MIC5225 Datasheet, PDF (7/9 Pages) Micrel Semiconductor – Ultra-Low Quiescent Current 150mA μCap Low Dropout Regulator

Micrel, Inc.
Application Information
Enable/Shutdown
The MIC5225 comes with an active-high enable pin that
allows the regulator to be disabled. Forcing the enable
pin lows 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.
Input Capacitor
The MIC5225 has a wide input voltage capability up to
16V. The input capacitor must be rated to sustain
voltages that may be used on the input. An input
capacitor may be required when the device is not near
the source power supply or when supplied by a battery.
Small, surface mount, ceramic capacitors can be used
for bypassing. Larger value may be required if the
source supply has high ripple.
Output Capacitor
The MIC5225 requires an output capacitor for stability.
The design requires 1.0µF or greater on the output to
maintain stability. The design is optimized for use with
low-ESR ceramic chip capacitors. High ESR capacitors
may cause high frequency oscillation. The maximum
recommended ESR is 300mΩ. The output capacitor can
be increased, but performance has been optimized for a
1.0µF ceramic output capacitor and does not improve
significantly with the use of a larger capacitor.
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
The MIC5225 will remain stable and in regulation with no
load unlike many other voltage regulators. This is
especially important in CMOS RAM keep-alive
applications.
MIC5225
Thermal Consideration
The MIC5225 is designed to provide 150mA of
continuous current in a very small package. Maximum
power dissipation can be calculated based on the output
current and the voltage drop across the part. To
determine the maximum power dissipation 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)/θ_JA
T_J(MAX) is the maximum junction temperature of the die,
125^°C, and T_A is the ambient operating temperature. θ_JA
is layout dependent; Table 1 shows examples of the
junction-to-ambient thermal resistance for the MIC5225.
Package
SOT-23-5
θ_JA Recommended
Minimum Footprint
235^oC/W
Table 1. SOT-23-5 Thermal Resistance
The actual power dissipation of the regulator circuit can
be determined using the equation:
P_D = (V_IN – V_OUT)I_OUT + V_INI_GND
Substituting P_D(MAX for P_D and solving for the operating
conditions that are critical to the application will give the
maximum operating conditions for the regulator circuit.
For example, when operating the MIC5225-3.0BMM at
50^°C with a minimum footprint layout, the maximum input
voltage for a set output current can be determined as
follows:
P_D(MAX) = (125^oC – 50^oC)/ 235^oC/W
P_D(MAX) = 319mW
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. Using the output voltage of 3.0V, and
an output current of 150mA, the maximum input voltage
can be determined.
319mW = (V_IN – 3.0V)150mA + V_IN × 3.0mA
319mW = V_IN × 153mA – 450mW
769mW = V_IN × 153mA
V_IN(MAX) = 5.02V
Therefore, a 3.0V application at 150mA of output current
can accept a maximum input voltage of 5.02V in the
SOT-23-5 package. For a full discussion of heat sinking
and thermal effects on the voltage regulators, refer to the
Regulator Thermals section of Micrel’s Designing with
Low-Dropout Voltage Regulators handbook:
http://www.onfulfillment.com/estore/pdf_download.asp?
s=2243381&p=18&pdf=842935-iecjdf-bicadii
July 2008
7
M9999-072908-A