LTC3576 Datasheet, PDF (37/48 Pages) Linear Technology – Switching Power Manager with USB On-the-Go + Triple Step-Down DC/DCs

LTC3576/LTC3576-1
APPLICATIONS INFORMATION
additional pole created by any PROG pin capacitance,
capacitance on this pin must be kept to a minimum. With
no additional capacitance on the PROG pin, the charger
is stable with program resistor values as high as 25k.
However, additional capacitance on this node reduces the
maximum allowed program resistor. The pole frequency at
the PROG pin should be kept above 100kHz. Therefore, if
the PROG pin has a parasitic capacitance, C_PROG, the fol-
lowing equation should be used to calculate the maximum
resistance value for R_PROG:
R_PROG
≤
2π
1
• 100kHz
• C_PROG
Alternate NTC Thermistors and Biasing
The LTC3576/LTC3576-1 provide 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 respec-
tively 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 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.
R25 = Value of the Thermistor at 25°C
R_NTC|COLD = Value of thermistor at the cold trip point
R_NTC|HOT = Value of the thermistor at the hot trip
point
r_COLD = Ratio of R_NTC|COLD to R25
r_HOT = Ratio of R_NTC|HOT to R25
R_NOM – Primary thermistor bias resistor
(see Figure 9)
R1 = Optional temperature range adjustment resistor
(see Figure 10)
NTCBIAS
3
R_NOM
100k
NTC
4
LTC3576/LTC3576-1
NTC BLOCK
0.765 • NTCBIAS
–
+
TOO_COLD
T
R_NTC
100k
–
0.349 • NTCBIAS
+
TOO_HOT
NTCBIAS
3
LTC3576/LTC3576-1
NTC BLOCK
R_NOM
105k
NTC
4
R1
12.7k
T
R_NTC
100k
0.765 • NTCBIAS
–
+
–
0.349 • NTCBIAS ₊
TOO_COLD
TOO_HOT
+
0.1V
–
NTC_ENABLE
3576 F09
Figure 9. Standard NTC Configuration
+
0.1V
–
NTC_ENABLE
Figure 10. Modified NTC Configuration
3576 F10
3576f
37