MIC3263 datasheet(24/32 Pages) MICREL

24 / 32 page

Micrel, Inc.

MIC3263

Switching Frequency Set RFSW

January 2010

M9999-012110

To fi

following equatio

)

− 0.3 × (1000) = 200kΩ

Dim

tor RDFS

FDIM

ode is used. Connect MODE

to VDD. To find RDFS (R8) use the following equation:

DIM

e 10

ency DR will

t D

d according to Table 2 for the actual d

ming frequency is high the filter R6

is not necessary. They may be used with no ill effe

Operating Duty Cycle

The operating duty cycle can be calculated

Equation 6.

Equation 6:

nd he value of RFSW use the

RFSW(kΩ) ≈ 500 − 0.3 × fSW(kHz

(kΩ)

≈ 5

FSW

Use 200kHz for RFSW (R5).

mi g Frequency Sele t Resis

is 5kHz therefore HF m

RDFS(in kΩ) = 432 − 20 × F (in kHz)

= 432

− 20 × 10 = 232(kΩ)

The input frequency to the DRC pin can b

40kHz and the internal dimming frequ

0Hz to

always be 5kHz.

The duty cycle of the input frequency a

converte

RC is

imming

and C6

ct.

using

duty cycle.

Since the dim

()

OUT(NOM)

IN(NOM)

NOM

V- eff × V

OUT(NOM)

()

OUT(MAX)

IN(MAX)

MAX

OUT(MAX)

V- eff V

()

OUT(MAX)

IN(MAX)

MIN

OUT(MAX)

V- eff V

Therefore DNOM = 66%, DMAX = 80% and DMIN = 58%.

ominal, minimum,

m given the operating

n example table. The

ue of the RMS input current is necessary to

ensure proper operation. Using Equation 7, the following

values have been calculated:

Equation 7

Inductor Selection

First calculate the RMS input current (n

and maximum) for the syste

conditions listed in the desig

minimum val

OUT(MIN)

IN_RMS(MAX)

(RMS)

IN(MAX)

V× I

0.43A

eff × V

OUT(NOM)

IN_RMS(NOM)

(RMS)

IN(NOM)

V× I

= 0.53A

eff × V

OUT(MAX)

IN_RMS(MIN)

eff × V

(RMS)

IN(MIN)

V× I

= 0.9A

eak-to-peak) IL_PP to be

ax), in this case 40%, we

.4 × 0.9 = 0.36APP

There is a trade off between the inductor value and the

minimum PWM dimming pulse. The larger the inductor,

the longer the PWM dimming pulse time will be. Due to

this, the percentage of the ripple current may be limited

by the required PWM dimming pulse. Also, the internal

current amplifiers will attenuate the LED ripple current by

more than a magnitude. It is recommended to operate in

the continuous conduction mode. The value of “L” in

Equation 8 represents Continuous Conduction Mode.

Equation 8:

IOUT is the same as ILED tot

Selecting the inductor current (p

between 20% to 50% of IIN_RMS(m

obtain:

IL_PP(MAX) = 0.40 × IIN_RMS(MAX) = 0

L_PP

V× D

I× F

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