Microchip TC1268 Manual

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Side 1/6
DS00776A-page 1
© 2002 Microchip Technology, Inc.
DC Performance Comparisons of CMOS vs. Bipolar LDOs when
Operating in "Dropout" (VIN = Nominal VOUT) Mode
More and more, battery operated systems are requiring lower
terminal voltages to power internal circuits. Multi-cell designs are
rapidly migrating to single-cell architectures to reduce system cost.
A prime example of this system type is digital cameras, which often
use a single-cell 3.6V Li-Ion battery for their power source. Digital
cameras contain high-speed memory ICs, which require tight
voltage regulation at moderate loads to meet the required timing
parameters of the system. Precision low dropout (LDO) regulator
devices can be used to meet these requirements but in doing so,
the LDO regulators must be able to successfully operate in the
‘dropout’ mode as the battery discharges. Dropout mode is entered
when the input voltage (from the battery source) is equal to the
“nominal output voltage” of the LDO; for example a 3.3V LDO
enters dropout mode when its input voltage at the V
IN pin is equal
to 3.3V. Minimal output voltage droop and minimal LDO power
dissipation are critical to meeting various system performance
parameters and extending the life of the battery.
This application note compares the performance of Microchip
Technology’s TC1015 CMOS family of LDOs to two of its bipolar
counterparts, the National Semiconductor LP2981 and the Micrel
MIC5205. Dropout measurements were taken on three different
popular output voltage options (5.0V, 3.3V, and 3.0V) under
varying load conditions ranging from 10mA to 150mA. All measure-
ments were made at ambient temperature (T
A = +25°C).
Author: Patrick Maresca,
Microchip Technology, Inc.
FIGURE 1: Bipolar vs. CMOS LDO regulator schematics.
Figures 1A and 1B compare the block diagram for a common
bipolar regulator with that of an equivalent regulator fabricated in
CMOS. The supply current to the bipolar device is composed of the
bias current, plus a “ground current” (I
GND) component shown in
Figure 1A, which is a fraction of the output current (determined by
the h
FE of the pass transistor) sunk through the output stage of the
error amplifier. The “ground current” component of the CMOS
regulator shown in Figure 1B is virtually zero, due to the extremely
large drain-to-gate impedance of the CMOS pass transistor.
The circuit shown in Figure 2 was used to measure output voltage
droop and device ground current with loads ranging from 10mA to
100mA (in 10mA increments), 125mA, and 150mA. Both the
TC1015 and the MIC5205 have optional reference bypass capaci-
tor connections from pin four to ground. Measurements were made
with and without a 470pF bypass capacitor on both of these devices
but the output voltage droop and ground current did not vary much
with the bypass capacitor connected (only the data taken without
a bypass capacitor is shown in this application note).
Tables I, II, and III show the performance of the TC1015, LP2981,
and MIC5205 for dropout mode operation. Table I contains the data
taken for 5.0V LDOs, Table II contains the data taken for 3.3V
LDOs, and Table III contains the data taken for 3V LDOs. Notice
that in each case, the ground current and power dissipation for the
+
–
VOUT
VREF
VIN
V
IN
B. CMOS Regulator
IGND = 0
~
+
VOUT
VREF
VIN
A. BiPolar Regulator
IGND = IOUT/hFEQ1
~
Q1
+
–
+
–
–
VIN

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Mærke: Microchip
Kategori: Ikke kategoriseret
Model: TC1268

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