MIC5211_05 Datasheet, PDF (8/9 Pages) Micrel Semiconductor – Dual μCap 80mA LDO Regulator

MIC5211
both regulators are dropping the voltage to 3.0V, then output
current will be limited at higher ambient temperatures.
For example, at 70°C ambient the first regulator can supply
3.0V at 50mA output from a 7.2V supply; however, the sec-
ond regulator will have limitations on output current to avoid
thermal shutdown. The dissipation of the first regulator is:
P_D1 = (7.2V – 3V) 50mA + 7.2V · 0.85mA
P_D1 = 216mW
Since maximum power dissipation for the dual regulator is
250mW at 70°C, the second regulator can only dissipate up
to 34mW without going into thermal shutdown. The amount
of current the second regulator can supply is:
P_D2(max) = 34mW
(7.2V – 3V) I_OUT2(max) = 34mW
4.2V • I_OUT2(max) = 34mW
I_OUT2(max) = 8mA
The second regulator can provide up to 8mA output current,
suitable for the keep-alive circuitry often required in hand-
held applications.
Refer to Application Hint 17 for heat sink requirements when
higher power dissipation capability is needed. Refer to Design-
ing with Low Dropout Voltage Regulators for a more thorough
discussion of regulator thermal characteristics.
Dual-Voltage Considerations
For configurations where two different voltages are needed
in the system, the MIC5211 has the option of having two
independent output voltages from the same input. For ex-
ample, a 3.3V rail and a 5.0V rail can be supplied from the
MIC5211 for systems that require both voltages. Important
considerations must be taken to ensure proper functionality
of the part. The input voltage must be high enough for the 5V
section to operate correctly, this will ensure the 3.3V section
proper operation as well.
Micrel, Inc.
Both regulators live off of the same input voltage, therefore
the amount of output current each regulator supplies may
be limited thermally. The maximum power the MIC5211 can
dissipate at room temperature is 455mW, as shown in the
“Thermal Considerations” section. If we assume 6V input
voltage and 50mA of output current for the 3.3V section
of the regulator, then the amount of output current the 5V
section can provide can be calculated based on the power
dissipation.
P_D = (V_GND – V_OUT) I_OUT + V_GND · I_GND
P_D(3.3V) = (6V – 3.3V) 50mA + 6V · 0.85mA
P_D(3.3V) = 140.1mW
P_D(max) = 455mW
P_D(max) – P_D(3.3V) = P_D(5V)
P_D(5V) = 455mW – 140.1mW
P_D(5V) = 314.9mW
Based on the power dissipation allowed for the 5V section, the
amount of output current it can source is easily calculated.
P_D(5V) = 314.9mW
314.9mW = (6V – 5V) I_MAX – 6V · I_GND
(I_GND typically adds less than 5% to the total power dissipa-
tion and in this case can be ignored)
314.9mW = (6V – 5V) I_MAX
I_MAX = 314.9mA
I_MAX exceeds the maximum current rating of the device.
Therefore, for this condition, the MIC5211 can supply 50mA
of output current from each section of the regulator.
MIC5211
8
May 2006