An372 – Cirrus Logic AN372 User Manual
Page 30
AN372
30
AN372REV1
The negative voltage on the auxiliary winding is calculated using Equation 67:
The recommended current into pin FBAUX is limited to 1mA. Choosing resistor R12 to equal 47k
limits the
current to 0.72mA.
To set the OVP, R13 must be chosen, yielding Equation 68:
where
R13 = 2.15k
is the closest standard value solution
The peak control threshold, V
PKMax(th)
, is specified in the data sheet as 1.4V, and the peak current through
R
Sense
is I
PK(FB)
=0.207A
Step 20) Choose Power Components
The drain current through FET Q4 is limited to 207mA. The smallest 600V FET in a package capable of
handling the power is 1A. The buck stage output diode D3 has a peak current of (5
0.207 = 1.04A), an
average DC current of less than 0.4A, and a max reverse voltage of (24 + ((445-25) / 4) = 129V). A 200V, 1A
fast-recovery diode, such as 1N4935, meets the requirements. The boost diode D1 has a peak current of 0.6A
and an average DC current of (10.1W/405V = 25mA). Choosing an STTH1R06 meets the requirements. The
maximum boost drain current in FET Q2 is 600mA due to the attach current. A 600V 1A FET is adequate.
Step 21) 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 R10. The bias circuit is optimized to work with the CS1612/13 across a range of
applications.
Step 22) 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
pin BSTAUX to less than 5V.
Step 23) Overvoltage Protection
The buck inductor auxiliary winding has the sufficient number of turns to produce a voltage that 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 auxiliary winding reaches +16.8V. Deducting the 0.4V drop across diode
D3 sets the OVP threshold at 16.4V, or approximately +10% of the nominal output voltage.
Step 24) 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
resistor R18 is 14k
, so the eOTP pin has a total resistance of 20.3k. Pin eOTP initiates protective dimming
V
BST
V
OUT
–
Z
N 1
+
------------------
–
445V 23.8V
–
0.4
5
--------
–
33.7V
–
=
=
[Eq. 67]
R12
29 1.25
–
-----------------------------
R13
1.25
-----------
=
[Eq. 68]
R
Sense
V
PKMax th
I
PK FB
-----------------------------
=
1.4V
0.207A
-------------------
=
6.76
=
[Eq. 69]
N
P
N
AUX
--------------
V
BST
22
--------------
=
[Eq. 70]