Cirrus Logic AN372 - Manual (Page 21)

AN372
AN372REV1 21
The BSTAUX pin and FBAUX pin currents must be limited to less than 1mA. A series resistor of at least 22 k 
must be used to limit the current.
Step 23) Overvoltage Protection
Output open circuit protection and output overvoltage protection (OVP) are implemented by monitoring the
output voltage through the buck inductor auxiliary winding. During switching time T2, the voltage across the
buck inductor L4 auxiliary winding is representative of the output voltage using a turns ratio relationship. The
buck auxiliary winding voltage is applied to the FBAUX pin. If the voltage on the FBAUX pin exceeds a
threshold V
OVP(th)
of 1.25V, a fault condition occurs. The IC output is disabled and the controller attempts to
restart after one second.
Since the buck auxiliary winding does not supply V
DD
, the auxiliary winding circuit has fewer design
constraints. A buck auxiliary output voltage of 10V to 20V during switching period T2 is appropriate. The
resistive divider between the buck auxiliary winding and the FBAUX pin must be sized to produce 1.25V when
an overvoltage fault occurs at the desired load. The FBAUX pin current must be limited to less than 1mA.
Step 24) External Overtemperature Protection
The 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 current I
CONNECT
into the NTC and
series resistor R
S
to maintain a constant reference voltage V
CONNECT(th)
of 1.25V. Figure 11 illustrates the
functional block diagram when connecting an optional 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 28:
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 29:
CS1612/13
+
-
I
CONNE CT
V
CONNE CT (th)
Comp_Out
eOTP
Control
eOTP
R
S
C
NTC
NTC
V
DD
10
(Optional )
Figure 11. eOTP Functional Diagram
I
CONNECT
V
CONNECT th  
R
------------------------------------- = [Eq. 28]
CODE
I
CONNECT
2
N
--------------------------- 
V
CONNECT th  
R
NTC
R
S
+
------------------------------------- = [Eq. 29]