Cirrus Logic AN375 - Manual (Page 23)

AN375
AN375REV4 23
Step 16) Zero-current Detection
The recommended design approach is to set the auxiliary voltage V
AUX
to 22V. A higher voltage is acceptable
to assure a stable steady state supply voltage V
DD
at a cost of slightly larger losses. The customer reference
design CRD1615-8W is designed with a 5.75:1:1 transformer ratio, hence the auxiliary voltage V
AUX
is
designed to be equal to the output voltage V
OUT
. Use Equation 23 on page 16 to calculate the turns ratio for
the auxiliary winding. See Equations 56:
Step 17) Overvoltage Protection
Resistor R6 must be greater than 31k
 to limit the current into the FBAUX pin to below 1mA. The customer
reference design CRD1615-8W uses a 69.8k  resistor for R6, which is also used to attenuate the voltage to
pin FBAUX to a voltage of 1.25V. Use Equation 57 to calculate the auxiliary voltage V
AUX(OVP)
when the output
voltage is 25% above the maximum output allowed:
To calculate the current I
AUX
flowing into the voltage divider formed by resistors R6 and R10 when an OVP
event occurs use Equation 58:
Calculate resistor R10 using Equation 59:
Resistor R10 is selected to be a 2.61k  standard value.
Step 18) External Overtemperature Protection
The external negative temperature coefficient (NTC) thermistor reference is a Murata NCP18WF104J03RB.
This NTC is 100k
 with a Beta of 4275. If the temperature exceeds 95°C, thermistor R
NTC
is approximately
6.3k  and series resistor R18 is 14k, so the eOTP pin has a total resistance of 20.3k. The eOTP pin
initiates protective dimming action by reducing the power dissipation. At 125°C, the thermistor R
NTC
is
approximately 2.26k  and series resistor R18 is equal to 14k so the eOTP pin has a total resistance of
16.26k  The eOTP circuit temperature reaches the point where a thermal shutdown fault intervenes. The
CS1615/16 will continue to monitor pin eOTP, and once the series resistor R18 plus the thermistor R
NTC
rise
above 19.23k , the device will resume power conversion.
Step 19) Optional Clamp Circuit
The clamp circuit is optional and not implemented in this design example.
N
S
N
AUX
--------------
V
F
V
OUT
+
V
AUX
----------------------------
0.9V 27.9V +
28.8V
----------------------------------- 1 = = = [Eq. 56]
V
AUX OVP  
1.25
N
AUX
N
S
--------------
 
 
V
F
V
OUT OVP  
+     1.25 1.0 0.9V 27.9V +     36V = = = [Eq. 57]
I
AUX
V
AUX max  
1.25V – 0.9V –
R6
--------------------------------------------------------------------
36V 1.25V – 0.9V –
69.8k 
--------------------------------------------------- 0.495mA = = = [Eq. 58]
R10
1.25V
I
AUX
----------------
1.25V
0.495mA
------------------------ 2.53k  = = = [Eq. 59]