MIC5231_08 Datasheet, PDF (6/7 Pages) Micrel Semiconductor – Micropower μCap LDO Regulator

Micrel, Inc.
Application Information
Input Capacitor
A 0.1µF (or larger) capacitor should be placed from the
IN (supply input) to GND (ground) if there is more than
20cm of wire between IN and the ac filter capacitor or if
supplied from a battery.
Output Capacitors
The MIC5231 does not require an output capacitor for
stability. A 1µF or larger capacitor is recommended
between OUT (output) and GND to improve the
regulator’s transient response. A 0.1µF capacitor can be
used to reduce overshoot recovery time at the expense
of overshoot amplitude. The ESR (effective series
resistance) of this capacitor has no effect on regulator
stability, but low-ESR capacitors improve high frequency
transient response. The value of this capacitor may be
increased without limit, but values larger than 10µF tend
to increase the settling time after a step change in input
voltage or output current.
The MIC5231 has no minimum load current; it will
remain stable and in regulation with no load (other than
the internal voltage divider). This is especially important
in real-time clock and CMOS RAM keep-alive
applications.
Minimum Load Current
The MIC5231 does not require a minimum load for
proper operation. This allows the device to operate in
applications where very light output currents are required
for keep-alive purposes. This is important for powering
SRAM or Flash memory in low-power modes for
handheld devices.
Safe Operating Conditions
The MIC5231 does not incorporate current limit or
thermal shutdown in the design. The output pass
element is approximately 15Ω, therefore, when a short
occurs from the output to ground, the current is self-
limited. The pass element has a positive temperature
coefficient, such that when the device gets hot, the
output impedance goes up, limiting the current even
further. The maximum junction temperature for the
device is +125°C, and it is important that this is not
exceeded for any length of time.
MIC5231
Thermal Considerations
The MIC5231 is not intended for sourcing currents that
would cause a large power loss in the device, but since it
is not current limited, it is possible to source more than
the rated 10mA. At this point, it is important to ensure
that the die temperature does not exceed +125°C.
Power dissipation in the regulator is calculated as
follows:
( ) P_D = V_IN − V_OUT I_OUT + V_IN ⋅ I_GND
The MIC5231 consumes only 0.65µA over load and
does not need to consider that contribution in the power
dissipation equation, therefore the equation is simplified.
( ) P_D = V_IN − V_OUT I_OUT
The MIC5231, in the IttyBitty SOT-23-5 package, has a
thermal resistance, junction-to-ambient, of 235°C/W.
Using this number, the power dissipation capability of
that package, without exceeding a +125°C junction
temperature rating, can easily be calculated.
P_D(MAX)
=
T_J(MAX) − T_A
θ _JA
P_D(MAX)
=
125°C − T_A
235°C/W
If the device is being operated at +85°C, the maximum
power dissipation allowed can easily be determined.
P_D(MAX)
=
125°C − 85°C
235°C/W
P_D(MAX) = 170mW
Therefore, the device can only dissipate 170mW
maximum. If the MIC5231 is powered off of a 12V
source and the output voltage is 3.3V, the maximum
output current can be calculated.
( ) P_D(MAX) = V_IN − V_OUT I_OUT
170mW = (12V − 3.3V )I_OUT
I_OUT
=
170mW
8.7V
I_OUT = 19.5mA
Therefore, the device can source almost 20mA at an
ambient of +85°C before the die temperature exceeds
+125°C.
November 2008
6
M9999-110408-B