LTC4099
Applications Information
Reverse-Voltage Protection
The LTC4099 can also be easily protected against the
application of reverse voltage, as shown in Figure 6. D1
and R1 are necessary to limit the maximum VGS seen by
MP1 during positive overvoltage events. D1’s breakdown
voltage must be safely below MP1’s BVGS. The circuit
shown in Figure 6 offers forward voltage protection up
to MN1’s BVDSS and reverse-voltage protection up to
MP1’s BVDSS.
USB/WALL
ADAPTER
MP1
MN1
D1
VBUS
C1
LTC4099
R1
R2
OVGATE
OVSENS
VBUS POSITIVE PROTECTION UP TO BVDSS OF MN1
VBUS NEGATIVE PROTECTION UP TO BVDSS OF MP1
4099 F06
Figure 6. Dual-Polarity Voltage Protection
Alternate NTC Thermistors and Biasing
The LTC4099 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 preprogrammed to
approximately 45°C and 0°C, respectively, when using a
Vishay curve 2 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 tem-
perature trip points can be independently programmed
with the constraint that the difference between the upper
and lower temperature thresholds must increase. Exam-
ples of each technique follow.
28
NTC thermistors have temperature characteristics which
are indicated on resistance-temperature conversion tables.
The Vishay-Dale thermistor NTHS0603N02N1002-FF, used
in the following examples, has a nominal value of 10k
and follows the Vishay curve 2 resistance-temperature
characteristic.
In the explanation below, the following notation is used.
R25 = Value of the thermistor at 25°C
RCOLD = Value of thermistor at the cold trip point
RHOT = Value of the thermistor at the hot trip point
aCOLD = Ratio of RCOLD to R25
aHOT = Ratio of RHOT to R25
RNOM = Primary thermistor bias resistor (see Figure 7)
R1 = Optional temperature range adjustment resistor
(see Figure 8)
The trip points for the LTC4099’s temperature qualification
are internally programmed at 0.326 • NTCBIAS for the hot
threshold and 0.738 • NTCBIAS for the cold threshold.
Therefore, the hot trip point is set when:
RHOT
RNOM + RHOT
• NTCBIAS =
0.326 • NTCBIAS
and the cold trip point is set when:
RCOLD
RNOM + RCOLD
• NTCBIAS = 0.738 • NTCBIAS
Solving these equations for RCOLD and RHOT results in
the following:
RHOT = 0.4839 • RNOM
and
RCOLD = 2.816 • RNOM
By setting RNOM equal to R25, the above equations result
in aHOT = 0.4839 and aCOLD = 2.816. Referencing these
ratios to the Vishay resistance-temperature curve 2 chart
gives a hot trip point of about 45°C and a cold trip point
of about 0°C. The difference between the hot and cold trip
points is approximately 45°C.
4099fd