.

The actual power dissipation of the regulator circuit can

be determined using the equation:

Because this device is CMOS and the ground current is

typically <100uA 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

D

T

T

P

θ

(max)

(max)