Cirrus Logic CS1613A - Manual (Page 13)

CS1610A/11A
CS1612A/13A
DS976F1 13
5.7.4 Overcurrent Protection
Overcurrent protection (OCP) is implemented by monitoring
the voltage across the second stage sense resistor. If this
voltage exceeds OCP threshold V
OCP(th)
of 1.69V, a fault
condition occurs. The IC output is disabled, and the controller
attempts to restart after one second.
5.7.5 Open Loop Protection
Both open loop protection (OLP) and protection against a
short of the second stage sense resistor are implemented by
monitoring the voltage across the sense resistor. If the voltage
on pin FBSENSE does not reach protection OLP threshold
V
OLP(th)
of 200mV, the IC output is disabled, and the controller
attempts to restart after one second.
5.8 Overtemperature Protection
The CS1610A/11A/12A/13A incorporates both internal over-
temperature protection (iOTP) and the ability to connect an ex-
ternal overtemperature sense circuit for IC protection.
Typically, a negative temperature coefficient (NTC) thermistor
is used.
5.8.1 Internal Overtemperature Protection
Internal overtemperature protection (iOTP) is activated, and
switching is disabled, when the die temperature of the device
exceeds 135°C. There is a hysteresis of about 14°C before
resuming normal operation.
5.8.2 External Overtemperature Protection
The external overtemperature protection (eOTP) pin is used to
implement overtemperature protection using an external 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, which adjusts
current I
CONNECT
into the NTC thermistor and series resistor
R
S
to maintain a constant reference voltage V
CONNECT(th)
of
1.25V. Figure 17 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 9:
When the loop is in equilibrium, the voltage on pin eOTP
fluctuates around voltage threshold V
CONNECT(th)
. The digital
‘CODE’ output by the ADC is used to generate
current I
CONNECT
. In normal operating mode,
current I
CONNECT
is updated once every seventh half
line-cycle by a single ± LSB step. See Equation 10:
Using Equation 10 solve for digital CODE. See Equation 11:
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 thermistor 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 thermistor with
a Beta of 4334 is used. The eOTP tracking circuit is designed
to function accurately with external capacitance 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 external 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.
CS1610A/11A/12A/13A
+
-
I
CONNE CT
V
CONNE CT (th)
Comp_Out
eOTP
Control
eOTP
R
S
C
NTC
NTC
V
DD
10
(Optional )
Figure 17. eOTP Functional Diagram
I
CONNECT
V
CONNECT th  
R
-------------------------------------
=
[Eq.9]
CODE
I
CONNECT
2
N
--------------------------- 
V
CONNECT th  
R
NTC
R
S
+
-------------------------------------
=
[Eq.10]
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.11]