Datasheet
LTC4008
17
4008fb
APPLICATIONS INFORMATION
used, with closed-loop feedback ensuring that adapter load
current remains within limits. Amplifi er CL1 in Figure 7
senses the voltage across R
CL
, connected between the
CLP and CLN pins. When this voltage exceeds 100mV,
the amplifi er will override programmed charging current
to limit adapter current to 100mV/R
CL
. A lowpass fi lter
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.
–
+
CLP
15nF
5k
R
CL
*
V
IN
LTC4008
16
CLN
100mV
15
4008 F07
C
IN
TO
SYSTEM
LOAD
CL1
*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%)
Table 5. Common R
CL
Resistor Values
ADAPTER
RATING (A)
R
CL
VALUE*
(Ω) 1%
R
CL
POWER
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 fi xed 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)
LTC4008
NTC
R9
C7 R
TH
4008 F08
8
Figure 8. Voltage Divider Thermistor Network