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Micrel, Inc.
Switching Frequency Set R FSW
To fi nd t he value of R FSW use the following equatio n:
R FSW (k ? ) ≈ 500 ? 0.3 × f SW (kHz )
R FSW (k ? ) ≈ 5 00 ? 0.3 × (1000) = 200k ?
Use 200kHz for R FSW (R5).
MIC3263
Inductor Selection
First calculate the RMS input current (n ominal, minimum,
and maximum) for the syste m given the operating
conditions listed in the desig n example table. The
minimum val ue of the RMS input current is necessary to
ensure proper operation. Using Equation 7, the following
values have been calculated:
Equation 7 :
Dim mi n g Frequency Sele c t Resis tor R DFS
F DIM is 5kHz therefore HF m ode is used. Connect MODE
to V DD . To find R DFS (R8) use the following equation:
R DFS (in k ? ) = 432 ? 20 × F DIM (in kHz)
= 432 ? 20 × 10 = 232(k ? )
The input frequency to the DRC pin can b e 10 0Hz to
I IN_RMS(MAX) =
I IN_RMS(NOM) =
I IN_RMS(MIN) =
V OUT(MIN) ×I OUT(MIN)
= 0.43A (RMS)
eff × V IN(MAX)
V OUT(NOM) ×I OUT(NOM)
= 0.53A (RMS)
eff × V IN(NOM)
V OUT(MAX) ×I OUT(MAX)
= 0.9A (RMS)
eff × V IN(MIN)
40kHz and the internal dimming frequ ency DR will
always be 5kHz.
The duty cycle of the input frequency a t D RC is
converte d according to Table 2 for the actual d imming
duty cycle.
Since the dim ming frequency is high the filter R6 and C6
is not necessary. They may be used with no ill effe ct.
Operating Duty Cycle
The operating duty cycle can be calculated using
Equation 6.
Equation 6:
I OUT is the same as I LED tot al
Selecting the inductor current (p eak-to-peak) I L_PP to be
between 20% to 50% of I IN_RMS(m ax) , in this case 40%, we
obtain:
I L_PP(MAX) = 0.40 × I IN_RMS(MAX) = 0 .4 × 0.9 = 0.36A PP
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
D NOM =
D MAX =
( V OUT(NOM) - eff × V IN(NOM) )
( V OUT(MAX) - eff × V IN(MAX) )
V OUT(NOM)
V OUT(MAX)
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:
D MIN =
( V OUT(MAX) - eff × V IN(MAX)
V OUT(MAX)
)
L=
V IN × D
I L_PP × F SW
Therefore D NOM = 66%, D MAX = 80% and D MIN = 58%.
January 2010
24
M9999-012110
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