MIC39300/39301
Micrel
resistor to drop excessive voltage and distribute the heat
between this resistor and the regulator. The low dropout
properties of Micrel Super
βeta
PNP regulators allow signifi-
cant reductions in regulator power dissipation and the asso-
ciated heat sink without compromising performance. When
this technique is employed, a capacitor of at least 1.0µF is
needed directly between the input and regulator ground.
Refer to
Application Note 9
for further details and examples
on thermal design and heat sink specification.
Output Capacitor
The MIC39300/1 requires an output capacitor to maintain
stability and improve transient response. Proper capacitor
selection is important to ensure proper operation. The
MIC39300/1 output capacitor selection is dependent upon
the ESR (equivalent series resistance) of the output capacitor
to maintain stability. When the output capacitor is 47µF or
greater, the output capacitor should have less than 1Ω of
ESR. This will improve transient response as well as promote
stability. Ultralow ESR capacitors, such as ceramic chip
capacitors may promote instability. These very low ESR
levels may cause an oscillation and/or underdamped tran-
sient response. A low-ESR solid tantalum capacitor works
extremely well and provides good transient response and
stability over temperature. Aluminum electrolytics can also
be used, as long as the ESR of the capacitor is < 1Ω.
The value of the output capacitor can be increased without
limit. Higher capacitance values help to improve transient
response and ripple rejection and reduce output noise.
Input Capacitor
An input capacitor of 1µF or greater is recommended when
the device is more than 4 inches away from the bulk ac supply
capacitance, or when the supply is a battery. Small, surface-
mount, ceramic chip capacitors can be used for the bypass-
ing. Larger values will help to improve ripple rejection by
bypassing the input to the regulator, further improving the
integrity of the output voltage.
Transient Response and 3.3V to 2.5V Conversion
The MIC39300/1 has excellent transient response to varia-
tions in input voltage and load current. The device has been
designed to respond quickly to load current variations and
input voltage variations. Large output capacitors are not
required to obtain this performance. A standard 47µF output
capacitor, preferably tantalum, is all that is required. Larger
values help to improve performance even further.
By virtue of its low-dropout voltage, this device does not
saturate into dropout as readily as similar NPN-based de-
signs. When converting from 3.3V to 2.5V, the NPN-based
regulators are already operating in dropout, with typical
dropout requirements of 1.2V or greater. To convert down to
2.5V without operating in dropout, NPN-based regulators
require an input voltage of 3.7V at the very least. The
MIC39300/1 regulator will provide excellent performance
with an input as low as 3.0V. This gives the PNP-based
regulators a distinct advantage over older, NPN-based linear
regulators.
Applications Information
The MIC39300/1 is a high-performance low-dropout voltage
regulator suitable for moderate to high-current voltage regu-
lator applications. Its 500mV dropout voltage at full load
makes it especially valuable in battery-powered systems and
as a high-efficiency noise filter in post-regulator applications.
Unlike older NPN-pass transistor designs, where the mini-
mum dropout voltage is limited by the base-to-emitter voltage
drop and collector-to-emitter saturation voltage, dropout per-
formance of the PNP output of these devices is limited only
by the low V
CE
saturation voltage.
A trade-off for the low dropout voltage is a varying base drive
requirement. Micrel’s Super
βeta
PNP™ process reduces
this drive requirement to only 2% to 5% of the load current.
The MIC39300/1 regulator is fully protected from damage
due to fault conditions. Current limiting is provided. This
limiting is linear; output current during overload conditions is
constant. Thermal shutdown disables the device when the
die temperature exceeds the maximum safe operating tem-
perature. Transient protection allows device (and load) sur-
vival 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.
V
IN
MIC39300-x.x
IN
C
IN
OUT
GND
C
OUT
V
OUT
Figure 1. Capacitor Requirements
Thermal Design
Linear regulators are simple to use. The most complicated
design parameters to consider are thermal characteristics.
Thermal design requires four application-specific param-
eters:
• Maximum ambient temperature (T
A
)
• Output Current (I
OUT
)
• Output Voltage (V
OUT
)
• Input Voltage (V
IN
)
• Ground Current (I
GND
)
Calculate the power dissipation of the regulator from these
numbers and the device parameters from this datasheet,
where the ground current is taken from the data sheet.
P
D
= (V
IN
– V
OUT
) I
OUT
+ V
IN
· I
GND
The heat sink thermal resistance is determined by:
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
MIC39300/39301
8
March 2000
MICREL [ MICREL SEMICONDUCTOR ]
