Micrel, Inc. MIC5264
May 2006
9
M9999-052406
(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 300mΩ. 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
5.0V, the V
OUT1
output voltage is 3.0V at 150mA; V
OUT2
output voltage is 2.8V at 100mA.
The actual power dissipation of the regulator circuit can
be determined using the equation:
P
D
= (V
IN
– V
OUT
) I
OUT
+ V
IN
I
GND
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.
P
D
= (5.0V-3.0V) x 150mA + (5.0V-2.8V) x 100mA
P
D
= 0.52W
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
AJ
D
TT
P
θ
(max)
(max)
T
J(max)
= 125°C, the max. junction temperature of the die
θ
JA
thermal resistance = 63°C/W
