LTC4088 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC4088' PDF : 24 Pages View PDF
LTC4088
Applications Information
power to charge the battery as quickly as possible, and
with minimal power dissipation within the charger.
Alternate NTC Thermistors and Biasing
The LTC4088 provides temperature qualified charging if
a grounded thermistor and a bias resistor are connected
to NTC. By using a bias resistor whose value is equal to
the room temperature resistance of the thermistor (R25)
the upper and lower temperatures are pre-programmed
to approximately 40°C and 0°C, respectively (assuming
a Vishay “Curve 1” thermistor).
The upper and lower temperature thresholds can be ad-
justed by either a modification of the bias resistor value
or by adding a second adjustment resistor to the circuit.
If only the bias resistor is adjusted, then either the upper
or the lower threshold can be modified but not both. The
other trip point will be determined by the characteristics
of the thermistor. Using the bias resistor in addition to an
adjustment resistor, both the upper and the lower tempera-
ture trip points can be independently programmed with
the constraint that the difference between the upper and
lower temperature thresholds cannot decrease. Examples
of each technique are given below.
NTC thermistors have temperature characteristics which
are indicated on resistance-temperature conversion tables.
The Vishay-Dale thermistor NTHS0603N01N1003, used
in the following examples, has a nominal value of 100k
and follows the Vishay “Curve 1” resistance-temperature
characteristic.
In the explanation below, the following notation is used.
R25 = Value of the Thermistor at 25°C
RNTC|COLD = Value of thermistor at the cold trip point
RNTC|HOT = Value of the thermistor at the hot trip point
rCOLD = Ratio of RNTC|COLD to R25
rHOT = Ratio of RNTC|COLD to R25
RNOM = Primary thermistor bias resistor (see Figure 2)
R1 = Optional temperature range adjustment resistor
(see Figure 3)
The trip points for the LTC4088’s temperature qualifica-
tion are internally programmed at 0.349 • VBUS for the hot
threshold and 0.765 • VBUS for the cold threshold.
Therefore, the hot trip point is set when:
RNTC|HOT
RNOM + RNTC|HOT
• VBUS
=
0.349 • VBUS
and the cold trip point is set when:
RNTC|COLD
RNOM + RNTC|COLD
•
VBUS
=
0.765 •
VBUS
Solving these equations for RNTC|COLD and RNTC|HOT
results in the following:
RNTC|HOT = 0.536 • RNOM
and
RNTC|COLD = 3.25 • RNOM
By setting RNOM equal to R25, the above equations result
in rHOT = 0.536 and rCOLD = 3.25. Referencing these ratios
to the Vishay Resistance-Temperature Curve 1 chart gives
a hot trip point of about 40°C and a cold trip point of about
0°C. The difference between the hot and cold trip points
is approximately 40°C.
By using a bias resistor, RNOM, different in value from R25,
the hot and cold trip points can be moved in either direc-
tion. The temperature span will change somewhat due to
the non-linear behavior of the thermistor. The following
equations can be used to easily calculate a new value for
the bias resistor:
RNOM
=
rHOT
0.536
•
R25
RNOM
=
rCOLD
3.25
• R25
where rHOT and rCOLD are the resistance ratios at the desired
hot and cold trip points. Note that these equations are
linked. Therefore, only one of the two trip points can be
chosen, the other is determined by the default ratios de-
signed in the IC. Consider an example where a 60°C hot
trip point is desired.
4088fb
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