MIC3975
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 MIC3975 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 MIC3975 features 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 or
1.65V Conversion
The MIC3975 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, 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 or 1.65V,
the NPN based regulators are already operating in dropout,
with typical 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 MIC3975 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 MIC3975 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.
Applications Information
The MIC3975 is a high-performance low-dropout voltage regu-
lator suitable for moderate to high-current voltage regulator
applications. Its 500mV dropout voltage at full load and over-
temperature makes it 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-to-emit-
ter voltage drop and collector-to-emitter saturation 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 MIC3975 regulator is fully protected from damage due to
fault conditions. Linear current limiting is provided. Output cur-
rent during 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 volt-
age 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
MIC3975x.x
IN
OUT
GND
V
OUT
C
IN
C
OUT
Figure 1. Capacitor Requirements
Output Capacitor
The MIC3975 requires an output capacitor for stable opera-
tion. As a
µCap
LDO, the MIC3975 can operate with ceramic
output capacitors as long as the amount of capacitance is
10µF or greater. For values of output capacitance lower than
10µF, the recommended ESR range is 200mΩ to 2Ω. The
minimum value of output capacitance recommended for the
MIC3975 is 4.7µF.
For 10µF or greater the ESR range recommended is less than
1Ω. Ultra-low ESR ceramic capacitors are recommended
for output capacitance of 10µF or greater to help improve
transient response and noise reduction at high frequency.
X7R/X5R dielectric-type ceramic capacitors are recom-
mended because of their temperature performance. X7R-type
capacitors change capacitance by 15% over their operating
temperature range and are the most stable type of ceramic
capacitors. Z5U and Y5V dielectric capacitors change value
by as much as 50% and 60% respectively over their operat-
ing temperature ranges. To use a ceramic chip capacitor with
Y5V dielectric, the value must be much higher than an X7R
ceramic capacitor to ensure the same minimum capacitance
over the equivalent operating temperature range.
February 2005
9
MIC3975
MICREL [ MICREL SEMICONDUCTOR ]
