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TYPICAL APPLICATIONS
APPLICATIONS
R U and R S Calculation
The two equations ( equ. 2 and equ. 3 ) on page 16 can be
further simplified as the following by substituting the K HOT
and K COLD with their typical values:
Temperature Tolerance
The equ. 6 is also the basis for tolerance calculation. The
errors of the internal voltage thresholds, external resistors
and the thermistor resistance all contribute to the
R S = ( R COLD – ( 4 × R HOT ) ) ? 3
R U = 2 × ( R COLD – R HOT ) ? 3
The R S equation requires
R COLD ≥ 4 × R HOT
,
equ. 4
equ. 5
temperature error. For the low temperature threshold, T COLD ,
the maximum thermistor resistance happens when the
internal threshold is at its maximum, R U at its maximum and
the R S at its minimum value. Assuming 1% accuracy for both
R U and R S and taking the maximum value for the low
--------------------------------------------------------------------------------------------------------------------------------------- )
otherwise, the R S calculation results in a negative value.
Assuming the target temperature window is from 0°C to
50°C, from Table 7 it can be found that R HOT = 33.1946 k Ω
and R COLD = 357.0117 k Ω . Using equ. 4 and equ. 5 , one can
find that
R S = 74.74k Ω
R U = 215.9k Ω
Temperature Hysteresis
The thermistor resistance can be found with equ. 1 on
temperature threshold from Table 4 , the maximum thermistor
resistance at the cold temperature is found to be
R COLD , MAX = equ. 7
0.6741 × ( 74.74 × 0.99 + 215.9 × 1.01 ) – ( 74.74 × 0.99
1 – 0.6741
= 377.0k Ω
Similarly, the minimum thermistor resistance at the hot
temperature, R HOT,MIN , happens when the internal threshold
is at its minimum, R U at this minimum, and the R S at its
R NTC = --------------------------------------------------
K X ? ( R S + R U ) – R S
1 – K X
equ. 6
maximum. Using the same method, the R HOT,MIN can be
found to be 29.73 k Ω , which corresponds to 53°C
approximately.
R HOT = 38.51k Ω
R COLD = 320.6k Ω
Since the R S and R U have already been determined, the
thermistor resistance can be found by replacing the K X with
the Low Temperature Falling Threshold and the High
Temperature Rising Threshold given in Table 4 . The
thermistor resistance at these two thresholds can be found as
′
′
From Table 7 it is found that rising threshold for the cold
temperature is about 2°C and the falling threshold for the hot
temperature is between 46 to 47°C. Therefore the hystereses
for the cold and the hot temperature is 2°C and 2 to 3°C
respectively.
Based on the above calculation, the tolerances for the cold
and the hot temperatures are about 1.4°C and 3°C
respectively.
ESD ENHANCEMENT
All pins in the MC34674 are rated 2.0 kV for the ESD
performance with the Human Body Model (HBM) . The end
product usually requires higher ESP performance for the
nodes that can be touched by human hands in normal usage
of the end product. Three additional capacitors can be used
to pass the ESD tests. Figure 28 shows how the three
capacitors (C 3 , C 4 , and C 5 ) are connected in the circuit.
APPLICATIONS
Current
VIN
BAT
VREF
R U
C 2
Limited
AC/DC
Regulator
C 1
RED
GRN
TEMP
GND
C 5
R S
NTC
C 3
C 4
EN
Figure 28. 34674 Typical Application Circuit
C1 and C2 are for decoupling purposes. C3, C4 and C5
are to enhance the ESD performance of the travel charger or
the cradle charger. C1 = 1.0 μ F/16 V/X5R, C2 = 1.0 μ F/6.3 V/
X5R, C3 = C4 = 0.1 μ F/16 V/X5R, C5 = 0.1 μ F/6.3 V/X5R.
34674
Analog Integrated Circuit Device Data
22
Freescale Semiconductor
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