TPS78327

GND

EN

IN

OUT

V

IN

V

OUT

1 F

m

1 F

m

4.2Vto5.5V

2.7V

On

Off

90

80

70

60

50

40

30

20

10

0

5 10

20

30

40

50

60

70

80

90

100

DutyCycle(TimeinActiveMode)(%)

Battery,V

=3.0V

TPS783,V

=2.2V

CC

CC

TPS783xx

www.ti.com

SBVS133 – FEBRUARY 2010

APPLICATION INFORMATION

X5R- and X7R-type capacitors are best because they

APPLICATION EXAMPLES

have minimal variation in value and ESR over

temperature. Maximum ESR should be less than

The TPS783 family of LDOs is factory-programmable

1.0

Ω. With tolerance and dc bias effects, the

to have a fixed output. Note that during startup or

minimum capacitance to ensure stability is 1mF.

steady-state conditions, it is important that the EN pin

EXTENDING BATTERY LIFE IN KEEP-ALIVE

CIRCUITRY APPLICATIONS FOR MSP430

AND OTHER LOW-POWER

MICROCONTROLLERS

One of the primary advantages of a low quiescent

current LDO is its extremely low energy requirement.

Counter-intuitively, this requirement enables a longer

battery life compared to using only the battery as an

unregulated

voltage

supply

for

low-power

microcontrollers such as the MSP430. Figure 19

illustrates

the

characteristic

performance

of

an

unregulated (3.0V) battery supply versus a regulated

TPS783 supply for a typical MSP430 application.

Figure 18. Typical Application Circuit

INPUT AND OUTPUT CAPACITOR

REQUIREMENTS

Although an input capacitor is not required for

stability, it is good analog design practice to connect

a 0.1mF to 1.0mF low equivalent series resistance

(ESR) capacitor across the input supply near the

regulator. This capacitor counteracts reactive input

sources and improves transient response, noise

rejection,

and

ripple

rejection.

A

higher-value

capacitor may be necessary if large, fast rise-time

load transients are anticipated, or if the device is not

located near the power source. If source impedance

is not sufficiently low, a 0.1mF input capacitor may be

necessary to ensure stability.

Calculated with an MSP430F model, operating at 6MHz.

The TPS783 series are designed to be stable with

Figure 19. Battery Life Comparison vs Duty Cycle

standard ceramic capacitors with values of 1.0mF or

for MSP430 Application

larger at the output.

Table 2 summarizes this comparison.

Table 2. Battery Life Comparison vs Active Mode Time for MSP430 Application

DUTY CYCLE

TPS783xx

BATTERY

1mA LDO

CONDITION/PERFORMANCE

(%)

(NO. OF DAYS)

(NO. OF DAYS)

(NO. OF DAYS)

—

73%

100%

73%

—

0.5mA

0

1mA

MSP430 active current

—

2.19mA

3.09mA

2.19mA

MSP430 low-power current

—

0.5mA

0.6mA

0.5mA

Active mode, 1 sec/hour

0.028

5742

6286

4373

Active mode, 10 sec/hour

0.28

1320

998

1085

Active mode, 100 sec/hour

2.8

151

106

148

Active mode, 1000 sec/hour

28

15.4

10.7

15.4

Active mode, 100% duty cycle (on all the time)

100

4.2

3.0

4.2

Copyright © 2010, Texas Instruments Incorporated

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