Cirrus Logic AN364 - Manual (Page 24)

AN364
24 AN364REV3
4.3 Final Design Steps
Step 19) Choose Power Components
The drain current through transistor Q4 is limited to 165mA. The smallest 800V MOSFET in a package
capable of handling the power is 1A. The flyback stage output diode D3 has a peak current of 14.3
 0.131 =
1.87A, an average DC current of 0.44A, and a maximum reverse voltage of 445/14.3 + 15 = 46V. A 1A 60V
Schottky diode meet the requirements. The boost diode D1 has a peak current of 0.6A and an average DC
current of 7.3W/405V = 18mA. The maximum boost drain current in transistor Q2 is 600mA due to the attach
current. A 600V 1A MOSFET is adequate.
Step 20) Bias Circuit
It is recommended to build a bias circuit that contains the following components: C2, C8, D7, D4, Z1, R9, Q1,
R12, D8, C6, D5, C9, and R11 (for more information about these values, see the Bill of Materials on page 27.)
The bias circuit is optimized to work with the CS1610/11 across a range of applications.
Step 21) Zero-current Detection
The auxiliary winding on inductor L3 must have sufficient voltage to produce 16V across capacitor C6 and be
current limited. Capacitor C9 limits the available current by reducing the charge transfer for each cycle. The
auxiliary secondary peak-to-peak voltage is V
BST
 (N
AUX
/N
P
) and must be greater than 16V, otherwise no
charge transfer will occur. Choosing V
AUX
= 22V allows sufficient charge transfer across capacitor C9 without
unnecessarily wasting power.
The current through pin BSTAUX must be limited to less than 1mA. A series resistor of at least 22k  needs
to be used to limit the current. Resistor R3 completes a resistor divider with resistor R11 to limit the voltage on
the BSTAUX pin to less than 5V.
Step 22) Overvoltage Protection
Transformer T1 auxiliary winding has the same number of turns as the power secondary. The voltage available
on transformer T1 SOURCE pin is between +15V and -31V. Resistor R22 limits the current to less than 0.5mA.
The voltage at pin FBAUX reaches the OVP threshold of +1.25V when the transformer T1 SOURCE pin
reaches +16.8V. Transformer T1 auxiliary terminal A reaches +16.8V at the same time (the turns ratio is 1:1).
Deducting the 0.4V drop across diode D3 sets the OVP threshold at 16.4V, or approximately +10% of the
nominal output voltage.
Step 23) 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, R
NTC
is approximately 6.3k  and
R18 is 14k , so the eOTP pin has a total resistance of 20.3k. The eOTP pin initiates protective dimming
action. At 125°C the thermistor has 2.5k  plus R18 = 14k present a resistance of 16.5k at the eOTP pin
reaching the point where a thermal shutdown fault intervenes.
Step 24) Clamp Circuit
Clamp load resistors R6 and R16 must each be 2k
2W resistors for 230V systems. This value has been
validated for optimal dimming performance.
N
P
N
AUX
--------------
V
BST
22
-------------- = [Eq. 46]