MCP1643 datasheet(13/28 Pages) MICROCHIP

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DS20005208B-page 13

MCP1643

5.6

Inductor Selection

The MCP1643 device is designed to be used with small

surface mount inductors. An inductance value of

4.7 µH is recommended to achieve a good balance

between the inductor size, converter load transient

response and minimized noise.

Several parameters are used to select the correct

inductor:

• maximum-rated current

• saturation current

• copper resistance (ESR)

For boost converters, the inductor current can be much

higher than the output current. The lower the inductor

ESR, the higher the efficiency of the converter, a com-

mon trade-off in size versus efficiency.

The saturation current typically specifies a point at

which the inductance has rolled off a percentage of the

rated value. This can range from a 20% to 40%

reduction in inductance. As the inductance rolls off, the

inductor ripple current increases, as does the peak

switch current. It is important to keep the inductance

from rolling off too much, causing switch current to

reach the peak limit.

5.7

Thermal Calculations

The MCP1643 is available in two different packages:

MSOP-8 and 2 mm x 3 mm DFN-8. By calculating the

power dissipation and applying the package thermal

resistance (

JA), the junction temperature is estimated.

The maximum continuous ambient temperature rating

for the MCP1643 family of devices is +85°C.

To quickly estimate the internal power dissipation for

the switching boost regulator, an empirical calculation

using measured efficiency can be used. Given the

measured efficiency, the internal power dissipation is

estimated by Equation 5-2:

EQUATION 5-2:

The difference between the first term, input power, and

the second term, power delivered, is the internal

MCP1643’s power dissipation. This is an estimate

assuming that most of the power lost is internal to the

MCP1643 device and not CIN, COUT and the inductor.

There is some percentage of power lost in the boost

inductor, with very little loss in the input and output

capacitors. For a more accurate estimation of the inter-

nal power dissipation, subtract the IINRMS2 xLDCR

power dissipation.

5.8

PCB Layout Information

Good printed circuit board layout techniques are

important to any switching circuitry, and switching

power supplies are no different. When wiring the

switching high current paths, short and wide traces

should be used. For the MCP1643, these paths are

from VIN pin to the VOUT, output capacitor, LED load,

RSET sense resistor, and SGND and PGND pins to the

input capacitor. Therefore, it is important that the input

and output capacitors be placed as close as possible to

the MCP1643, to minimize the loop area.

The feedback track should be routed away from the

switching node and close to the VFB pin. RSET must be

connected as close as possible to the VFB pin, unless

regulation issues appears. When possible, ground

planes and traces should be used to help shield the

feedback signal and minimize noise and magnetic

interference.

TABLE 5-1:

MCP1643 RECOMMENDED

INDUCTORS

Part Number

Value

(µH)

DCR

(

– typ)

ISAT

(A)

Size

WxLxH

(mm)

Wurth® Group

744025004

4.7

0.100

1.7

2.8x2.8x2.8

744042004

4.7

0.070

1.65 4.8x4.8x1.8

Coilcraft

ME3220

4.7

0.190

1.5

2.5x3.2x2.0

LPS4018

4.7

0.125

1.9

4x4x1.8

XFL4020

4.7

0.052

2.7

4x4x2.1

TDK Corporation

B82462 G4472M

4.7

0.035

2.2

6x6x3

B82462 A4472M

4.7

0.055

3.2

6x6x3

SLF6028-

4R7M1R6

4.7

0.028

1.6

6x6x2.8

VOUT IOUT



Efficiency

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





VOUT IOUT





–

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