Datasheet

12
LT1510/LT1510-5
APPLICATIONS INFORMATION
WUU
U
Figure 7. Current Comparator for Initiating Float Time-Out
0.18Ω
GND
NEGATIVE EDGE
TO TIMER
INTERNAL
SENSE
RESISTOR
1510 F06
3.3V OR 5V
ADAPTER
OUTPUT
3
8
7
1
4
2
LT1510
D1
1N4148
C1
0.1µF
BAT
SENSE
R1*
1.6k
R4
470k
R3
430k
R2
560k
LT1011
D2
1N4148
* TRIP CURRENT =
R1(V
BAT
)
(R2 + R3)(0.18Ω)
+
–
voltage decrease is detected as an indication of near full
charge. The charging current is then reduced to a much
lower value and maintained as a constant trickle charge.
An intermediate “top off” current may be used for a fixed
time period to reduce 100% charge time.
NiMH batteries are similar in chemistry to NiCd but have
two differences related to charging. First, the inflection
characteristic in battery voltage as full charge is ap-
proached is not nearly as pronounced. This makes it more
difficult to use dV/dt as an indicator of full charge, and
change of temperature is more often used with a tempera-
ture sensor in the battery pack. Secondly, constant trickle
charge may not be recommended. Instead, a moderate
level of current is used on a pulse basis (≈ 1% to 5% duty
cycle) with the time-averaged value substituting for a
constant low trickle.
Thermal Calculations
If the LT1510 is used for charging currents above 0.4A, a
thermal calculation should be done to ensure that junction
temperature will not exceed 125°C. Power dissipation in
the IC is caused by bias and driver current, switch resis-
tance, switch transition losses and the current sense
resistor. The following equations show that maximum
practical charging current for the 8-pin SO package
(125° C/W thermal resistance) is about 0.8A for an 8.4V
battery and 1.1A for a 4.2V battery. This assumes a 60°C
maximum ambient temperature. The 16-pin SO, with a
thermal resistance of 50°C/W, can provide a full 1.5A
charging current in many situations. The 16-pin PDIP falls
between these extremes. Graphs are shown in the Typical
Performance Characteristics section.
P mA V mA V
V
V
mA I
P
IV
V
V
P
IRV
V
tVI f
P
BIAS IN BAT
BAT
IN
BAT
DRIVER
BAT BAT
BAT
IN
SW
BAT SW BAT
IN
OL IN BAT
SENSE
=
()()
+
()
+
()
+
()()
[]
=
()( )
+






()
=
()( )( )
+
()()( )()
=
35 15
75 0012
1
30
55
0
2
2
2
..
..
.. 18
2
Ω
()()
I
BAT
R
SW
= Switch ON resistance ≈ 0.35Ω
t
OL
= Effective switch overlap time ≈ 10ns
f = 200kHz (500kHz for LT1510-5)