Datasheet

MCP16331
DS20005308C-page 16  2014-2016 Microchip Technology Inc.
Shunt boost supply regulation is used for low output
voltage converters operating from a wide ranging input
source. A regulated 3.0V to 5.5V supply is needed to
provide high-side drive bias. The shunt uses a Zener
diode to clamp the voltage within the 3.0V to 5.5V
range using the resistance shown in
Figure 4-3.
To calculate the shunt resistance, the boost drive
current can be estimated using
Equation 4-1.
I
BOOST_TYP
for 3.3V Boost Supply = 0.6 mA
I
BOOST_TYP
for 5.0V Boost Supply = 0.8 mA.
EQUATION 4-1: BOOST CURRENT
To calculate the shunt resistance, the maximum I
BOOST
and I
Z
current are used at the minimum input voltage
(
Equation 4-2).
EQUATION 4-2: SHUNT RESISTANCE
V
Z
and I
Z
can be found on the Zener diode
manufacturer’s data sheet. Typically, I
Z
= 1 mA.
Series regulator applications use a Zener diode to drop
the excess voltage. The series regulator bias source
can be input or output voltage derived, as shown in
Figure 4-4. The boost supply must remain between
3.0V and 5.5V at all times for proper circuit operation.
FIGURE 4-4: Series Regulator Boost Supply.
I
BOOST
I
BOOST_TYP
1.5  mA=
R
SH
V
INMIN
V
Z
–
I
Boost
I
Z
+
------------------------------=
C
B
V
OUT
V
IN
C
IN
C
OUT
SW
BOOST
GND
EN
FB
L
R
TOP
R
BOT
V
IN
Boost Diode
FW Diode
12V
15V to 50V
C
B
V
IN
C
IN
SW
BOOST
GND
EN
FB
L
V
IN
Boost Diode
FW Diode
2V
12V
VZ = 7.5V
VZ = 7.5V
V
OUT
R
TOP
R
BOT
C
OUT
MCP16331
MCP16331