Cirrus Logic AN372 - Manual (Page 22)

AN372
22 AN372REV1
Solving Equation 29 for CODE:
The tracking range of this resistance ADC is approximately 15.5k  to 4M. The series resistor R
S
is used to
adjust the resistance of the NTC to fall within this ADC tracking range so that the entire 8-bit dynamic range of
the ADC is used. A 14k
 (±1% tolerance) series resistor is required to allow measurements of up to 130°C to
be within the eOTP tracking range when a 100k  NTC with a Beta of 4334 is used. The eOTP tracking circuit
is designed to function accurately with an external capacitance of up to 470pF. A higher 8-bit code output
reflects a lower resistance and hence a higher external temperature.
The ADC output code is filtered to suppress noise and compared against a reference code that corresponds
to 125/130°C. If the temperature exceeds this threshold, the chip enters an overtemperature state and shuts
down. This is not a latched protection state, and the ADC keeps tracking the temperature in this state in order
to clear the fault state once the temperature drops below 110°C. If an external overtemperature protection
thermistor is not used, connect the eOTP pin to GND using a 50k
 to 500k resistor to disable the eOTP
feature.
When exiting reset, the chip enters startup and the ADC quickly (<5ms) tracks the external temperature to
check if it is below the 110°C reference code before the boost and second stages are powered up. If this check
fails, the chip will wait until this condition becomes true before initializing the rest of the system.
For external overtemperature protection, a second low-pass filter with a time constant of 2 seconds filters the
ADC output and uses it to scale down the internal dim level of the system (and hence the LED current, I
LED
)
if the temperature exceeds 95 °C (see Figure 12).
The large time constant for this filter ensures that the dim scaling does not happen spontaneously and is not
noticeable (suppress spurious glitches). The I
LED
starts reducing when R
NTC
~ 6.3k  (assuming a 14k1%
tolerance, series resistor), which corresponds to a temperature of 95°C for a 100k  NTC (100k at 25°C).
The I
LED
current is scaled until the NTC value reaches 2.5k  (125°C). The CS1612/13 uses this calculated
value to scale the output LED current, I
LED
, as shown in Figure 12. Beyond this temperature, the IC shuts
down. If the external overtemperature protection feature is not required, connect the eOTP pin to GND using
a 50k
-to-500k resistor to disable the eOTP feature.
CODE
2
N
V
CONNECT th  

I
CONNECT
R
NTC
R
S
+   
------------------------------------------------------------------- =
256 1.25 V 
80 A   R
NTC
R
S
+   
----------------------------------------------------------- =
4M 
R
NTC
R
S
+  
--------------------------------- =
[Eq. 30]
Temperature (°C)
C
u
rr
e
n
t
(I
LE
D
, N
o
m
.)
125 95
50%
100%
0
25
Figure 12. LED Current vs. Temperature