Microchip Technology ADM00352 Data Sheet
2011-2013 Microchip Technology Inc.
DS20005004C-page 17
MCP16301/H
5.0
APPLICATION INFORMATION
5.1
Typical Applications
The MCP16301/H step-down converters operate over
a wide input voltage range, up to 36V maximum.
Typical applications include generating a bias or V
a wide input voltage range, up to 36V maximum.
Typical applications include generating a bias or V
DD
voltage for the PIC
®
microcontroller product line, digital
control system bias supply for AC-DC converters, 24V
industrial input and similar applications.
industrial input and similar applications.
5.2
Adjustable Output Voltage
Calculations
Calculations
To calculate the resistor divider values for the
MCP16301/H devices,
MCP16301/H devices,
can be used.
R
TOP
is connected to V
OUT
, R
BOT
is connected to GND
and both are connected to the V
FB
input pin.
EQUATION 5-1:
EXAMPLE 5-1:
EXAMPLE 5-2:
The transconductance error amplifier gain is controlled
by its internal impedance. The external divider resistors
have no effect on system gain, so a wide range of
values can be used. A 10 k
by its internal impedance. The external divider resistors
have no effect on system gain, so a wide range of
values can be used. A 10 k
resistor is recommended
as a good trade-off for quiescent current and noise
immunity.
immunity.
5.3
General Design Equations
The step-down converter duty cycle can be estimated
using
using
, while operating in Continuous
Inductor Current mode. This equation also counts the
forward drop of the freewheeling diode and internal
N-Channel MOSFET switch voltage drop. As the load
current increases, the switch voltage drop and diode
voltage drop increase, requiring a larger PWM duty
cycle to maintain the output voltage regulation. Switch
voltage drop is estimated by multiplying the switch
current times the switch resistance or R
forward drop of the freewheeling diode and internal
N-Channel MOSFET switch voltage drop. As the load
current increases, the switch voltage drop and diode
voltage drop increase, requiring a larger PWM duty
cycle to maintain the output voltage regulation. Switch
voltage drop is estimated by multiplying the switch
current times the switch resistance or R
DSON
.
EQUATION 5-2:
CONTINUOUS INDUCTOR
CURRENT DUTY CYCLE
CURRENT DUTY CYCLE
The MCP16301/H devices feature an integrated slope
compensation to prevent the bimodal operation of the
PWM duty cycle. Internally, half of the inductor current
down slope is summed with the internal current sense
signal. For the proper amount of slope compensation,
it is recommended to keep the inductor down-slope
current constant by varying the inductance with V
compensation to prevent the bimodal operation of the
PWM duty cycle. Internally, half of the inductor current
down slope is summed with the internal current sense
signal. For the proper amount of slope compensation,
it is recommended to keep the inductor down-slope
current constant by varying the inductance with V
OUT
,
where K = 0.22V/µH.
EQUATION 5-3:
For V
OUT
= 3.3V, an inductance of 15 µH is
recommended.
R
TOP
R
BOT
V
OUT
V
FB
-------------
1
–
=
V
OUT
=
3.3V
V
FB
=
0.8V
R
BOT
=
10 k
R
TOP
=
31.25 k
(standard value = 31.2 k)
V
OUT
=
3.3V
V
OUT
=
5.0V
V
FB
=
0.8V
R
BOT
=
10 k
R
TOP
=
52.5 k
(standard value = 52.3 k)
V
OUT
=
4.98V
TABLE 5-1:
RECOMMENDED INDUCTOR
VALUES
VALUES
V
OUT
K
L
STANDARD
2.0V
0.20
10 µH
3.3V
0.22
15 µH
5.0V
0.23
22 µH
12V
0.21
56 µH
15V
0.22
68 µH
D
V
OUT
V
Diode
+
V
IN
I
SW
R
DSON
–
-------------------------------------------------------
=
K
V
OUT
L
=