AN1894 Datasheet, PDF (2/14 Pages) STMicroelectronics – BUCK-BOOST CONVERTER REFERENCE BOARD

AN1894 - APPLICATION NOTE
As soon as C4 voltage reaches the start-up threshold the internal 60 kHz oscillator sets the internal flip-
flop and through output driver turns-on the internal high voltage power MOSFET. The power MOSFET
applies the bulk capacitor C1 and C2 high voltage to the cathode of the power diode and to one terminal
of the inductor. Since the voltage at the output capacitor C3 connected to the inductor’s second terminal
is much lower than input bulk capacitor voltage the inductor current will ramp-up. As soon as the inductor
current ramp reaches the VIPer’s internal set-point defined by feedback loop, the internal power switch
turns off. The inductor keeps the direction of the current flowing and it reverses the voltage at C3. The
inductor current then flows through the forward biased D5 diode and charges the output capacitor C3. In
this switch-off phase the source terminal of the VIPer12 sees a negative level of the forward biased D5
(when referenced to ground) so it can be considered as grounded. This allows the inductor current to
flow also through D4 and supply the VIPer12 and give the feedback information about output voltage.
Figure 1: Schematic diagram of non isolated buck converter with positive output voltage
R1
D1
D2
L1 BC
10R
90...264V~
3W
CON1
L
1L
N
2
N
clamp
GL1M
1000V
1A
GL1M
1000V
1A
1.5mH 100mA
C1
+ 4.7uF
400V
KMG
C2
+ 4.7uF
400V
KMG
Layout Hints: C4, C6 have
to be close to VIPer12A
Assembly options:
(1a): +5V/60mA, +15V/100mA
(1b): +5V/20mA, +15V/100mA
note: all voltages refer to neutral
note: sum of currents is 100mA max.
U1
VDD ⁴
FB ³
VIPer12AS
D7
VDD
C4
10uF
50V
KME
LL4148
+
D3
ZMM13
R3
C6
22nF
1k
C7
100nF
D4
RGL34J
600V
0.5A
+15V
R6
L2
1mH
200mA
LBC
0R
U3 L78M05CDT (1a)
VIN VOUT
GND
D5
RGL34J
600V
0.5A
R5
0R
C3
+ 56uF
35V
LXY
D6
ZMM18
U2
8
VIN
(1b)
L78L05CD
1
VOUT
CON2
+5V 3
2
1
C8
100nF
clamp
The output voltage of the converter at the 15V terminal is determined by the voltage drop across zener
diode D3 together with voltage drop across the resistor R3 and FB pin voltage. Resistor R3 limits the
feedback current to a safe value lower than the maximum rating specified in the data sheet. Capacitor C6
protects the FB input against EMI. One has to take into account the slight variation of the output voltage
with the load. It is because the feedback current reacts to the output load change to adopt switching duty
cycle. The variable feedback current creates different FB voltage, different voltage drop across the
resistor R3 and D3 zener voltage. The feedback current can change from 0mA (full output power) to
about 0.9mA at no output load. The R3 voltage variation is 0.9V and FB pin voltage about 1.2V. The D3
voltage variation depends on the diode V-I characteristics. Diode D6 limits the output voltage at light load
condition and it also protects the U2 voltage regulator. Regulator U2 accommodated in DPAK or SO-8
package is optional and can be assembled if the power supply for a microcontroller or logic part is
required. The DPAK package version of U2 is dedicated for 60mA output current option while U3 in SO-8
can provide max. 20mA.
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