Datasheet

9
LT1304/LT1304-3.3/LT1304-5
OPERATIO
U
V
LBO
2V/DIV
V
LBI
200mV/DIV 1304 F08
LOAD CURRENT (mA)
HOURS (H)
1 100 200
1304 F10
10
1000
100
10
1
Figure 10. Battery Life vs Load Current. Dots Specify
Actual Measurements
LOAD CURRENT (mA)
1
WATT HOURS (WH)
6
5
4
3
2
1
0
10 100
1304 F11
200
Figure 11. Output Watt Hours vs Load Current. Note
Rapid Fall-Off at Higher Discharge Rates
Figure 8. Low-Battery Detector Transfer Function.
Pull-Up R = 22k, V
IN
= 2V, Sweep Frequency = 10Hz
V
IN
SW
GND
I
LIM
SENSESHDN
LB0LB1
LT1304-5
C2
100µF
C1
100µF
B1
2 CELLS
L1
22µH
D1
V
OUT

5V
200mA
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
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 = I
LOAD
(5V)(H)
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.