LTC4008_15 Datasheet, PDF (17/24 Pages) Linear Technology – 4A, High Effi ciency, Multi-Chemistry Battery Charger

LTC4008
APPLICATIONS INFORMATION
used, with closed-loop feedback ensuring that adapter load
current remains within limits. Amplifier CL1 in Figure 7
senses the voltage across R_CL, connected 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_CL. A lowpass filter
formed by 5kΩ and 15nF is required to eliminate switch-
ing noise. If the current limit is not used, CLN should be
connected to CLP.
Note that the I_CL pin will be asserted when the voltage
across R_CL is 93mV, before the adapter limit regulation
threshold.
LTC4008
CL1
CLP
16
100mV
CLN
15
V_IN
R_CL*
15nF
+
5k
TO
SYSTEM
LOAD
C_IN
4008 F07
*R_CL
=
100mV
ADAPTER CURRENT
LIMIT
Figure 7. Adapter Current Limiting
Setting Input Current Limit
To set the input current limit, you need to know the mini-
mum wall adapter current rating. Subtract 7% for the input
current limit tolerance and use that current to determine
the resistor value.
R_CL = 100mV/I_LIM
I_LIM = Adapter Min Current –
(Adapter Min Current • 7%)
LTC4008
R9
NTC 8
C7
R_TH
4008 F08
Figure 8. Voltage Divider Thermistor Network
Table 5. Common R_CL Resistor Values
ADAPTER
RATING (A)
R_CL VALUE*
R_CL POWER
(Ω) 1% DISSIPATION (W)
R_CL POWER
RATING (W)
1.5
0.06
0.135
0.25
1.8
0.05
0.162
0.25
2
0.045
0.18
0.25
2.3
0.039
0.206
0.25
2.5
0.036
0.225
0.5
2.7
0.033
0.241
0.5
3
0.03
0.27
0.5
*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 resistance
ratio of 1:7. If this happy circumstance is true for you, then
simply set R9 = R_TH(LOW)
4008fb
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