MCP1603B datasheet(17/34 Pages) MICROCHIP

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 2007-2012 Microchip Technology Inc.

DS22042B-page 17

MCP1603/B/L

5.0

APPLICATION INFORMATION

5.1

Typical Applications

The

MCP1603/B/L

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 2 x 3 DFN packages provide a small footprint with

minimal external components.

5.2

Fixed Output Voltage Applications

The Typical Application Circuit shows a fixed

MCP1603/B/L 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 solution 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/B/L can be used. The circuit listed in

Figure 6-2 shows an adjustable device being used to

convert a 5V rail to 1.0V @ 500 mA. The output voltage

is adjustable by using two external resistors as a volt-

age divider. For adjustable-output voltages, it is

recommended that the top resistor divider value be

200 k

. 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:

Table 5-1 contains the recommend range for the output

capacitor value.

BOT

TOP

OUT

–

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







Example:

RTOP =200 k

VOUT =1.0V

VFB =0.8V

RBOT =200 k x (0.8V/(1.0V – 0.8V))

RBOT =800 k(Standard Value = 787 k)

RCOMP =4.99 k

CCOMP =33 pF

TABLE 5-1:

CAPACITOR VALUE RANGE

CIN

COUT

Minimum

4.7 µF

Maximum

—

22 µF

CIN RMS

OUT MAX

OUT

–





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









V

OUT

I

ESR



I



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