MCP16331 datasheet(18/40 Pages) MICROCHIP

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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 VDD 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. RTOP is

connected to VOUT, RBOT is connected to GND and

both are connected to the VFB 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 RDSON.

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 VOUT,

where K = 0.22V/µH.

EQUATION 5-3:

TOP

BOT

OUT

-------------1

–







VOUT =3.3V

VFB =0.8V

RBOT =10 k

RTOP = 31.25 k (standard value = 31.6 k)

VOUT = 3.328V (using standard value)

VOUT =5.0V

VFB =0.8V

RBOT =10 k

RTOP = 52.5 k (standard value = 52.3 k)

VOUT = 4.98V (using standard value)

TABLE 5-1:

RECOMMENDED INDUCTOR

VALUES

VOUT

LSTANDARD

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

OUT

Diode



DSON





–



-------------------------------------------------------

OUT L



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