MIC5335 Datasheet, PDF (8/10 Pages) Micrel Semiconductor – Dual, High Performance 300mA UCap ULDO

Micrel, Inc.
MIC5335
Applications Information
Enable/Shutdown
The MIC5335 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.
Input Capacitor
The MIC5335 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 MIC5335 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 on the market. 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 MIC5335
will remain stable and in regulation with no load. This
is especially important in CMOS RAM keep-alive
applications.
Thermal Considerations
The MIC5335 is designed to provide 300mA of
continuous current for both outputs in a very small
package. Maximum ambient operating temperature
can be calculated based upon 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 = 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 = (3.3V – 2.8V) × 300mA + (3.3V – 2.5V) × 300mA
P_D = 0.39W
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)
=
⎜⎛ ^{TJ(max) − TA}
⎜⎝
θ _JA
⎟⎟⎠⎞
T_J(max) = 125°C, the maximum junction temperature of
the die θ_JA thermal resistance = 100°C/W.
The table that follows shows junction-to-ambient
thermal resistance for the MIC5335 in the Thin MLF^®
package.
θ_JA
Package
Recommended
Minimum
θ_JC
Footprint
6-Pin 1.6 X1.6
Thin MLF™
100°C/W
2°C/W
Thermal Resistance
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 100°C/W.
The maximum power dissipation must not be
exceeded for proper operation.
For example, when operating the MIC5335-MFYML at
an input voltage of 3.3V and 300mA loads at each
January 2007
8
M9999-010407
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