Micrel, Inc.
MIC29311
First, calculate the power dissipation of the regulator
from these numbers and the device parameters from this
datasheet.
P
D
= I
OUT
(1.02V
IN
– V
OUT
)
Where the ground current is approximated by 2% of I
OUT
.
Then the heat sink thermal resistance is determined by:
θ
SA
=
T
J(max)
−
T
A
P
D
−
θ
JC
+
θ
CS
Application Information
The MIC29311 is a high-performance low-dropout
voltage regulator suitable for all moderate to high-current
voltage regulator applications. The 600mV dropout
voltage at full load makes it especially valuable in
battery-powered systems and as high-efficiency noise
filters in “post-regulator” applications. Its unique output
voltage makes the MIC39311 ideal for Universal Serial
Bus (USB) power switching applications. Unlike older
NPN-pass transistor designs, where the minimum
dropout voltage is limited by the base-emitter voltage
drop and collector-emitter saturation voltage, dropout
performance of the PNP output of these devices is
limited merely by the low VCE saturation voltage.
A trade-off for the low dropout voltage is a varying base
drive requirement. But Micrel’s Super
βeta
PNP™
process reduces this drive requirement to merely 1% to
5% of the load current.
The MIC29311 regulator is fully protected from damage
due to fault conditions. Current limiting is provided. This
limiting is linear; output current under overload
conditions is constant. Thermal shutdown disables the
device when the die temperature exceeds the maximum
safe operating temperature. Transient protection allows
device (and load) survival even when the input voltage
spikes above and below nominal. The output structure of
these regulators allows voltages in excess of the desired
output voltage to be applied without reverse current flow.
The MIC29311 version offers a logic level on-off control:
when disabled, the device draws nearly zero current.
Where T
J(max)
≤
125°C and
θ
CS
is between 0 and 2°C/W.
The heat sink may be significantly reduced in
applications where the minimum input voltage is known
and is large compared with the dropout voltage. Use a
series input resistor to drop excessive voltage and
distribute the heat between this resistor and the
regulator. The low dropout properties of Micrel Super
ßeta PNP regulators allow very significant reductions in
regulator power dissipation and the associated heat sink
without compromising performance. When this technique
is employed, a capacitor of at least 1µF is needed
directly between the input and regulator ground.
Please refer to Application Note 9 for further details and
examples on thermal design and heat sink specification.
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 MIC29311
regulator is stable with a minimum capacitor value of
10µF 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.
Where the regulator is powered from a source with high
AC impedance, a 0.1µF capacitor connected between
Input and GND is recommended. This capacitor should
have good characteristics to above 250kHz. When the
regulator is located more than 3 inches from the ac bulk
supply capacitors, a 1µF or greater input capacitor is
recommended.
Minimum Load Current
The MIC29311 regulator is specified between finite
loads. If the output current is too small, leakage currents
dominate and the output voltage rises. A 10mA minimum
load current is necessary for proper regulation.
Figure 1. Input and Output Capacitors
Thermal Design
Linear regulators are simple to use. The most
complicated design parameters to consider are thermal
characteristics. Thermal design requires the following
application-specific parameters:
Maximum ambient temperature, T
A
Output Current, I
OUT
Output Voltage, V
OUT
Input Voltage, V
IN
September 2007
8
M9999-090507
MICREL [ MICREL SEMICONDUCTOR ]
