LT1304CS8-5 View Datasheet(PDF) - Linear Technology

Part Name

Description

MFG CO.

LT1304CS8-5

Micropower DC/DC Converters with Low-Battery Detector Active in Shutdown

Linear Technology 

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U

OPERATIO

LT1304/LT1304-3.3/LT1304-5

1000

VLBO

100

2V/ DIV

VLBI

200mV/DIV

1304 F08

Figure 8. Low-Battery Detector Transfer Function.

Pull-Up R = 22k, VIN = 2V, Sweep Frequency = 10Hz

Battery Life

How may hours does it work? This is the bottom line

question that must be asked of any efficiency study. AA

alkaline cells are not perfect power sources. For efficient

power transfer, energy must be taken from AA cells at a

rate that does not induce excessive loss. AA cells internal

impedance, about 0.2Ω fresh and 0.5Ω end-of-life, results

in significant efficiency loss at high discharge rates. Figure

10 illustrates battery life vs load current of Figure 9’s

LT1304, 2-cell to 5V DC/DC converter. Note the acceler-

ated decrease in hours at higher power levels. Figure 11

plots total watt hours vs load current. Watt hours are

determined by the following formula:

WH = ILOAD(5V)(H)

L1

22µH

D1

10

1

1

10

100 200

LOAD CURRENT (mA)

1304 F10

Figure 10. Battery Life vs Load Current. Dots Specify

Actual Measurements

6

5

4

3

2

1

0

1

10

100 200

LOAD CURRENT (mA)

1304 F11

Figure 11. Output Watt Hours vs Load Current. Note

Rapid Fall-Off at Higher Discharge Rates

B1

2 CELLS

+

VIN

SHDN

SW

SENSE

LT1304-5

C1

100µF

LB1

ILIM

LB0

GND

VOUT

5V

200mA

+ C2

100µF

B1 = 2× EVEREADY INDUSTRIAL

ALKALINE AA CELLS #EN91

C1, C2 = AVX TPSD107M010R0100

D1 = MOTOROLA MBRS130L

L1 = SUMIDA CD54-220

1304 F09

Figure 9. 2-Cell to 5V Converter Used in Battery Life Study

Figure 11’s graph varies significantly from electrical effi-

ciency plot pictured on the first page of this data sheet.

Why? As more current is drawn from the battery, voltage

drop across the cells’ internal impedance increases. This

causes internal power loss (heating), reducing cell termi-

nal voltage. Since the regulator input acts as a negative

resistance, more current is drawn from the battery as the

terminal voltage decreases. This positive feedback action

compounds the problem.

9

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