Datasheet

MCP16331
DS20005308B-page 18  2014 Microchip Technology Inc.
5.0 APPLICATION INFORMATION
5.1 Typical Applications
The MCP16331 step-down converter operates over a
wide input voltage range, up to 50V 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.
5.2 Adjustable Output Voltage
Calculations
To calculate the resistor divider values for the
MCP16331,
Equation 5-1 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 resistor is recommended
as a good trade-off for quiescent current and noise
immunity.
5.3 General Design Equations
The step-down converter duty cycle can be estimated
using
Equation 5-2, 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
DSON
.
EQUATION 5-2: CONTINUOUS INDUCTOR
CURRENT DUTY CYCLE
The MCP16331 device features 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
OUT
,
where K
= 0.22V/µH.
EQUATION 5-3:
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 = 31.6 k)
V
OUT
= 3.328V (using standard value)
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 (using standard value)
TABLE 5-1: RECOMMENDED INDUCTOR
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
24V 0.24 100 µH
D
V
OUT
V
Diode
+
V
IN
I
SW
R
DSON
 –
-------------------------------------------------------=
KV
OUT
L  =