MIC2874YCS datasheet(17/22 Pages) MICREL

17 / 22 page

Micrel, Inc.

MIC2874

Component Selection

Inductor

Inductor selection should strike a balance between

efficiency, stability, cost, size, and rated current. Since the

boost

converter

compensated

internally,

the

recommended inductance of L is limited from 0.47µH to

1µH to ensure system stability, and a 0.47µH inductor is

typically recommended. It is usually a good balance

between these considerations.

A large inductance value reduces the peak-to-peak

inductor ripple current hence the output ripple voltage and

the LED ripple current. This also reduces both the DC loss

and the transition loss at the same inductor’s DC

resistance (DCR). However, the DCR of an inductor

usually increases with the inductance in the same package

size. This is due to the longer windings required for an

increase in inductance. Since the majority of the input

current passes through the inductor, the higher the DCR

the lower the efficiency is, and more significantly at higher

load currents. On the other hand, inductor with smaller

DCR but the same inductance usually has a larger size.

The saturation current rating of the selected inductor must

be higher than the maximum peak inductor current to be

encountered and should be at least 20% to 30% higher

than the average inductor current at maximum output

current.

Input Capacitor

A ceramic capacitor of 4.7µF or larger with low ESR is

recommended to reduce the input voltage ripple to ensure

a clean supply voltage for the device. The input capacitor

should be placed as close as possible to the MIC2874 VIN

pin with short trace for good noise performance. X5R or

X7R type ceramic capacitors are recommended for better

tolerance over temperature. The Y5V and Z5U type

temperature

rating

ceramic

capacitors

are

not

recommended due to their large reduction in capacitance

over temperature and increased resistance at high

frequencies. These reduce their ability to filter out high-

frequency noise. The rated voltage of the input capacitor

should be at least 20% higher than the maximum

operating input voltage over the operating temperature

range.

Output Capacitor

Output capacitor selection is also a trade-off between

performance, size, and cost. Increasing output capacitor

will lead to an improved transient response, however, the

size and cost also increase. The output capacitor is

preferred in the range of 2.2µF to 10µF with ESR from

10mΩ to 50mΩ, and a 4.7µF ceramic capacitor is typically

recommended. X5R or X7R type ceramic capacitors are

recommended for better tolerance over temperature. The

Y5V

and

Z5U

type

ceramic

capacitors

are

not

recommended due to their wide variation in capacitance

over temperature and increased resistance at high

frequencies. The rated voltage of the output capacitor

should be at least 20% higher than the maximum

operating output voltage over the operating temperature

range.

Power Dissipation Consideration

As with all power devices, the ultimate current rating of the

output is limited by the thermal properties of the device

package and the PCB on which the device is mounted.

There is a simple, Ohm’s law type relationship between

thermal resistance, power dissipation and temperature

which are analogous to an electrical circuit:

Figure 5. Series Electrical Resistance Circuit

From this simple circuit we can calculate VX if we know

ISOURCE, VZ and the resistor values, RXY and RYZ using

Equation 1:

SOURCE

)

Eq. 1

Thermal circuits can be considered using this same rule

and can be drawn similarly by replacing current sources

with power dissipation (in watts), resistance with thermal

resistance (in °C/W) and voltage sources with temperature

(in °C).

Figure 6. Series Thermal Resistance Circuit

Now replacing the variables in the equation for VX, we can

find

the

junction

temperature

(TJ) from the power

dissipation, ambient temperature and the known thermal

resistance of the PCB (

CA) and the package (θJC).

August 26, 2014

Revision 1.0

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