MIC5307_07 Datasheet, PDF (7/9 Pages) Micrel Semiconductor – 300mA Micropower μCap Baseband LDO

Micrel, Inc.
Application Information
Input Capacitance
A 1µF capacitor should be placed from IN to GND if
there is more than 10 inches of wire between the input
and the ac filter capacitor or if a battery is used as the
input.
Output Capacitance
The MIC5307 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 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.
Enable
Forcing EN (enable/shutdown) high (>1V) enables the
regulator. EN is compatible with CMOS logic gates. If the
enable/shutdown feature is not required, connect EN
(pin 3) to IN (supply input, pin 1).
Current Limit
There is overcurrent protection circuitry built into the
MIC5307. Even with the output grounded, current will be
limited to approximately 500mA. Further protection is
provided by thermal shutdown.
Thermal Considerations
The MIC5307 is designed to provide 300mA of
continuous current 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 3.8V, the output
voltage is 2.8V and the output current equals 300mA.
The actual power dissipation of the regulator circuit can
be determined using the equation:
P_D = (V_IN – V_OUT) I_OUT + V_IN I_GND
MIC5307
Because this device is CMOS and the ground current is
typically < 50µA over the load range, the power dissipa-
tion contributed by the ground current is < 1% and can
be ignored for this calculation.
P_D = (3.8V – 2.8V) ⋅ 300mA
P_D = 0.3W
To determine the maximum ambient operating tempera-
ture of the package, use the junction-to-ambient thermal
resistance of the device and the following basic
equation:
T_J(max) – T_A
P_D(max) =
JA
T_J(max) = 125°C, the maximum junction temperature of
the die θ_JA thermal resistance = 235°C/W
Table 1 shows junction-to-ambient thermal resistance for
the MIC5307 in the TSOT23-5 package.
Package
TSOT23-5
θ_JA Recommended
Minimum Footprint
235°C/W
Table 1. TSOT23-5 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
235°C/W, from Table 1. The maximum power dissipation
must not be exceeded for proper operation.
For example, when operating the MIC5307-2.8 at an
input voltage of 3.8V and 300mA load with a minimum
footprint layout, the maximum ambient operating
temperature T_A can be determined as follows:
0.3W = (125°C - T_A) / 235C°/W
T_A = 54.5°C
Therefore, a 2.8V application at 300mA of output current
can accept an ambient operating temperature of 89.8°C
in a TSOT23-5 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
August 2007
7
M9999-082407-B