LTC4007_1 Datasheet, PDF (17/24 Pages) Linear Technology – 4A, High Efficiency, Standalone Li-Ion Battery Charger

LTC4007
APPLICATIO S I FOR ATIO
Adapter Limiting
An important feature of the LTC4007 is the ability to
automatically adjust charging current to a level which
avoids overloading the wall adapter. This allows the prod-
uct to operate at the same time that batteries are being
charged without complex load management algorithms.
Additionally, batteries will automatically be charged at the
maximum possible rate of which the adapter is capable.
This feature is created by sensing total adapter output
current and adjusting charging current downward if a
preset adapter current limit is exceeded. True analog
control is used, with closed-loop feedback ensuring that
adapter load current remains within limits. Amplifier CL1
in Figure 8 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
switching noise. If the current limit is not used, CLP should
be connected to DCIN.
Note that the I_CL pin will be asserted when the voltage
across R_CL is 93mV, before the adapter limit regulation
threshold.
LTC4007
CL1
100m₊V
CLP
18
15nF
5k
CLN
19
+
*R_CL
=
100mV
ADAPTER CURRENT
LIMIT
R_CL*
C_IN
AC ADAPTER
INPUT
V_IN
4007 F08
Figure 8. Adapter Current Limiting
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.
R_CL = 100mV/I_LIM
I_LIM = Adapter Min Current –
(Adapter Min Current • 7%)
Table 5. Common R_CL Resistor Values
ADAPTER RCL VALUE*
RCL POWER
RATING (A)
(Ω) 1%
DISSIPATION (W)
RCL 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 9. 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_TH(LOW).
LTC4007
R9
NTC 9
C7
R_TH
4007 F09
Figure 9. Voltage Divider Thermistor Network
4007fa
17