MIC29150 Datasheet, PDF (14/20 Pages) Micrel Semiconductor – High-Current Low-Dropout Regulators

MIC29150/29300/29500/29750
Applications Information
The MIC29150/29300/29500/29750 are high performance
low-dropout voltage regulators suitable for all moderate to
high-current voltage regulator applications. Their 300mV to
400mV dropout voltage at full load make them especially
valuable in battery powered systems and as high efficiency
noise filters in “post-regulator” 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
V_CE saturation voltage.
A trade-off for the low dropout voltage is a varying base drive
requirement. But Micrel’s Super ßeta PNP™ process re-
duces this drive requirement to merely 1% of the load current.
The MIC29150–29750 family of regulators 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 de-
vice when the die temperature exceeds the 125°C maximum
safe operating temperature. Transient protection allows de-
vice (and load) survival even when the input voltage spikes
between –20V and +60V. When the input voltage exceeds
about 35V to 40V, the overvoltage sensor temporarily dis-
ables the regulator. The output structure of these regulators
allows voltages in excess of the desired output voltage to be
applied without reverse current flow. MIC29xx1 and MIC29xx2
versions offer a logic level ON/OFF control: when disabled,
the devices draw nearly zero current.
An additional feature of this regulator family is a common
pinout: a design’s current requirement may change up or
down yet use the same board layout, as all of these regulators
have identical pinouts.
V_IN
IN
OUT
V_OUT
GND
Figure 3. Linear regulators require only two capacitors
for operation.
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
Micrel
First, we calculate the power dissipation of the regulator from
these numbers and the device parameters from this datasheet.
( ) ^P_D ^{= I}_OUT ^1.01V_IN ^{− V}_OUT
Where the ground current is approximated by 1% of I_OUT.
Then the heat sink thermal resistance is determined with this
formula:
( ) ^θ_SA
=
T_{J MAX} − T_A
P_D
−
θ_JC + θ_CS
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
0.1µF is needed directly between the input and regulator
ground.
Please refer to Application Note 9 and Application Hint 17 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. MIC29150—29750 regulators are stable
with the following minimum capacitor values at full load:
Device
Full Load Capacitor
MIC29150 .............................. 10µF
MIC29300 .............................. 10µF
MIC29500 .............................. 10µF
MIC29750 .............................. 22µF
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 ca-
pacitors are recommended for systems where fast load
transient response is important.
Where the regulator is powered from a source with a high AC
impedance, a 0.1µF capacitor connected between Input and
GND is recommended. This capacitor should have good
characteristics to above 250kHz.
Minimum Load Current
The MIC29150–29750 regulators are specified between fi-
nite loads. If the output current is too small, leakage currents
dominate and the output voltage rises. The following mini-
mum load current swamps any expected leakage current
across the operating temperature range:
MIC29150/29300/29500/29750
14
March 2000