Cirrus Logic AN368 - Manual (Page 46)

AN368
46 AN368REV2
j. External Overtemperature Protection
The external overtemperature protection (eOTP) pin is used to implement overtemperature protection using a
negative temperature coefficient (NTC) thermistor. The total resistance on the eOTP pin is converted to an 8-
bit digital ‘CODE’ (which gives an indication of the temperature) using a digital feedback loop, adjusting the
current I
CONNECT
into the NTC and series resistor R
S
to maintain a constant reference voltage of 1.25V
(V
CONNECT(th)
). Figure 20 illustrates the functional block diagram when connecting an optional external NTC
temperature sensor to the eOTP circuit.
Current I
CONNECT
is generated from an 8-bit controlled current source with a full-scale current of 80 A. See
Equation 74:
When the loop is in equilibrium, the voltage on the eOTP pin fluctuates around V
CONNECT(th)
. The digital
‘CODE’ output by the ADC is used to generate I
CONNECT
. In normal operating mode, the I
CONNECT
current is
updated once every seventh half line-cycle by a single ±LSB step. See Equation 75:
Solving Equation 75 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 well 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 shutoff temperature Temp
Shutdown
. If the temperature exceeds this threshold, the controller enters an
overtemperature state and stops converting. 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 a wakeup
temperature Temp
Wakeup
.
+
-
I
CONNE CT
V
CONNE CT (th)
Comp_Out
eOTP
Control
eOTP
R
S
C
NTC
NTC
V
DD
10
(Optional )
CS1630/31
GND
Figure 20. eOTP Functional Diagram
I
CONNECT
V
CONNECT th  
R
---------------------------------- = [Eq. 74]
CODE
I
CONNECT
2
N
------------------------ 
V
CONNECT th  
R
NTC
R
S
+
---------------------------------- = [Eq. 75]
CODE
2
N
V
CONNECT th  

I
CONNECT
R
NTC
R
S
+   
------------------------------------------------------------- =
256 1.25 V 
80 A   R
NTC
R
S
+   
-------------------------------------------------------- = [Eq. 76]
4M 
R
NTC
R
S
+  
-------------------------------- =