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 microam-
peres 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
saturate into dropout as readily as similar NPN-based de-
signs. 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 respec-
tively. This gives the PNP based regulators a distinct advan-
tage 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
regulator suitable for moderate to high-current voltage regu-
lator applications. Its 500mV dropout voltage at full load and
overtemperature makes it especially valuable in battery-
powered systems and as high-efficiency noise filters in post-
regulator applications. Unlike older NPN-pass transistor de-
signs, 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 MIC3975 regulator is fully protected from damage due to
fault conditions. Linear current limiting is provided. Output
current during overload conditions is constant. Thermal shut-
down disables the device when the die temperature exceeds
the maximum safe operating temperature. Transient protec-
tion allows device (and load) survival even when the input
voltage spikes above and below nominal. The output struc-
ture of these regulators allows voltages in excess of the
desired output voltage to be applied without reverse current
flow.
V
IN
MIC3975x.x
IN
C
IN
OUT
GND
C
OUT
V
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 tran-
sient 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 oper-
ating 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
operating temperature ranges. To use a ceramic chip capaci-
tor with Y5V dielectric, the value must be much higher than an
X7R ceramic capacitor to ensure the same minimum capaci-
tance over the equivalent operating temperature range.
February 2003
9
MIC3975
MICREL [ MICREL SEMICONDUCTOR ]
