MIC2846A-MFYMT TR【PDF 16/19 页】IC LED DRIVR WHITE BCKLGT 14TMLF

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? ?

P TOTAL(max) = ? ?

Micrel Inc.

LDO

MIC2846A LDOs are low noise 150mA LDOs. The

MIC2846A LDO regulator is fully protected from damage

due to fault conditions, offering linear current limiting and

thermal shutdown.

Input Capacitor

The MIC2846A 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 MIC2846A 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 MIC2846A

LDOs will remain stable and in regulation with no load.

Thermal Considerations

The MIC2846A LDOs are each designed to provide

150mA of continuous current. Maximum ambient

operating temperature can be calculated based on the

MIC2846A

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) × 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:

? T J(max) ? T A ?

? θ JA ?

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 MIC2846A 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) × 150mA = 0.120W

P LDO2 = (3.6V – 1.5V) × 150mA = 0.315W

P TOTAL =0.120W+ 0.315W = 0.435W

= (125°C – T A )/(60°C/W)

T A = 125°C – 0.435W × 60°C/W

T A = 98.9°C

Therefore, under the above conditions, the maximum

ambient operating temperature of 98.9°C is allowed.

April 2010

16

M9999-041210-D

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