MCP16251 datasheet(16/34 Pages) MICROCHIP

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MCP16251/2

DS25173A-page 16

 2013 Microchip Technology Inc.

Several parameters are used to select the correct

inductor: maximum rated current, saturation current

and copper resistance (ESR). For boost converters, the

inductor current can be much higher than the output

current. The lower the inductor ESR, the higher the

efficiency of the converter, a common trade-off in size

versus efficiency.

The saturation current typically specifies a point at

which the inductance has rolled off a percentage of the

rated value. This can range from a 20% to 40%

reduction in inductance. As the inductance rolls off, the

inductor ripple current increases, as does the peak

switch current. It is important to keep the inductance

from rolling off too much, causing switch current to

reach the peak limit.

5.6

Thermal Calculations

The MCP16251/2 is available in two different packages

(SOT-23-6 and 2 x 3 TDFN-8). By calculating the

power dissipation and applying the package thermal

resistance (



JA), the junction temperature is estimated.

The maximum continuous junction temperature rating

for the MCP16251/2 family of devices is +125oC.

To quickly estimate the internal power dissipation for

the switching boost regulator, an empirical calculation

using measured efficiency can be used. Given the

measured efficiency, the internal power dissipation is

estimated by Equation 5-3.

EQUATION 5-3:

The difference between the first term, input power, and

the second term, power delivered, is the internal

MCP16251/2’s power dissipation. This is an estimate

assuming that most of the power lost is internal to the

MCP16251/2 and not CIN, COUT and the inductor.

There is some percentage of power lost in the boost

inductor, with very little loss in the input and output

capacitors. For a more accurate estimation of internal

power dissipation, subtract the IINRMS

2 x L

ESR power

dissipation.

5.7

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 MCP16251/2 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.

FIGURE 5-1:

MCP16251/2 SOT-23-6 Recommended Layout.

OUT



Efficiency

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





OUT





–

Dis

COUT

CIN

+VIN

GND

+VOUT

Via to GND Plane

MCP1625X

Via for Enable

RTOP

RBOT

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