MSS7341 datasheet(20/38 Pages) MICROCHIP

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MCP16301

DS25004A-page 20

5.9

Boost Capacitor

The boost capacitor is used to supply current for the

internal high side drive circuitry that is above the input

voltage. The boost capacitor must store enough energy

to completely drive the high side switch on and off. A

0.1 µF X5R or X7R capacitor is recommended for all

applications. The boost capacitor maximum voltage is

5.5V, so a 6.3V or 10V rated capacitor is recom-

mended.

5.10

Thermal Calculations

The MCP16301 is available in a SOT-23-6 package. By

calculating the power dissipation and applying the

package thermal resistance (

JA), the junction temper-

ature is estimated. The maximum continuous junction

temperature rating for the MCP16301 is +125°C.

To quickly estimate the internal power dissipation for

the switching step-down regulator, an empirical calcu-

lation using measured efficiency can be used. Given

the measured efficiency, the internal power dissipation

is estimated by Equation 5-7. This power dissipation

includes all internal and external component losses.

For a quick internal estimate, subtract the estimated

Schottky diode loss and inductor ESR loss from the

PDIS calculation in Equation 5-7.

EQUATION 5-7:

TOTAL POWER

DISSIPATION ESTIMATE

The difference between the first term, input power, and

the second term, power delivered, is the total system

power dissipation. The freewheeling Schottky diode

losses are determined by calculating the average diode

current and multiplying by the diode forward drop. The

inductor losses are estimated by PL = IOUT

2 x L

ESR.

EQUATION 5-8:

DIODE POWER

DISSIPATION ESTIMATE

EXAMPLE 5-5:

5.11

PCB Layout Information

Good printed circuit board layout techniques are

important to any switching circuitry, and switching

power supplies are no different. When wiring the

switching high-current paths, short and wide traces

should be used. Therefore, it is important that the input

and output capacitors be placed as close as possible to

the MCP16301 to minimize the loop area.

The feedback resistors and feedback signal should be

routed away from the switching node and the switching

current loop. When possible, ground planes and traces

should be used to help shield the feedback signal and

minimize noise and magnetic interference.

A good MCP16301 layout starts with CIN placement.

CIN supplies current to the input of the circuit when the

switch is turned on. In addition to supplying high-

frequency switch current, CIN also provides a stable

voltage source for the internal MCP16301 circuitry.

Unstable PWM operation can result if there are

excessive transients or ringing on the VIN pin of the

MCP16301 device. In Figure 5-1, CIN is placed close to

pin 5. A ground plane on the bottom of the board

provides a low resistive and inductive path for the

return current. The next priority in placement is the

freewheeling current loop formed by D1, COUT and L,

while strategically placing COUT return close to CIN

return. Next, CB and DB should be placed between the

boost pin and the switch node pin SW. This leaves

space close to the MCP16301 VFB pin to place RTOP

and RBOT. RTOP and RBOT are routed away from the

Switch node so noise is not coupled into the high-

impedance VFB input.

OUT

Efficiency

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

⎝⎠

⎛⎞

OUT

()

–

Dis

Diode

–

() I

OUT

()

VIN

=10V

VOUT

=5.0V

IOUT

=0.4A

Efficiency

= 90%

Total System Dissipation

= 222 mW

LESR

=0.15

=24 mW

Diode VF

= 0.50

D= 50%

PDiode

=125 mW

MCP16301 internal power dissipation estimate:

PDIS - PL - PDIODE = 73 mW

=198°C/W

Estimated Junction

Temperature Rise

=+14.5°C

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