MIC5265_07 Datasheet, PDF (9/11 Pages) Micrel Semiconductor – 150mA μCap LDO Regulator

Micrel, Inc.
Applications Information
Enable/Shutdown
The MIC5265 comes with an active-high enable pin that
allows the regulator in each output to be disabled.
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 MIC5265 is a high performance, high bandwidth
device. Therefore, it requires well-bypassed input
supplies 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.
Output Capacitor
The MIC5265 requires an output capacitor for stability.
The design requires 1µF 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
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. X7Rtype 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 0.01µF 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 MIC5265 to drive a
February 2007
MIC5265
large capacitor on the bypass pin without significantly
slowing turn-on time.
Active Shutdown
The MIC5265 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 MIC5265 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 MIC5265 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 output voltage is 2.8V, and the output current
is 100mA.
The actual power dissipation of the regulator circuit can
be determined using the equation:
PD = (VIN – VOUT) IOUT + VIN IGND
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.
PD = (5.0V – 2.8V) x 150mA
PD = 0.33W
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:
P_D(max)
=
⎜⎜⎝⎛
T_J
(max) ^−
θ _JA
T_A
⎟⎟⎠⎞
TJ(max) = 125°C, the max. junction temperature of the
die θJA thermal resistance = 235°C/W
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M9999-022307