Datasheet

LTC4099
28
4099fd
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 V
GS
seen by
MP1 during positive overvoltage events. D1’s breakdown
voltage must be safely below MP1’s BV
GS
. The circuit
shown in Figure 6 offers forward voltage protection up
to MN1’s BV
DSS
and reverse-voltage protection up to
MP1’s BV
DSS
.
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
R
COLD
= Value of thermistor at the cold trip point
R
HOT
= Value of the thermistor at the hot trip point
a
COLD
= Ratio of R
COLD
to R25
a
HOT
= Ratio of R
HOT
to R25
R
NOM
= 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:
R
HOT
R
NOM
+ R
HOT
• NTCBIAS = 0.326 • NTCBIAS
and the cold trip point is set when:
R
COLD
R
NOM
+ R
COLD
• NTCBIAS = 0.738 • NTCBIAS
Solving these equations for R
COLD
and R
HOT
results in
the following:
R
HOT
= 0.4839 • R
NOM
and
R
COLD
= 2.816 • R
NOM
By setting R
NOM
equal to R25, the above equations result
in a
HOT
= 0.4839 and a
COLD
= 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.
R2R1
USB/WALL
ADAPTER
4099 F06
C1D1
MN1MP1
V
BUS
POSITIVE PROTECTION UP TO BV
DSS
OF MN1
V
BUS
NEGATIVE PROTECTION UP TO BV
DSS
OF MP1
V
BUS
OVSENS
OVGATE
LTC4099
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.
applicaTions inForMaTion