LTC4099 Linear Technology Corporation, LTC4099 Datasheet - Page 28

• Manufacturer Part Number: LTC4099
• Description: I2C Controlled USB Power Manager/Charger
• Manufacturer: Linear Technology Corporation

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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 V
MP1 during positive overvoltage events. D1’s breakdown
voltage must be safely below MP1’s BV
shown in Figure 6 offers forward voltage protection up
to MN1’s BV
MP1’s BV
Alternate NTC Thermistors and Biasing
The LTC4099 provides temperature-qualifi ed 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 modifi cation 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 modifi ed, 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
USB/WALL
ADAPTER
V
V
BUS
BUS
DSS
Figure 6. Dual-Polarity Voltage Protection
POSITIVE PROTECTION UP TO BV
NEGATIVE PROTECTION UP TO BV
.
DSS
and reverse-voltage protection up to
MP1
R1
D1
R2
MN1
DSS
DSS
OF MN1
OF MP1
C1
GS
V
OVGATE
OVSENS
. The circuit
LTC4099
BUS
GS
4099 F06
seen by
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.
The trip points for the LTC4099’s temperature qualifi cation
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:
and the cold trip point is set when:
Solving these equations for R
the following:
and
By setting R
in
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.
R25 = Value of the thermistor at 25°C
R
R
R
R1 = Optional temperature range adjustment resistor
(see Figure 8)
R
R
R
R
HOT
COLD
HOT
COLD
HOT
NOM
HOT
COLD
NOM
NOM
R
R
= Ratio of R
= 0.4839 • R
= 0.4839 and
= Value of the thermistor at the hot trip point
= Primary thermistor bias resistor (see Figure 7)
HOT
COLD
= 2.816 • R
= Value of thermistor at the cold trip point
= Ratio of R
R
R
NOM
HOT
COLD
equal to R25, the above equations result
•
•
NTCBIAS
NTCBIAS
HOT
NOM
NOM
COLD
COLD
to R25
to R25
= 2.816. Referencing these
COLD
0 326
0 738
.
.
and R
•
•
NTCBIAS
NTCBIAS
HOT
results in
4099f