MIC5331 Datasheet, PDF (7/10 Pages) Micrel Semiconductor – Micro-Power High Performance Dual 300mA ULDO™

Micrel, Inc.
MIC5331
Application Information
MIC5331 is a tiny dual low quiescent current 300mA
LDO. The MIC5331 regulator is fully protected from
damage due to fault conditions, offering linear current
limiting and thermal shutdown.
Input Capacitor
The MIC5331 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.
X5R or X7R dielectrics are recommended for the input
capacitor. Y5V dielectrics lose most of their capacitance
over temperature and are therefore, not recommended.
Output Capacitor
The MIC5331 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
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.
No-Load Stability
Unlike many other voltage regulators, the MIC5331 will
remain stable and in regulation with no load. This is
especially important in CMOS RAM keep-alive
applications.
Enable/Shutdown
The MIC5331 comes with dual active-high enable pins
that allow each regulator to be disabled independently.
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.
Thermal Considerations
The MIC5331 is designed to provide 300mA 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. For example if the input
voltage is 3.6V, the output voltage is 3.0V for V_OUT1, 2.8V
for V_OUT2 and the output current = 300mA. 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 – 3.0V) × 300mA + (3.6V -2.8) × 300mA
P_D = 0.42W
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
⎟⎟⎠⎞
T_J(max) = 125°C, and the maximum junction temperature
of the die, θ_JA, thermal resistance = 90°C/W.
Substituting P_D for P_D(max) and solving for the ambient
operating temperature will give the maximum operating
conditions for the regulator circuit. The junction-to-
ambient thermal resistance for the minimum footprint is
90°C/W.
The maximum power dissipation must not be exceeded
for proper operation.
For example, when operating the MIC5331-PMYMT at
an input voltage of 3.6V and 300mA loads at each output
with a minimum footprint layout, the maximum ambient
operating temperature T_A can be determined as follows:
0.42W = (125°C – T_A)/(90°C/W)
T_A = 87.2°C
Therefore, a 3.0V/2.8V application with 300mA at each
output current can accept an ambient operating
temperature of 87°C in a 2mm x 2mm MLF^® package.
For a full discussion of heat sinking and thermal effects
on voltage regulators, refer to the “Regulator Thermals”
section of Micrel’s Designing with Low-Dropout Voltage
Regulators handbook. This information can be found on
Micrel's website at:
http://www.micrel.com/_PDF/other/LDOBk_ds.pdf
February 2008
7
M9999-021408-A