MCP1253 datasheet(11/18 Pages) MICROCHIP

11 / 18 page

 2002 Microchip Technology Inc.

DS21752A-page 11

MCP1252/3

5.1

Capacitor Selection

The style and value of capacitors used with the

MCP1252 and MCP1253 family of devices determine

several important parameters such as output voltage

ripple and charge pump strength. To minimize noise

and ripple, it is recommended that low ESR (0.1

Ω)

capacitors be used for both CIN and COUT. These

capacitors should be either ceramic or tantalum and

should be 10 µF or higher. Aluminum capacitors are not

recommended because of their high ESR.

If the source impedance to VIN is very low, up to several

megahertz, CIN may not be required. Alternatively, a

somewhat smaller value of CIN may be substituted for

the recommended 10 µF, but will not be as effective in

preventing ripple on the VIN pin.

The value of COUT controls the amount of output volt-

age ripple present on VOUT. Increasing the size of

COUT will reduce output ripple at the expense of a

slower turn-on time from shutdown and a higher in-rush

current.

The flying capacitor (CFLY) controls the strength of the

charge pump. In order to achieve the maximum rated

output current (120 mA), it is necessary to have at least

1 µF of capacitance for the flying capacitor. A smaller

flying capacitor delivers less charge per clock cycle to

the output capacitor, resulting in lower output ripple.

The output ripple is reduced at the expense of maxi-

mum output current and efficiency.

5.2

Output Voltage Setting

The MCP1252-33X50 and MCP1253-33X50 feedback

controllers select between an internally-set, regulated

output voltage (3.3V or 5.0V). Connect SELECT to

GND for a regulated 5.0V output and connect SELECT

to VIN for a regulated 3.3V output.

The MCP1252-ADJ and MCP1253-ADJ utilize an

external resistor divider that allows the output voltage

to be adjusted between 1.5V and 5.5V. For an adjust-

able output, connect a resistor between VOUT and FB

(R1) and another resistor between FB and GND (R2). In

the following equation, choose R2 to be less than or

equal to 30 k

Ω and calculate R1 from the following

formula:

EQUATION

and

EQUATION

Note that the tolerance of the external resistors will

have an effect on the accuracy of the output voltage.

For optimum results, it is recommended that the

external resistors have a tolerance no larger than 1%.

5.3

Recommended Layout

The MCP1252 and MCP1253 family of devices transfer

charge at high switching frequencies, producing fast,

high peak, transient currents. As a result, any stray

inductance in the component layout will produce

unwanted noise in the system. Proper board layout

techniques are required to ensure optimum perfor-

mance. Figure 5-3 depicts the recommended board

layout. The input capacitor connected between VIN and

GND, and the output capacitor connected between

VOUT and GND, are 10 µF ceramic, X7R dielectric, in

1206 packages. The flying capacitor connected

between C+ and C- is a 1 µF ceramic, X7R dielectric in

a 0805 package. The layout is scaled 3:1.

FIGURE 5-3:

Recommended Printed

Circuit Board Layout.

R1 R2 VOUT VFB

⁄

() 1

–

[]

VOUT

VFB 1R1 R2

⁄

()

where:

VOUT is the desired output voltage from 1.5V to 5.5V

VFB is the internal regulation voltage, nominally 1.21V

PGOOD

VOUT

GND

VIN

C+

C-

SHDN

SELECT

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