Micrel, Inc. MIC5357
January 2011 7
M9999-011311
Applications Information
Enable/Shutdown
The MIC5357 is provided with dual active high enable
pins that allow each regulator to be enabled
independently. Forcing both enable pins low 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 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 MIC5357 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
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 MIC5357 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
does not improve significantly with larger capacitance.
X7R/X5R dielectric type ceramic capacitors are
recommended because of their superior 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 0.1µF 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 MIC5357 to drive
a large capacitor on the bypass pin without significantly
slowing the turn on time.
No-Load Stability
Unlike many other voltage regulators, the MIC5357 will
remain stable with no load. This is especially important
in CMOS RAM keep alive applications.
Thermal Considerations
The MIC5357 is designed to provide 500mA of
continuous current for V
OUT1
and 500mA for V
OUT2
in a
small package. The 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 3.6V, the output voltage is 3.3V for
V
OUT1
, 2.8V for V
OUT2
and the output current of 500mA
and 500mA respectively. 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 <100µA over the load range, the power
dissipation contributed by the ground current is < 1%
and can be ignored for this calculation.
P
D
= (3.6V – 3.3V) × 500mA + (3.6V -2.8) × 500mA
P
D
= 0.55W
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)
- T
A
JA
⎝
T
J(max)
= 125°C, the maximum junction temperature of
the die θ
JA
thermal resistance = 64.4°C/W.
