LT1304/LT1304-3.3/LT1304-5
U
OPERATIO
1000
100
10
VLBO
2V/DIV
VLBI
200mV/DIV
1304 F08
1
Figure 8. Low-Battery Detector Transfer Function.
Pull-Up R = 22k, VIN = 2V, Sweep Frequency = 10Hz
1
10
100 200
LOAD CURRENT (mA)
1304 F10
Figure 10. Battery Life vs Load Current. Dots Specify
Actual Measurements
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,about0.2Ωfreshand0.5Ωend-of-life,results
insignificantefficiencylossathighdischargerates.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:
6
5
4
3
2
1
0
1
10
100 200
LOAD CURRENT (mA)
1304 F11
WH = ILOAD(5V)(H)
Figure 11. Output Watt Hours vs Load Current. Note
Rapid Fall-Off at Higher Discharge Rates
L1
22µH
D1
V
SW
IN
V
OUT
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.
SHDN
SENSE
5V
200mA
B1
LT1304-5
2 CELLS
+
C2
100µF
LB1
LB0
GND
+
C1
100µF
I
LIM
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
9
Linear [ Linear ]
