MIC5321_08 Datasheet, PDF (9/12 Pages) Micrel Semiconductor – Dual, High Performance 150mA μCap ULDO™

Micrel, Inc.
Applications Information
Enable/Shutdown
The MIC5321 comes with a single active-high enable
pin that allows both regulators to be disabled
simultaneously. 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 MIC5321 is a high-performance, high bandwidth
device. Therefore, it requires a well-bypassed input
supply 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 MIC5321 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 perform-
ance. 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 0.1µF
capacitor is recommended for applications that require
low-noise outputs. The bypass capacitor can be
increased, further reducing noise and improving
May 2008
MIC5321
PSRR. Turn-on time increases slightly with respect to
bypass capacitance. A unique, quick-start circuit
allows the MIC5321 to drive a large capacitor on the
bypass pin without significantly 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 MIC5321
will remain stable and in regulation with no load. This
is especially important in CMOS RAM keep-alive
applications.
Thermal Considerations
The MIC5321 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. Given that the input
voltage is 3.3V, the output voltage is 2.8V for V_OUT1,
2.5V for V_OUT2 and the output current = 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
P_D = 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 MIC5321 in the Thin MLF^® package.
Package
6-Pin 1.6x1.6 Thin MLF^®
θ_JA Recommended
Minimum Footprint
100°C/W
Thermal Resistance
θ_JC
2°C/W
9
M9999-051508-E