Micrel, Inc.
Next, calcualte the junction temperature for the expected
power dissipation:
T
J
= (θ
JA
×P
D
) + T
A
= (35°C/W × 2.3472W)
+ 75°C = 157.15°C
Now determine the maximum power dissipation allowed
that would not exceed the IC’s maximum junction
temperature (125°C) without the use of a heat sink by:
P
D(MAX)
= (T
J(MAX)
– T
A
) /
θ
JA
= (125°C – 75°C) / (35°C/W)
= 1.428W
Capacitor Requirements
For stability and minimum output noise, a capacitor on
the regulator output is necessary. The value of this
capacitor is dependent upon the output current; lower
currents allow smaller capacitors. The MIC29302A is
stable with a 10μF capacitor at full load.
This capacitor need not be an expensive low-ESR type;
aluminum electrolytics are adequate. In fact, extremely
low-ESR capacitors may contribute to instability.
Tantalum capacitors are recommended for systems
where fast load transient response is important.
When the regulator is powered from a source with high
AC impedance, a 0.1µF capacitor connected between
input and GND is recommended.
Transient Response and 5V to 3.3V Conversion
The MIC29302A has excellent response to variations in
input voltage and load current. By virtue of its low
dropout voltage, the device does not saturate into
dropout as readily as similar NPN-based designs. A 3.3V
output Micrel LDO will maintain full speed and
performance with an input supply as low as 4.2V, and
will still provide some regulation with supplies down to
3.8V, unlike NPN devices that require 5.1V or more for
good performance and become nothing more than a
resistor under 4.6V of input. Micrel’s PNP regulators
provide superior performance in “5V to 3.3V” conversion
applications than NPN regulators, especially when all
tolerances are considered.
Minimum Load Current
The MIC29302A regulator operates within a specified
load range. If the output current is too small, leakage
currents dominate and the output voltage rises.
A minimum load current of 10mA is necessary for proper
regulation and to swamp any expected leakage current
across the operating temperature range.
MIC29302A
For best performance the total resistance (R1+R2)
should be small enough to pass the minimum regulator
load current of 10mA.
Adjustable Regulator Design
The output voltage can be programmed anywhere
between 1.25V and the 15V. Two resistors are used.
The resistor values are calculated by:
⎛
V
⎞
R
1
=
R
2
× ⎜
OUT
−
1
⎟
⎜
1.240
⎟
⎝
⎠
where V
OUT
is the desired output voltage.
Figure 4 shows component definition. Applications with
widely varying load currents may scale the resistors to
draw the minimum load current required for proper
operation (see “Minimum Load Current” section).
Figure 4. Adjustable Regulator with Resistors
Enable Input
MIC29302A features an enable (EN) input that allows
ON/OFF control of the device. Special design allows
“zero” current drain when the device is disabled—only
microamperes of leakage current flows. The EN input
has TTL/CMOS compatible thresholds for simple
interfacing with logic, or may be directly tied to V
IN
.
Enabling the regulator requires approximately 20µA of
current into the EN pin.
August 2011
11
M9999-080411-A
MICREL [ MICREL SEMICONDUCTOR ]
