MIC5323 Datasheet, PDF (9/15 Pages) Micrel Semiconductor – High PSRR Low Noise 300mA μCap Ultra-Low Dropout LDO Regulator

Micrel, Inc.
Application Information
Enable/Shutdown
The MIC5323 comes with an active high enable pin that
allows the regulator 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. 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.
Input Capacitor
The MIC5323 is a high performance, high bandwidth
device. Therefore, it requires a well-bypassed input
supply for optimal performance. A 2.2µF capacitor is
required from the input-to-ground to provide stability.
Low ESR ceramic capacitors provide optimal perform-
ance using a minimum amount of space. Additional high
frequency capacitors, such as small valued NPO
dielectric type capacitors, help filter out high-frequency
noise, and are a good practice in any RF based circuit.
Output Capacitor
The MIC5323 requires an output capacitor of 2.2µF 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 2.2µF ceramic output capacitor,
and it does not improve significantly with larger
capacitance.
X7R/X5R dielectric type ceramic capacitors are recom-
mended because of their temperature performance. X7R
type capacitors change capacitance by 15% over their
operating temperature range, making them 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 0.1µF 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 MIC5323 to drive a
large capacitor on the bypass pin without significantly
MIC5323
slowing turn on time. Refer to the Typical Characteristics
section for performance with different bypass capacitors.
No Load Stability
Unlike many other voltage regulators, the MIC5323 will
remain stable and in regulation with no load. This is
especially important in CMOS RAM keep alive
applications.
Adjustable Regulator Application
Adjustable regulators use the ratio of two resistors to
multiply the reference voltage to produce the desired
output voltage. The MIC5323 can be adjusted from
1.25V to 5.5V by using two external resistors (Figure 1).
The resistors set the output voltage based on the
following equation:
VOUT
=
VREF

^{1 +}

R1
R2


VREF = 1.25V
Figure 1. Adjustable Voltage Application
November 2011
9
M9999-110311B