MCP1612 datasheet(13/22 Pages) MICROCHIP

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DS21921B-page 13

MCP1612

Solving for C:

There will also be some ripple voltage caused by the

ESR of the capacitor. The ripple is defined as follows.

EQUATION 5-7:

For this example:

5.2.4

INPUT CAPACITOR

For the buck topology, the input current is pulled from

the source and the input capacitor in pulses. The size

of the input capacitor will determine the amount of

current pulled from the source. For most applications,

a 10 µF ceramic capacitor connected between the

MCP1612’s VIN and PGND is recommended to filter the

current pulses. Less capacitance can be used for

applications that have low source impedance. The

ripple current rating for ceramic capacitors are typically

very high due to their low loss characteristics. Low-cost

electrolytic capacitors can be used, but their ripple

current rating should not be exceeded.

5.2.5

VCC INPUT

The VCC input is used to bias the internal MCP1612

circuitry. A 10

Ω resistor is recommended between the

unregulated inputs VIN and VCC, along with a 0.1 µF

capacitor to ground to help isolate the VCC pin from the

switching noise.

5.2.6

COMPENSATION COMPONENTS

An internal transconductance error amplifier is used to

compensate the buck converter. An external resistor

(RC) and capacitor (CC), connected between COMP

and GND, are all that is needed to provide a high-

bandwidth loop.

Table 5-1 identifies values for RC and CC for standard

buck inductor (L) and output capacitor (COUT) values.

5.3

Printed Circuit Board (PCB)

Layout

The MCP1612 is capable of switching over 1A at

1.4 MHz. As with all high-frequency switching power

supplies, good PCB layout techniques are essential to

prevent noise generated by the switching power-train

from interfering with the sensing circuitry.

There are two ground pins (PGND and AGND) on the

MCP1612 to separate the large-signal ground current

from the small-signal circuit ground. These two

grounds should be kept separate, only connecting near

the input bulk capacitor.

Care must also be taken to minimize the length and

loop area of the large signal connections. Components

connected to this loop consist of the input bulk

capacitor, VIN, PGND and LX pins of the MCP1612, the

buck inductor and the output filter capacitor.

IC = 165 mA

C=4.7 µF

Δt = 260 ns

ESR = 8 m

ΔVC = (260 ns x 165 mA)/4.7 µF

ΔVC =9.13 mV

VESRRIPPLE =8 mΩ x 165 mA

VESRRIPPLE =1.32 mV

ΔVOUT = ΔVC + VESRRIPPLE

ΔVOUT =9.13 mV + 1.32 mV

ΔVOUT = 10.45 mV

Δt

ΔV

-----------

Where:

IC = peak-to-peak ripple current

Δt = on-time of P-channel MOSFET

ΔVC = output ripple voltage

ESRR IPP LE

ESR

TABLE 5-1:

RC and CC VALUES

LCOUT

3.3 µH

10.0 µF

25 k

1000 pF

2.2 µH

4.7 µF

10 k

1000 pF

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