MIC3263 datasheet(25/32 Pages) MICREL

25 / 32 page

Micrel, Inc.

MIC3263

January 2010

M9999-012110

Using the nominal values, one gets:

12V × 0.66

=22μH

0.36A ×1MHz

t a standard value, use the next higher st

value. Select the standard inductor value of 22µH

Going back and calculating the actual ripple current

gives:

If no

andard

IN(NOM)

NOM

L_PP

V× D

12V × 0.66

= 0.3

L× F

22μH× 1MHz

The average input current is different than the RMS input

ecause of the ripple current. If the ripple current

is low, then the average input current nearly equals the

RMS input current. In the case wh re the average input

current is different than the RMS, Equation 9 shows the

following:

current b

Equation 9

() ()

IN_PP

N_AVE(MAX)

IN_RMS(MAX)

() () ≈

IN_AVE(MAX)

0.36

0.9

0.9A

found

Equation 10

IL_PK(MAX) = IIN_AVE(MAX) + 0.5 ×IL_PP(MAX) = 1.0A

The saturation current (ISAT) at the highest operating

temperature the inductor must be rated higher than this.

The power dissipated in the inductor is:

Equation 11:

PINDUCTOR(max) = IIN_RMS(MAX)

2 × DCR

t # DO3316P-223ML is used in this example.

Its DCR is 85 m

5 mΩ = 67mW

e ILED ripple current is a

impo

r compared to that of the output

hough the two are directly related). To

find the COUT for a required ILED ripple use the following

calculation:

For an output ripple ILED(RIPPLE) = 20mA.

Equation 12:

The Maximum Peak input current IL_PK can

using

Equation 10:

A Coilcraf

Ω, ISAT =2.6A.

PINDUCTOR(MAX) = 0.9 × 8

Output Capacitor

In this LED driver application, th

rtant facto

ripple voltage (alt

LED

OUT

(total)

Ripple

I× D

V× F

V PPLE can usually be kept below 50mV:

0mA

ILED(total) = 6 × 30mA = 18

OUT

0.18A × 0.76

= 2.7μF

50mV ×1Mhz

arge depends upon the

luses and the size of the

se COUT

full output voltage VOUT

flows.

put Capacitor

The input capacitor is shown in the Typical Application.

For superior performance, ceramic capacitors should be

used because of their low Equivalent Series Resistance

(ESR). The input capacitor C ripple current is equal to

the ripple in the inductor. The ripple voltage across the

input capacitor, is the ESR of CIN times the inductor

ripple. The input capacitor will also bypass the EMI

generated by the converter as well as any voltage spikes

generated by the inductance of the input line. For a

required VIN(RIPPLE).

Equation 13:

Use 2.7µF or higher.

The amount that COUT will disch

time between PWM Dimming p

output capacitor. At the next PWM Dimming pul

has to be charged up to the

before the desired LED current

()

IN_PP

IN(RIPPLE)

0.36A

= 0.8μF

8× V

8×50mV × 1MHz

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