LTC4015_15 Datasheet, PDF (43/76 Pages) Linear Technology – Multichemistry Buck Battery Charger Controller with Digital Telemetry System

LTC4015
Applications Information
NTC Resistor Selection
NTCBIAS
NTC
R_NTCBIAS
R_SERIES
THERMISTOR T R_NTC
R_PARALLEL
R_NTCEFF =
1
1+
1
R_PARALLEL R_NTC +R_SERIES
4015 F11
Figure. 11
With minor modifications to the thermistor bias network,
it is possible to adjust the effective temperature profile
of the thermistor. Note that this technique can generally
only reduce the slope of the temperature profile—it is
not possible to increase the sensitivity of the thermistor.
The temperature based charging characteristics of the
LTC4015 are based on the ADC reading NTC_RATIO. For
the alternate thermistor bias network shown in Figure 11,
the value of NTC_RATIO is determined by:
NTC_RATIO= 21845 •
R_NTCEFF
R_NTCBIAS +R_NTCEFF
The values of R_NTCBIAS, R_PARALLEL, and R_SERIES can be
selected in order to achieve a desired temperature profile
for NTC_RATIO. Note that thermistor temperature profiles
are highly nonlinear; consult manufacturers’ documenta-
tion for data on a specific thermistor. Two examples are
included here as a demonstration
Example 1: For a lithium chemistry battery with a 100kΩ
Vishay NTHS0402N01N1003J NTC thermistor, using
R_NTCBIAS =100kΩ, R_PARALLEL = 2MΩ, and R_SERIES = 5kΩ
will approximately mimic the profile of a thermistor β value
of 3490k over the range 0°C to 60°C, resulting in less than
1°C of error (typical) for the default JEITA temperature
thresholds (defined by JEITA_Tn, see JEITA Temperature
Controlled Charging). This error is significantly less than
the error tolerance of most thermistors.
Example 2: For a lead-acid battery with a 100kΩ Vishay
NTHS0402N01N1003J NTC thermistor, using R_NTCBIAS =
95kΩ, R_PARALLEL = 5MΩ, and R_SERIES = 2kΩ will approxi-
mately mimic the profile of a thermistor β value of 3490k
over the range –40 to 125°C, resulting in less than 5°C
of error (typical) for the lead-acid temperature charging
profile (see Lead-Acid Temperature Compensated Charg-
ing), which in turn results in a battery voltage error of less
than 20mV/cell over temperature.
Setting the R_T Resistor
A resistor on the RT pin sets the LTC4015’s step down
regulator switching frequency. To keep the inductor size
down and ensure optimum efficiency and stability the
LTC4015 switching frequency can be optimized (see In-
ductor Selection section). An R_T value of 95.3k resistor
sets the frequency to 500kHz:
f_OSC(MHz)
=
47.65
R_T (kΩ)
Setting Input and Charge Currents
As mentioned previously, maximum average charge current
is determined by the value of the sense resistor R_SNSB,
connected between CSP and CSN, which is in series with
the inductor. The maximum average input current is de-
termined by the resistance R_SNSI, connected between the
CLP and CLN pins. The input and charge current loops
servo the voltages across their respective sense resistors
to a maximum of 32mV. Therefore the maximum input
and charge average currents are:
I_IN(MAX
)⁼
32mV
R_SNSI
I
CHG(MAX
)⁼
32mV
R_SNSB
Compensation
The input current, charge current, V_BAT voltage and UVCL
voltage loops all require a 6.8nF to 14.7nF capacitor from
the V_C node to ground.
When using the MPPT feature with resistive sources in
excess of 0.5Ω, the required V_C capacitor (C_VC) may be
in the 100's of nF, with an additional series resistor in the
100Ω to 1000Ω range. If a series R is used, a smaller
cap, C_VC/10, should be placed directly from V_C to ground.
For more information www.linear.com/LTC4015
4015f
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