MIC5322 Datasheet, PDF (7/9 Pages) Micrel Semiconductor – Dual, High Performance 150mA μCap ULDO™

Micrel, Inc.
Applications Information
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The MIC5322 comes with a single active-low enable
pin that allows both regulators to be disabled
simultaneously. Forcing the enable pin high disables
the regulators 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
low enables the output voltages. The active-low
enable pin cannot be left floating since a floating
enable pin may cause an indeterminate state on the
output.
Input Capacitor
The MIC5322 is a high-performance, high bandwidth
device. Therefore optimal performance can be
achieved by providing a well-bypassed input supply. 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 MIC5322 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 stable
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.
MIC5322
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 MIC5322 to drive a large capacitor on the
bypass pin without significantly slowing turn-on time.
Refer to the Typical Characteristics section of this
datasheet for performance with different bypass
capacitors.
No-Load Stability
Unlike many other voltage regulators, the MIC5322
will remain stable and in regulation with no load. This
is especially important in CMOS RAM keep-alive
applications.
Thermal Considerations
The MIC5322 is designed to provide 150mA 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. As an example: Given
that the input voltage is 3.3V, the output voltage is
2.8V for V_OUT1, 1.5V for V_OUT2 and the output current at
150mA. The actual power dissipation of the regulator
circuit can be determined using the equation:
P_D = (V_IN – V_OUT1) I_OUT1 + (V_IN – V_OUT2) I_OUT2+ V_IN I_GND
Because this device is CMOS and the ground current
is typically <150µA over the load range, the power
dissipation contributed by the ground current is < 1%
and can be ignored for this calculation.
P_D = (3.3V – 2.8V) × 150mA + (3.3V -1.5) × 150mA
PD = 0.345W
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:
⎛ ^TJ(MAX) ^{- T}A
⎝ ^{PD(MAX) =}
JA
T_J(max) = 125°C, the maximum junction temperature of
the die θ_JA thermal resistance = 100°C/W.
The table below shows junction-to-ambient thermal
resistance for the MIC5322 in the Thin MLF^® package.
March 2008
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M9999-030608-A