Application Note

VISHAY BCCOMPONENTS
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Resistive Products
Application Note
Fast Charging Control with NTC Temperature Sensing
1. INTRODUCTION
The need for increased autonomy for new models of laptops
and cellular phones has resulted in high-energy density
power packs - Ni MH and Li-ion batteries.
These batteries can be charged quickly, on the condition
that the fast charging complies with several criteria.
2. THE FAST CHARGE ALGORITHM FOR THE
BQ2005
Referring to the notice of the BQ2005 IC, we will focus on
the design part related to the temperature control of the
charge operation (see figure 1).
VCC
The techniques used are the following:
PACK +
• For the Ni MH cells, the quick and fast charging operation
uses the - V, d2V/dt2, the maximum time, the TCO
(Temperature Cut Off), or the T/t techniques. The
measurement of high temperature is used as a protection,
but the temperature variation (T/t) can also be used for
monitoring.
• For the Li-ion cells, the fast charging uses the CCCV
techniques (Constant Current Constant Voltage). The
initial temperature is measured in order to allow initiation
of fast charging. If the temperature reaches a high
threshold (TCO), the fast charging would stop.
The sophistication of the electronic system depends
principally upon cost and upon the requirements of the
batteries. Often, the fast charging is monitored by an IC,
measuring the voltage of the batteries, the char-ging
current via a sense resistor, and measuring the temperature
of the batteries via one or several Negative Temperature
Coefficient (NTC) thermistor(s). The IC's are almost always
in the chargers or integrated in the battery pack (Li-ion). The
thermistors are almost always integrated in the battery
packs, sometimes placed in the charger, and/or in the final
apertures (low cost cellular phones).
The computation methods performed here are
sufficiently general to be extended to a lot of other
configurations.
BQ2005
RT2
TS
RT1
SNS
N
T
C
PACK -
Figure 1
An NTC thermistor, together with fixed resistors RT1 and
RT2, is used in a voltage divider between Vcc and the current
sense resistor input VSNS of the IC.
At the beginning of a new charge cycle, the IC checks if the
voltage Vtemp = VTS - VSNS is within the limits designed by the
IC manufacturer (low temperature: 0.4 Vcc and high
temperature: 0.1 Vcc + 0.75 VTCO).
VTCO is a cut off threshold defined by external resistors (not
represented in figure 1): If after starting the fast charge
phase, Vtemp becomes lower than VTCO, then the return to
trickle mode is operated.
During the fast charge period, the IC samples the voltage
Vtemp and the return to trickle mode can also be operated
when the variation in time of Vtemp is going over a threshold.
This is called the T/t termination: each 34 s, Vtemp is
sampled and if Vtemp has fallen by 16 mV ± 4 mV
compared to the value measured two samples earlier, then
the fast charge is terminated.
The following table summarizes the voltage levels applicable
here:
SYMBOL
PARAMETER
AVERAGE
TOLERANCE
Vcc
Supply voltage
5V
± 10 %
VTCO
Cut off voltage
Adjustable between 0.1 Vcc and 0.2 Vcc
Vlow temp
Low temperature fault
0.4 Vcc
± 30 mV
Vhigh temp
High temperature fault
0.1 Vcc + 0.75 VTCO
± 30 mV
TS input change for T/t termination
16 mV/period of 2 x 34 s
± 4 mV
Vtherm
Revision: 16-Sep-14
Document Number: 29089
1
For technical questions, contact: [email protected]
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
APPLICATION NOTE
This application note explains how to design an NTC
thermistor from Vishay BCcomponents for a BQ2005 from
TEXAS INSTRUMENTS dual Ni MH batteries charging IC.
VTCO
Application Note
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Vishay BCcomponents
Fast Charging Control with NTC Temperature Sensing
We can derive from equations (1) and (3):
3. CONFIGURATION OF EXTERNAL
THERMISTOR/RESISTOR NETWORK
ΔVTS ΔVTS ΔT
- B RT1RT22RNTCVCC
ΔT
=
=
Δt
ΔT Δt T2 (RT1RT2 + RT1RNTC + RT2RNTC)2 Δt
The voltage around the TS input is:
VTS - VSNS =
RT2RNTC
(V - VSNS) (1)
RT1RT1 + RT1RNTC + RT2RNTC CC
The voltage around the NTC for the low fault, high fault, and
cut off temperatures has to comply to the thresholds
designed for the BQ2005. This is expressed by equations
(1a), (1b) and (1c).
VTS (T low) - VSNS = 0.4 Vcc
(1a)
VTS (T high) - VSNS = 0.1 Vcc + 0.75 VTCO
(1b)
VTS (T cut off) - VSNS = VTCO
(1c)

Normally VSNS is of the order of 0.1 V. For simplicity, we will
consider here that VSNS = 0. Should this approximation not
be valid, then the computations hereunder must be
modified.
Let us call RNTC (low temperature fault), RNTC (high
temperature fault) and RNTC (cut off temperature) respectively RnL, RnH, and RTCO.
Introducing
obtain:
(1)
in
(1a)
and solving with respect to RT2, we
RT2 =
0.666 RT1RnL
(RnL - 0.66 RT1)
RTCORnL
(RnL - RTCO)
V
CC
- 2.5 )
( VTCO
The characteristics of the thermistor are defined by Vishay
BCcomponents Tlow and TCO values. The B value can be
found in the catalog or by using the Steinhart & Hart
interpolation polynoms calculation.
These parameters are given in the appendix for several
currently used Vishay BCcomponents thermistors.
On this base, all the remaining parameters can be defined
with the help of relations (2a), (2b), and (4) which have to be
verified simultaneously: RT1 and RT2 are chosen to respect
Tlow and TCO via equation (2a) and (2b).
VTCO will be defined so that the required T/t (equation (4))
will be respected.
At last, T high fault will be computed with equation (1b).
4. NUMERICAL EXAMPLE
Example 1
• T low fault = 10 °C
• T cut off = 50 °C
(2a)
• T/t = 1 °C/min ± 0.3 °C/min
Once the thermistor characteristics and VTCO are defined,
RT1 and RT2 will be defined.
Then:
We also have to compute the speed of variation of
temperature on the thermistor, which will induce the
voltage Vtherm operating the T/t termination.
• Designing for the sensor the Vishay BCcomponents
leaded thermistor NTCLE203E3103JB0: 
R25 = 10 k ± 5 % B25/85 = 3977K ± 0.75 %
Assuming the exponential dependence of the electrical
resistance of the thermistor in function of the
temperature:
• Using VTCO = 1.6 V arbitrarily
• Using Vcc = 5 V, dV/dt = 16 mV / (2 x 34 s)
We derive RT1 = 2753  and RT2 = 2020 
Then we compute T/t for different temperatures from
10 °C to TCO. The results are shown in the following table:
Rntc (T) = R25 exp(B (1/T - 1/298.15)) (3)
APPLICATION NOTE
T/t, Tlow and TCO are given by the battery manufacturer.
VTS/t is defined by TI.
The following data are currently applicable to Ni MH
batteries:
(2a)
Introducing (1) and (2a) in (1c) we obtain:
RT1 =
(4)
where R25 is the electrical resistance of the NTC at 25 °C, B
is the B25/85 characteristic of the component (K), and T is
the absolute temperature (K).
Vthreshold
(V)
VTS/T
(mV/°C)
T/t
(°C/Min)
TEMP
(°C)
RNTC
()
Low fault
10
19872
1.999
2.000
-5
2.57
High fault
42.5
4824
1.704
1.700
- 13
1.07
50
3605
1.599
1.600
- 15
0.95
CHARACTERISTICS
Cut off
Revision: 16-Sep-14
VTS
(V)
Document Number: 29089
2
For technical questions, contact: [email protected]
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
Application Note
www.vishay.com
Vishay BCcomponents
Fast Charging Control with NTC Temperature Sensing
We see that the T/t falls into the range of 1 °C/min ± 0.3 °C/min. If it would not be the cause, then one should have let the
VTCO slightly change.
The tolerances on the electrical characteristics introduce also a variation on the thresholds:
For the limit case: Let us make the calculations for the value of the thermistor being at the limits ± 5 % and the B value at
± 0.75 %. We will also take into account the errors introduced by the tolerances on the fixed resistors (supposed ± 1 %).
The error T in the thresholds (low fault temperature and TCO) due to these tolerances are simply obtained by performing the
calculations of the VTS at the fixed temperature (10 °C and 50 °C) and by comparing these values with the requested ones, and
dividing these differences by the sensitivity VTS/T.
The results are summarized in the following tables:
RNTC (25 °C) = 10 500 
RT1 = - 1 %
B25/85 = 3977K - 0.75 %
RT2 = + 1 %
TEMP
(°C)
RNTC
()
VTS
(V)
Vthreshold
(V)
VTS/T
(mV/°C)
T/t
(°C/Min)
V
(mV)
T
(°C)
Low fault
10
20 755.49
2.027
2.000
-5
2.66
27
- 5.01
Cut off
50
3814.942
1.639
1.600
- 15
0.97
39
- 2.70
VTS/T
(mV/°C)
T/t
(°C/Min)
V
(mV)
T
(°C)
RNTC (25 °C) = 9500 
RT1 = + 1 %
B25/85 = 3977K + 0.75 %
RT2= - 1 %
TEMP
(°C)
RNTC
()
VTS
(V)
Vthreshold
(V)
Low fault
10
18 978.88
1.971
2.000
-6
2.48
- 29
5.12
Cut off
50
3398.598
1.558
1.600
- 15
0.93
- 42
2.73
With these tolerances:
• Low temperature fault will fall in the range 10 °C± 5 °C approx.
• Temperature cut off will fall in the range 50 °C ± 2.7 °C approx.
APPLICATION NOTE
If such variations should not be acceptable, then design a thermistor with R25 tolerance down to ± 1 % (code number:
NTCLE203E3103FB0) instead of ± 5 %: The tolerances on the definition of threshold will become negligible compared to
inherent tolerances of the IC. 

Example 2
The same calculations for all the SMD NTC thermistors (NiSn terminations, sizes 0805, 0603, or 0402 described in the appendix)
give the following results:

Adjusting slightly VTCO to 1.55 V, in order to keep T/t nominal at 1 °C/min at the high fault temperature, we then can compute:
CHARACTERISTIC
TEMP
(°C)
RNTC
()
VTS
(V)
Vthreshold
(V)
VTS/T
(mV/°C)
T/t
(°C/Min)
RT1
()
RT2
()
NTCS0805E3103xMT
SMD 0805
NiSn terminations
Low fault
High fault
Cut off
10
41.8
50
18 515
5331
4004
1.999
1.668
1.549
2.000
1.663
1.550
-7
- 14
- 15
1.98
1.01
0.93
3708
2850
NTCS0603E3103xMT
SMD 0603
NiSn terminations
Low fault
High fault
Cut off
10
41.9
50
18 664
5271
3960
1.999
1.668
1.549
2.000
1.663
1.550
-7
- 14
- 15
2.01
1.01
0.92
3649
2794
NTCS0402E3103xLT
SMD 0402
NiSn terminations
Low fault
High fault
Cut off
10
41.75
50
18 290
5408
4079
1.999
1.668
1.549
2.000
1.663
1.550
-7
- 14
- 15
1.95
1.02
0.94
3811
2947
COMPONENT
Revision: 16-Sep-14
Document Number: 29089
3
For technical questions, contact: [email protected]
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
Application Note
www.vishay.com
Vishay BCcomponents
Fast Charging Control with NTC Temperature Sensing
5. CONCLUSION AND GENERAL COMMENTS
Due to their low tolerances, low cost, and high sensitivity, NTC thermistors are perfectly suited for fast charging monitoring and
protection of the battery packs.
The notes and calculations described in this note can be easily extrapolated to other IC's, for example the BQ2954 for Li-ion
packs. In this case, the T/t charge termination is not of application, which makes it even more simple.
The greatest care should be used when positioning the thermistor into the pack to ensure close contact between the thermistor
and the batteries. Otherwise, all calculations about tolerances on will not be applicable.
Further information of the different mechanical executions (insulated leads, SMD version) suitable for these applications are
available from the Vishay BCcomponents offices.
6. APPENDIX
Different thermistors Steinhart & Hart characteristics
Formula : Ln (R(T)/R25) = A + B/T +C/T2 + D/T3 where T is expressed in Kelvins (°C + 273.15)
APPLICATION NOTE
CODE NUMBER
TOL.
R (25 °C)
TYPE
B25/85
TOLERANCE
(K)
STEINHART & HART COEFFICIENTS
A
B
C
D
NTCLE203E3103xB0
x=F
x=G
x=H
x=J
1%
2%
3%
5%
Leaded
3977
0.75 %
- 14.63372
4791.842
- 115 334
- 3 730 535
NTCS0805E3103xMT
x=F
x=G
x=H
x=J
1%
2%
3%
5%
SMD 0805
NiSn
terminations
3570
3%
- 13.40886
4547.961
- 176 965.9
3 861 154
NTCS0603E3103xMT
x=F
x=G
x=H
x=J
1%
2%
3%
5%
SMD 0603
NiSn
terminations
3610
1%
- 13.40957
4481.799
- 150 521.7
1 877 103
NTCS0402E3103xLT
x=F
x=G
x=H
x=J
1%
2%
3%
5%
SMD 0402
NiSn
terminations
3490
3%
- 12.0714
3503.902
109 391
- 24 154
454.74
Revision: 16-Sep-14
Document Number: 29089
4
For technical questions, contact: [email protected]
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000