MCP1603-ADJISLASHOS datasheet(14/26 Pages) MICROCHIP

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MCP1603

DS22042A-page 14

5.0

APPLICATION INFORMATION

5.1

Typical Applications

The MCP1603 500 mA synchronous buck regulator

operates over a wide input voltage range (2.7V to 5.5V)

and is ideal for single-cell Li-Ion battery powered

applications, USB powered applications, three cell

NiMH or NiCd applications and 3V or 5V regulated

input applications. The 5-lead TSOT and 8-lead 2x3

DFN packages provide a small footprint with minimal

external components.

5.2

Fixed Output Voltage Applications

Typical Application Circuit shows a fixed MCP1603

in an application used to convert three NiMH batteries

into a well regulated 1.8V @ 500 mA output. A 4.7 µF

input capacitor, 4.7 µF output capacitor, and a 4.7 µH

inductor make up the entire external component solu-

tion for this application. No external voltage divider or

compensation is necessary. In addition to the fixed

1.8V option, the MCP1603 is also available in 1.2V,

1.5V, 2.5V, or 3.3V fixed voltage options.

5.3

Adjustable Output Voltage

Applications

When the desired output for a particular application is

not covered by the fixed voltage options, an adjustable

MCP1603 can be used. The circuit listed in Figure 6-2

shows an adjustable MCP1603 being used to convert a

5V rail to 1.0V @ 500 mA. The output voltage is adjust-

able by using two external resistors as a voltage

divider.

For

adjustable-output

voltages,

recommended that the top resistor divider value be

200 k

Ω. The bottom resistor value can be calculated

using the following equation:

EQUATION 5-1:

For adjustable output applications, an additional R-C

compensation network is necessary for control loop

stability. Recommended values for any output voltage

are:

Refer to Figure 6-2 for proper placement of RCOMP and

CCOMP.

5.4

Input Capacitor Selection

The input current to a buck converter, when operating

in continuous conduction mode, is a squarewave with

a duty cycle defined by the output voltage (VOUT) to

input voltage (VIN) relationship of VOUT/VIN. To prevent

undesirable input voltage transients, the input capacitor

should be a low ESR type with an RMS current rating

given by Equation 5.5. Because of their small size and

low ESR, ceramic capacitors are often used. Ceramic

material X5R or X7R are well suited since they have a

low temperature coefficient and acceptable ESR.

EQUATION 5-2:

Table 5-1 contains the recommend range for the input

capacitor value.

5.5

Output Capacitor Selection

The output capacitor helps provide a stable output

voltage during sudden load transients, smooths the

current that flows from the inductor to the load, and

reduces the output voltage ripple. Therefore, low ESR

capacitors are a desirable choice for the output capac-

itor. As with the input capacitor, X5R and X7R ceramic

capacitors are well suited for this application.

The output ripple voltage is often a design specifica-

tion. A buck converters’ output ripple voltage is a

function of the charging and discharging of the output

capacitor and the ESR of the capacitor. This ripple

voltage can be calculated by Equation 5-3.

EQUATION 5-3:

RBOT

RTOP

VFB

VOUT VFB

–

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

⎝⎠

⎛⎞

Example:

RTOP

=200 k

VOUT

=1.0V

VFB

=0.8V

RBOT

=200 k

Ω x (0.8V/(1.0V - 0.8V))

RBOT

=800 k

(Standard Value = 787 k

Ω)

RCOMP

=4.99 k

CCOMP

=33 pF

ICIN RMS

IOUT MAX

VOUT

VIN VOUT

–

()

VIN

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

⎝⎠

⎜⎟

⎛⎞

ΔV

OUT

ΔI

ESR

ΔI

8fC

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

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