MIC39100_05 Datasheet, PDF (9/12 Pages) Micrel Semiconductor – 1A Low-Voltage Low-Dropout Regulator

MIC39100/39101/39102
Applications Information
The MIC39100/1/2 is a high-performance low-dropout volt-
age regulator suitable for moderate to high-current voltage
regulator applications. Its 630mV dropout voltage at full load
and overtemperature makes it especially valuable in bat-
tery-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-to-emitter voltage drop and collector-to-emitter satura-
tion voltage, dropout performance 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% of the load current.
The MIC39100/1/2 regulator is fully protected from damage
due to fault conditions. Linear current limiting is provided.
Output current during overload conditions is constant. Ther-
mal 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.
V_IN
MIC39100-x.x
V_OUT
IN OUT
GND
C _IN
C_OUT
Figure 1. Capacitor Requirements
Output Capacitor
The MIC39100/1/2 requires an output capacitor to maintain
stability and improve transient response. Proper capaci-
tor selection is important to ensure proper operation. The
MIC39100/1/2 output capacitor selection is dependent upon
the ESR (equivalent series resistance) of the output capacitor
to maintain stability. When the output capacitor is 10µF or
greater, the output capacitor should have an ESR less than
2Ω. This will improve transient response as well as promote
stability. Ultra-low-ESR capacitors (<100mΩ), such as ceramic
chip capacitors, may promote instability. These very low ESR
levels may cause an oscillation and/or underdamped transient
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 <2Ω.
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.
Micrel
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 bypassing.
Larger values will help to improve ripple rejection by bypass-
ing the input to the regulator, further improving the integrity
of the output voltage.
Error Flag
The MIC39101 features an error flag (FLG), which monitors
the output voltage and signals an error condition when this
voltage drops 5% below its expected value. The error flag is
an open-collector output that pulls low under fault conditions
and may sink up to 10mA. Low output voltage signifies a
number of possible problems, including an overcurrent fault
(the device is in current limit) or low input voltage. The flag
output is inoperative during overtemperature conditions. A
pull-up resistor from FLG to either V_IN or V_OUT is required
for proper operation. For information regarding the minimum
and maximum values of pull-up resistance, refer to the graph
in the typical characteristics section of the data sheet.
Enable Input
The MIC39101 and MIC39102 versions feature an active-high
enable input (EN) that allows on-off control of the regulator.
Current drain reduces to “zero” when the device is shutdown,
with only microamperes of leakage current. The EN input has
TTL/CMOS compatible thresholds for simple logic interfacing.
EN may be directly tied to V_IN and pulled up to the maximum
supply voltage
Transient Response and 3.3V to 2.5V or 2.5V to 1.8V
Conversion
The MIC39100/1/2 has excellent transient response to
variations 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 10µF 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 satu-
rate into dropout as readily as similar NPN-based designs.
When converting from 3.3V to 2.5V or 2.5V to 1.8V, the NPN
based regulators are already operating in dropout, with typi-
cal dropout requirements of 1.2V or greater. To convert down
to 2.5V or 1.8V without operating in dropout, NPN-based
regulators require an input voltage of 3.7V at the very least.
The MIC39100 regulator will provide excellent performance
with an input as low as 3.0V or 2.5V respectively. This gives
the PNP based regulators a distinct advantage over older,
NPN based linear regulators.
Minimum Load Current
The MIC39100/1/2 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.
August 2005
9
M9999-082505-B