LTC4060_15 Datasheet, PDF (17/20 Pages) Linear Technology – Standalone Linear NiMH/NiCd Fast Battery Charger

LTC4060
TYPICAL APPLICATIO S
Trickle Charge
The trickle charge function is normally not required due to
the automatic recharge feature. However, the LTC4060
does provide a modest pull-up current (I_BRD) as part of its
battery removal detection method. If additional current is
required for trickle charge or to support battery removal
detection with current loads greater than I_BRD, then the
simple circuit of Figure 4 will facilitate that. The diode
insures no reverse discharge current when V_IN is removed
and the resistor sets the trickle current.
Extending Charge Current
Extending the charge current beyond 2A can be accom-
plished by paralleling an external current sense resistor,
R_ISET, with the internal current sense resistor as shown in
Figure 5. Bond wire, lead frame and PCB interconnect
resistance and mismatches in the two sense resistor’s
value will cause charge current variability to increase in
proportion to the extension in current. Resistor R_ISET
should be connected directly to the LTC4060 to reduce
errors. The total current sense resistor, bond wire and lead
frame resistance is approximately 0.08Ω (T.C. ≅ 3500ppm/
°C). The formula for extended fast charge current is:
I_MAX(EXT)
=
I_MAX
•
⎛⎝⎜₁₊
0.08
R_ISET
⎞
⎠⎟
= 2A • 1.5 = 3A
for R_ISET = 0.16Ω and R_PROG = 698Ω.
Adequate PNP beta is required to meet the DRIVE pin
capability and the increased PNP power dissipation will
require additional heat sinking.
V_IN
1N4001
14
V_CC
LTC4060
3
SENSE
1
DRIVE
2
BAT
3.3k
+ 2-CELL
NiMH
BATTERY
4060 F04
Figure 4. Adding Trickle Charge
V_IN
14
V_{CC 0.08Ω}
R_ISET
0.16Ω
3
SENSE
1
DRIVE
2
BAT
LTC4060
+ _2-CELL
NiMH
BATTERY
4060 F05
Figure 5. Extended Charge Current Operation
4060f
17