Datasheet

 2004-2013 Microchip Technology Inc. DS21876B-page 17
MCP1650/51/52/53
5.2 Design Considerations
When developing switching power converter circuits,
there are numerous things to consider and the
MCP1650/51/52/53 family is no exception. The gated
oscillator architecture does provide a simple control
approach so that stabilizing the regulator output is an
easier task than that of a fixed-frequency regulator.
The MCP1650/51/52/53 controller utilizes an external
switch and diode allowing for a very wide range of
conversion (high voltage gain and/or high current gain).
There are practical, as well as power-conversion,
topology limitations. The MCP1650/51/52/53 gated
oscillator hysteretic mode converter has similar
limitations, as do fixed-frequency boost converters.
5.2.1 DESIGN EXAMPLE
Setting the output voltage:
By adjusting the external resistor divider, the output
voltage of the boost converter can be set to the desired
value. Due to the RC delay caused by the resistor
divider and the device input capacitance, resistor
values greater than 100 kare not recommended. The
feedback voltage is typically 1.22V.
For this example:
5.2.1.1 Calculations
For gated oscillator hysteretic designs, the switching
frequency is not constant and will gate several pulses
to raise the output voltage. Once the upper hysteresis
threshold is reached, the gated pulses stop and the
output will coast down at a rate determined by the out-
put capacitor and the load. Using the gated oscillator
switching frequency and duty cycle, it is possible to
determine what the maximum boost ratio is for
continuous inductor current operation.
This relationship assumes that the output load current
is significant and the boost converter is operating in
Continuous Inductor Current mode. If the load is very
light or a small boost inductance is used, higher boost
ratio’s can be achieved.
Calculate at minimum V
IN
:
The ideal maximum output voltage is 14V. The actual
measured result will be less due to the forward voltage
drop in the boost diode, as well as other circuit losses.
For applications where the input voltage is above and
below 3.8V, another point must be checked to deter-
mine the maximum boost ratio. At 3.8V, the duty cycle
changes from 80% to 56% to minimize the peak current
in the inductor.
For this case, V
OUTMAX
= 8.63V less than the required
12V output specified. The size of the inductor has to
decrease in order to operate the boost regulator in
Discontinuous Inductor Current mode.
Input Voltage = 2.8V to 4.2V
Output Voltage = 12V
Output Current = 100 mA
Oscillator Frequency = 750 kHz
Duty cycle = 80% for V
IN
< 3.8V
Duty cycle = 56% for V
IN
> 3.8V
R
BOT
=10k
V
OUT
= 12V
V
FB
=1.22V
R
TOP
= 88.4 k
90.9 K was selected as the closest standard value.
R
TOP
R
BOT
V
OUT
V
FB
-------------


1–


=
Where:
R
TOP
= Top Resistor Value
R
BOT
= Bottom Resistor Value
P
OUT
V
OUT
I
OUT
=
Where:
P
OUT
= 12V X 100 mA
P
OUT
= 1.2 Watts
P
IN
P
OUT
Efficiency=
Where:
P
IN
= 1.2W/80%
P
IN
= 1.5 Watts
(80% is a good efficiency estimate)
V
OUT
1
1D–
-------------


V
IN
=
V
OUTMAX
1
10.8–
----------------


2.8=
V
OUTM AX
1
10.56–
-------------------


3.8=