LTC3557EUF#PBF Linear Technology, LTC3557EUF#PBF Datasheet - Page 23

• Manufacturer Part Number: LTC3557EUF#PBF
• Description: IC USB POWER MANAGE 28-QFN
• Manufacturer: Linear Technology

Specifications of LTC3557EUF#PBF

• Applications: Handheld/Mobile Devices
• Voltage - Supply: 4.35 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-QFN
• Operating Temperature (max): 85C
• Operating Temperature (min): -40C
• Pin Count: 28
• Mounting: Surface Mount
• Package Type: QFN EP
• Case Length: 4mm
• Screening Level: Industrial
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Supply: -
• Lead Free Status / Rohs Status: Compliant

APPLICATIONS INFORMATION
Alternate NTC Thermistors and Biasing
The LTC3557/LTC3557-1 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 pre-programmed to approximately 40°C and 0°C,
respectively (assuming a Vishay “Curve 1” thermistor).
The upper and lower temperature thresholds can be
adjusted 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
temperature 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 NTHS0603N011-N1003F , 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.
The trip points for the LTC3557/LTC3557-1’s temperature
qualifi cation are internally programmed at 0.349 • V
R25 = Value of the Thermistor at 25°C
R
R
point
r
r
R
R1 = Optional temperature range adjustment resistor
(see Figure 9)
COLD
HOT
NTC|COLD
NTC|HOT
NOM
= Ratio of R
= Ratio of R
= Primary thermistor bias resistor (see Figure 8)
= Value of the thermistor at the hot trip
= Value of thermistor at the cold trip point
NTC|HOT
NTC|COLD
to R25
to R25
VNTC
for the hot threshold and 0.765 • V
threshold.
Therefore, the hot trip point is set when:
and the cold trip point is set when:
Solving these equations for R
in the following:
and
By setting R
in r
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, R
R25, the hot and cold trip points can be moved in either
direction. 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:
where r
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
designed in the IC. Consider an example where a 60°C
hot trip point is desired.
R
R
R
R
R
R
HOT
NTC|HOT
NTC|COLD
NOM
NOM
NOM
NOM
= 0.536 and r
R
R
HOT
NTC|HOT
NTC|COLD
=
=
+ R
+ R
r
0.536
NOM
3.25
= 0.536 • R
r
COLD
and r
NTC|HOT
NTC|COLD
HOT
= 3.25 • R
LTC3557/LTC3557-1
equal to R25, the above equations result
• R25
COLD
• R25
COLD
• V
• V
NOM
NOM
are the resistance ratios at the
VNTC
= 3.25. Referencing these ratios
VNTC
NOM
NTC|COLD
= 0.349 • V
= 0.765 • V
, different in value from
and R
VNTC
VNTC
NTC|HOT
VNTC
for the cold
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