Microchip TC1268 Manual

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Side 1/6

DS00776A-page 1

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

B. CMOS Regulator

IGND = 0

VOUT

VREF

VIN

A. BiPolar Regulator

IGND = IOUT/hFEQ1

–

VIN

FIGURE 2: Dropout mode test circuit.

TC1015 CMOS devices is several orders of magnitude better than

the bipolar LP2981/MIC5205 devices. The TC1015 has a slightly

better output voltage droop in dropout mode than the LP2981 for

all load currents and has slightly better droop performance than the

MIC5205 for load currents up to 60mA. The TC1015 has similar

droop performance compared to the MIC5205 for load currents

between 70mA and 100mA and slightly poorer droop performance

for load currents greater than 100mA. However, the extremely high

power dissipation of the MIC5205 makes it a hazardous liability in

systems where extending battery life is critical. The CMOS archi-

tecture of the TC1015 family tends to be the best fit for these types

of battery powered applications requiring regulators to operate in

the dropout mode.

In battery powered systems requiring lower terminal voltages (such

as digital cameras), LDO regulators must often operate in the

‘dropout’ mode to enhance battery life. The TC1015 series of

CMOS LDOs provide superior performance to bipolar LDOs in

minimizing device power dissipation (through lower ground cur-

rents) when operating in the dropout mode. The TC1015 series has

equivalent if not superior performance to bipolar LDOs in minimiz-

ing output voltage droop (under most load conditions) when

operating in dropout.

–

VIN

GND

SHDN BYPASS

(NC on LP2981)

VOUT

–

Ammeter Connection

for Ground Current Measurement

VIN = VOUT Nominal

1 Fµ

10K

Voltmeter Connection

for VIN Measurement Voltmeter Connection

for VOUT Measurement

1 Fµ

RL (varied from 10mA to 150mA)

OPEN

LDO

D.U.T.

–

Ammeter Connection

for Load Current Measurement

Produkt Specifikationer

Mærke:	Microchip
Kategori:	Ikke kategoriseret
Model:	TC1268

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