LTC4007_15 Datasheet, PDF (17/24 Pages) Linear Technology – 4A, High Effi ciency, Standalone Li-Ion Battery Charger

LTC4007
APPLICATIONS INFORMATION
Adapter Limiting
An important feature of the LTC4007 is the ability to auto-
matically adjust charging current to a level which avoids
overloading the wall adapter. This allows the product to
operate at the same time that batteries are being charged
without complex load management algorithms. Addition-
ally, 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 and CLN should
be connected together and tied 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 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%)
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 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 resistance
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
4007fc
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