LTC4008 Linear Technology, LTC4008 Datasheet - Page 16

• Manufacturer Part Number: LTC4008
• Description: Multi-Chemistry Battery Charger
• Manufacturer: Linear Technology

www.DataSheet4U.com
APPLICATIO S I FOR ATIO
LTC4008
control is used, with closed-loop feedback ensuring that
adapter load current remains within limits. Amplifier CL1
in Figure 7 senses the voltage across R
between the CLP and CLN pins. When this voltage exceeds
100mV, the amplifier will override programmed charging
current to limit adapter current to 100mV/R
filter formed by 5kΩ and 15nF is required to eliminate
switching noise. If the current limit is not used, CLN
should be connected to CLP.
Note that the I
across R
threshold.
Setting Input Current Limit
To set the input current limit, you need to know the
minimum wall adapter current rating. Subtract 7% for the
input current limit tolerance and use that current to deter-
mine the resistor value.
16
R
I
*R
LIM
LTC4008
CL
CL
=
= 100mV/I
= Adapter Min Current –
ADAPTER CURRENT LIMIT
(Adapter Min Current • 7%)
CL
CL1
is 93mV, before the adapter limit regulation
Figure 7. Adapter Current Limiting
100mV
CL
–
+
LIM
pin will be asserted when the voltage
100mV
U
16
15
U
4008 F07
CLP
CLN
15nF
R
V
W
CL
IN
*
5k
+
CL
CL
, connected
. A lowpass
C
IN
U
TO
SYSTEM
LOAD
Table 5. Common R
* Values shown above are rounded to nearest standard value.
As is often the case, the wall adapter will usually have at
least a +10% current limit margin and many times one can
simply set the adapter current limit value to the actual
adapter rating (see Table 5).
Designing the Thermistor Network
There are several networks that will yield the desired
function of voltage vs temperature needed for proper
operation of the thermistor. The simplest of these is the
voltage divider shown in Figure 8. Unfortunately, since the
HIGH/LOW comparator thresholds are fixed internally,
there is only one thermistor type that can be used in this
network; the thermistor must have a HIGH/LOW resis-
tance ratio of 1:7. If this happy circumstance is true for
you, then simply set R9 = R
RATING (A)
ADAPTER
1.5
1.8
2.3
2.5
2.7
2
3
Figure 8. Voltage Divider Thermistor Network
R CL VALUE*
LTC4008
(Ω) 1%
0.045
0.039
0.036
0.033
0.06
0.05
0.03
CL
NTC
Resistor Values
8
R9
DISSIPATION (W)
TH(LOW)
R CL POWER
0.135
0.162
0.206
0.225
0.241
0.18
0.27
C7
4008 F08
R
TH
R CL POWER
RATING (W)
0.25
0.25
0.25
0.25
0.5
0.5
0.5
4008fa