Datasheet
AN4165 Circuit description
Doc ID 023661 Rev 1 7/37
2 Circuit description
The power supply is set in flyback topology. The complete schematic is given in
Figure 2
. A
simplified schematic for V
OUT
≥12 V and the relevant BOM are given in
Figure 3
and in
Table 2
respectively. The input section includes a resistor R1 and an NTC for inrush current
limiting, a diode bridge (D0) and a Pi filter for EMC suppression (C1, L2, C2). The
transformer core is a standard E20. A Transil™ clamp network (D1, D4) is used for leakage
inductance demagnetization. The output voltage value is set simply through the R3-R4
voltage divider between the output terminal and the FB pin, according to the following
formula:
Equation 1
In fact, the FB pin is the input of an error amplifier and is an accurate 3.3 V voltage
reference. In the schematic the resistor R4 has been split into R4a and R4b in order to allow
better tuning of the output voltage value. The compensation network is connected between
the COMP pin (which is the output of the error amplifier) and the GND pin and consists of
C7, C8 and R7. The output rectifier D3 has been selected according to the calculated
maximum reverse voltage, forward voltage drop and power dissipation and is a power
Schottky. A resistor has been connected between the LIM and GND pins in order to reduce
the IDLIM to the value needed to supply the required output power, limiting the stress on the
power components.
At power-up the DRAIN pin supplies the internal HV startup current generator which
charges the VDD capacitor, C4, up to V
DDon
. At this point, the Power MOSFET starts
switching, the generator is turned off and the IC is powered by the energy stored in C4. If the
nominal value of V
OUT
exceeds the V
DDcson
threshold of the VIPER26 by a small signal
diode forward voltage drop, the IC can be supplied directly from the output, selecting jumper
J2 which is shown as open in
Figure 2
(ie. no selection of either jumper J1 or J2 is
indicated). In this case jumper J1 is open because the auxiliary winding of the transformer is
not needed and the schematic can be simplified, as shown in
Figure 3
. Since
V
DDcsonmax
= 11.5 V, the minimum value of V
OUT
allowing this connection is 12 V. If
V
OUT
< 12 V, the VIPER26 must be supplied through the auxiliary winding of the transformer
(J1 selected, J2 open in
Figure 2
), delivering to the V
DD
pin a voltage higher than V
DDcson
.
The voltage generated by the auxiliary winding increases with the load on the regulated
output. An external clamp (D5, R1) can be added in this case, in order to avoid exceeding
the V
DD
operating range.
The figures and measurements in this document refer to a case in which V
DD
is supplied
from the output, i.e. to the simplified schematic shown in
Figure 3
.
⎟
⎠
⎞
⎜
⎝
⎛
+⋅=
4
3
13.3
R
R
VV
OUT