MCP1661 datasheet(17/34 Pages) MICROCHIP

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 2014-2015 Microchip Technology Inc.

DS20005315B-page 17

MCP1661

Several parameters are used to select the correct

inductor: maximum rated current, saturation current

and copper resistance (DCR). For boost converters,

the inductor current is much higher than the output

current. The average inductor current is equal to the

input current. The inductor’s peak current is 30-40%

higher than the average. The lower the inductor DCR,

the higher the efficiency of the converter: a common

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 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.

5.6

Rectifier Diode Selection

Schottky diodes are used to reduce losses. The diode’s

current rating has to be equal or higher than the

maximum output current. The diode’s reverse

breakdown voltage must be higher than the internal

switch rating voltage of 36V.

The converter’s efficiency will be improved if the

voltage drop across the diode is lower. The forward

voltage rating is forward-current dependent, which is

equal in particular to the load current.

For high currents and high ambient temperatures, use

a diode with good thermal characteristics.

5.7

SEPIC Converter Considerations

One of the advantages of using MCP1661 in SEPIC

topology is the usage of an output disconnect feature.

Also, the output voltage may be lower or higher than

the input voltage, resulting in buck or boost operation.

Input voltage is limited to the 2.4-5.5V range.

One major advantage is that the SEPIC converter allows

3.0V or 3.3V buck-boost application from a Li-Ion battery

with load disconnect. Also, SEPIC is recommended for

higher output voltages where an input-to-output isolation

is necessary (due to the coupling capacitor). An

application example is shown in Figure 6-3.

The maximum output voltage, VOUTmax, must be

limited to the sum of (VIN +VOUT) < 36V, which is the

maximum internal switch DC rating. VIN must be ≤ 5.5V.

Some extra aspects need to be taken into account

when choosing the external components:

• the DC voltage rating of the coupling capacitor

should be at least equal to the maximum input

voltage

• the average current rating of the rectifier diode’s is

equal to the output load current

• the peak current of the rectifier diode is the same

as the internal switch current, ISW =IIN +IOUT.

See the notes on Figure 6-3 in Section 6.0 “Typical

Application Circuits” for some recommended 1:1

coupled inductors.

5.8

Thermal Calculations

The MCP1661 device is available in two different

packages (5-lead SOT-23 and 8-lead 2x3 TDFN). By

calculating the power dissipation and applying the

package thermal resistance (

JA), the junction

temperature is estimated. The maximum continuous

junction temperature rating for the MCP1661 device is

+125°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.

TABLE 5-2:

MCP1661 RECOMMENDED

INDUCTORS FOR BOOST

CONVERTERS

Part Number

Value

(µH)

DCR



(typ.)

ISAT

(A)

Size

WxLxH (mm)

Coilcraft

MSS5131-472

4.7

0.038

1.42

5.1x5.1x3.1

XFL4020-472

4.7

0.057

2.7

4.2x4.2x2.1

LPS5015-562

5.6

0.175

1.6

5.0x5.0x1.5

LPS6235-103

0.065

1.5

6.2x6.2x3.5

XAL4040-103

0.092

1.9

4.3x4.3x4.1

Würth Elektronik

744025004 WE-TPC

4.7

0.1

1.7

2.8x2.8x2.8

744043004 WE-TPC

4.7

0.05

1.7

4.8x4.8x2.8

744773112 WE-PD2

0.156

1.6

4.0x4.5x3.2

74408943100 WE-SPC

0.082

2.1

4.8x4.8x3.8

TDK Corporation

B82462G4472

4.7

0.04

1.8

6.3x6.3x3.0

B82462G4103

0.062

1.3

6.3x6.3x3.0

VLCF4024T-4R7

4.7

0.087

1.43

4.0x4.0x2.4

TABLE 5-3:

RECOMMENDED SCHOTTKY

DIODES

Type

VOUTmax

PMEG2005

18V

< 85°C

PMEG4005

36V

< 85°C

MBR0520

18V

< 125°C

MBR0540

36V

< 125°C

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