MIC2845 Datasheet, PDF (17/21 Pages) Micrel Semiconductor – 6 Channel High Side Current Source WLED driver

Micrel Inc.
LDO
MIC2845/6 LDOs are low noise 150mA LDOs. The
MIC2845/6 LDO regulator is fully protected from damage
due to fault conditions, offering linear current limiting and
thermal shutdown.
Input Capacitor
The MIC2845/6 LDOs are high-performance, high
bandwidth devices. Stability can be maintained using a
ceramic input capacitor of 1µF. Low-ESR ceramic
capacitors provide optimal performance at a minimum
amount 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 noise sensitive 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 MIC2845/6 LDOs require an output capacitor of at
least 1µF or greater to maintain stability, however, the
output capacitor can be increased to 2.2µF to reduce
output noise without increasing package size. The
design is optimized for use with low-ESR ceramic chip
capacitors. High ESR capacitors are not recommended
because they may cause high frequency oscillation.
X7R/X5R dielectric-type ceramic capacitors are
recommended due to their improved temperature
performance compared to Z5U and Y5V capacitors.
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 MIC2845/6
LDOs will remain stable and in regulation with no load.
This is especially important in CMOS RAM keep-alive
applications.
MIC2845/46
Thermal Considerations
The MIC2845/6 LDOs are each designed to provide
150mA of continuous current. 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
2.8V, and the output current = 150mA. The actual power
dissipation of the regulator circuit can be determined
using the equation:
P_LDO1 = (V_IN – V_OUT1) I _OUT + V_IN I_GND
Because this device is CMOS and the ground current
(I_GND) 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_LDO1 = (3.6V – 2.8V) x 150mA
P_LDO1 = 0.120W
Since there are two LDOs in the same package, the
power dissipation must be calculated individually and
then summed together to arrive at the total power
dissipation.
P_TOTAL = P_LDO1 + P_LDO2
To determine the maximum ambient operating
temperature of the package, use the junction-to-ambient
thermal resistance (θ_JA = 60°C/W) of the device and the
following basic equation:
P_TOTAL(max)
=
⎜⎜⎝⎛
T_J(max) −
θ _JA
T_A
⎟⎟⎠⎞
T_J(max) = 125°C, is the maximum junction temperature of
the die and θ_JA, is the thermal resistance = 60°C/W.
Substituting P_TOTAL for P_TOTAL(max) and solving for the
ambient operating temperature will give the maximum
operating conditions for the regulator circuit.
For example, when operating the MIC2845/6 LDOs
(LDO1 = 2.8V and LDO2 = 1.5V) at an input voltage of
3.6V with 150mA load on each, the maximum ambient
operating temperature T_A can be determined as follows:
P_LDO1 = (3.6V – 2.8V) x 150mA = 0.120W
P_LDO2 = (3.6V – 1.5V) x 150mA = 0.315W
P_TOTAL=0.120W+ 0.315W = 0.435W
= (125°C – T_A)/(60°C/W)
T_A = 125°C – 0.435W x 60°C/W
T_A = 98.9°C
Therefore, under the above conditions, the maximum
ambient operating temperature of 98.9°C is allowed.
January 2008
17
M9999-010808-A