LT1304-5 LINER [Linear Technology], LT1304-5 Datasheet - Page 9

• Manufacturer Part Number: LT1304-5
• Description: Micropower DC/DC Converters with Low-Battery Detector Active in Shutdown
• Manufacturer: LINER [Linear Technology]

OPERATIO
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 0.2 fresh and 0.5 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:
Figure 9. 2-Cell to 5V Converter Used in Battery Life Study
WH = I
B1 = 2 EVEREADY INDUSTRIAL
C1, C2 = AVX TPSD107M010R0100
D1 = MOTOROLA MBRS130L
L1 = SUMIDA CD54-220
2V/DIV
Figure 8. Low-Battery Detector Transfer Function.
Pull-Up R = 22k, V
B1
2 CELLS
V
ALKALINE AA CELLS #EN91
LBO
LOAD
+
(5V)(H)
C1
100 F
U
SHDN
LB1
IN
I
V
LIM
200mV/DIV
= 2V, Sweep Frequency = 10Hz
IN
LT1304-5
V
22 H
LBI
L1
SENSE
GND
SW
LB0
D1
+
1304 F08
C2
100 F
V
5V
200mA
OUT
1304 F09
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.
Figure 10. Battery Life vs Load Current. Dots Specify
Actual Measurements
Figure 11. Output Watt Hours vs Load Current. Note
Rapid Fall-Off at Higher Discharge Rates
LT1304/LT1304-3.3/LT1304-5
1000
100
10
1
6
5
4
3
2
1
0
1
1
LOAD CURRENT (mA)
LOAD CURRENT (mA)
10
10
100 200
100
1304 F10
1304 F11
200
9