MIC3263 Datasheet, PDF (25/32 Pages) Micrel Semiconductor – Six-Channel WLED Driver for Backlighting Applications with Flicker-Free Dimming

Micrel, Inc.
Using the nominal values, one gets:
12V × 0.66
L=
= 22μH
0.36A × 1MHz
If not a standard value, use the next higher standard
value. Select the standard inductor value of 22µH.
Going back and calculating the actual ripple current
gives:
I_{L_PP}
=
V_IN(NOM) × D_NOM
L × F_SW
=
12V × 0.66
22μH × 1MHz
=
0.36A _PP
The average input current is different than the RMS input
current because of the ripple current. If the ripple current
is low, then the average input current nearly equals the
RMS input current. In the case where the average input
current is different than the RMS, Equation 9 shows the
following:
Equation 9:
( ) ^{( ) I}_{IN_AVE(MAX)} ⁼
2
IIN_RMS(MAX) -
I_{IN_PP}
12
2
( ) ^{( )} I_{IN_AVE(MAX)} =
2
2 0.36
0.9 -
≈ 0.9A
12
The Maximum Peak input current I_{L_PK} can found using
Equation 10:
Equation 10:
I_{L_PK(MAX)} = I_{IN_AVE(MAX)} + 0.5 ×I_{L_PP(MAX)} = 1.0A
The saturation current (I_SAT) at the highest operating
temperature the inductor must be rated higher than this.
The power dissipated in the inductor is:
Equation 11:
P_{INDUCTOR(max)} = I_{IN_RMS(MAX)2} × DCR
MIC3263
A Coilcraft # DO3316P-223ML is used in this example.
Its DCR is 85 mΩ, I_SAT =2.6A.
P_{INDUCTOR(MAX)} = 0.9² × 85 mΩ = 67mW
Output Capacitor
In this LED driver application, the I_LED ripple current is a
more important factor compared to that of the output
ripple voltage (although the two are directly related). To
find the C_OUT for a required I_LED ripple use the following
calculation:
For an output ripple I_LED(RIPPLE) = 20mA.
Equation 12:
C_OUT
=
I_LED(total) × D
V_Ripple × Fsw
V_RIPPLE can usually be kept below 50mV:
I_LED(total) = 6 × 30mA = 180mA
0.18A × 0.76
C_OUT = _{50mV × 1Mhz} = 2.7μF
Use 2.7µF or higher.
The amount that C_OUT will discharge depends upon the
time between PWM Dimming pluses and the size of the
output capacitor. At the next PWM Dimming pulse C_OUT
has to be charged up to the full output voltage V_OUT
before the desired LED current flows.
Input 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_IN ripple current is equal to
the ripple in the inductor. The ripple voltage across the
input capacitor, is the ESR of C_IN 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 V_IN(RIPPLE).
Equation 13:
( ) _{CIN =}
I_{IN_PP}
8 × V_IN(RIPPLE) × FSW
0.36A
=
8 × 50mV × 1MHz
= 0.8μF
January 2010
25
M9999-012110