ADVANCED INFORMATION
TK654xx
DESIGN CONSIDERATIONS (CONT.)
The thresholds for the Low Battery Input (LBI) and the turnoff (V
) can be set independently using a single resistive
OFF
divider. Equations and a chart for selecting the LBI and V
thresholds are provided below:
OFF
LBI threshold = 1.22 V X (R + R ) / R
1
2
2
V
threshold = LBI threshold - 720 nA X R
OFF
1
LBI THRESHOLD
4.0 V
3.5 V
V
THRESHOLD (R = 243 K)
1
OFF
(R = 487 K)
1
3.0 V
2.5 V
2.0 V
(R = 732 K)
1
(R = 976 K)
1
0.6
1.0
1.4
1.8
RATIO OF R /R
1
2
DROPOUT CHARACTERISTICS
In battery powered applications, the input supply is not typically a constant value. As the battery discharges, the input
voltage will approach the output voltage. As the voltage on the input of the converter drops, the following two
characteristics can be observed: (a) as the battery voltage drops, output ripple will increase, (b) at very low battery levels
and high current loads, the output voltage may drop significantly below the regulation threshold.
In some applications, the above characteristics may not present a problem. For those applications that may be sensitive
to these characteristics, the LBO output can provide an early warning system indicating that battery failure is near. As
the battery voltage continues to drop, the turnoff feature of the TK654xx will ultimately turn the converter off (see LBI
Indicator section).
Page 14
September 1999 TOKO, Inc.
TOKO [ TOKO, INC ]
