MIC3230_11 Datasheet, PDF (14/19 Pages) Micrel Semiconductor – Constant Current Boost Controller for Driving High Power LEDs

Micrel, Inc.
MIC3230/1/2
(see the current waveforms in Figure 5).
It can be difficult to find large inductor values with high
saturation currents in a surface mount package. Due to
this, the percentage of the ripple current may be limited
by the available inductor. It is recommended to operate
in the continuous conduction mode. The selection of L
described here is for continuous conduction mode.
Eq. (13)
L = ^{VIN × D ×T}
I_{in _ PP}
Using the nominal values, we get:
L = ^{12V × 0.56 × 2μs} = 43μH
0.31A
Select the next higher standard inductor value of 47µH.
Going back and calculating the actual ripple current
gives:
Eq.
(13a)
I_{in _ PP}
= ^{VIN _ nom × Dnom ×T}
L
= ^{12v × 0.56 × 2us}
47uh
= 0.29A_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 10 shows the
following:
Eq. (13b)
( ) ^{( )} I_{IN _ AVE _ max} =
I_{IN _ RMS _ max}
2_−
I_{IN _ PP}
12
2
I_{IN _ AVE _ max} = (1.64)² − (0.29)² / 12 ≈ 1.64A
The Maximum Peak input current I_{L_PK} can found using
equation 11:
I _{L _ PK _ max} = I _{IN _ AVE _ max} + 0.5 × I _{L _ PP _ max} = 1.78A
The saturation current (I_SAT) at the highest operating
temperature of the inductor must be rated higher than
this.
The power dissipated in the inductor is:
Eq.
(13c)
P_INDUCTOR
=
I_in
_
RMS
_
2
max
× DCR
Current Limit and Slope Compensation
Having calculated the I_{L_pk} above, We can set the current
limit 20% above this maximum value:
I_{L _ pkLimit} = 1.2 × 1.6A = 1.9A
The internal current limit comparator reference is set at
0.45V, therefore when V_{IS _ PIN} = 0.45 , the IC enters
current limit.
Eq. (14)
( ) 0.45 = V_APK + Vcs_PK
Where V_APK is the peak of the V_A waveform and
Vcs_PK is the peak of the Vcs waveform
Eq. (14a)
0.45 = I _RAMP × R_SLC × D + I _{L _ pkLimit} × R_CS
To calculate the value of the slope compensation resistance,
R_SLC, we can use Equation 5:
( ) _RSLC
=
V_OUTMAX − V_INMIN × R_CS
L × 250μA × F_SW
First we must calculate R_CS, which is given below in
Equation 15:
Eq. (15)
( ) ^{RCS =}
0.45
VOUT_MAX −VIN_MIN
L × F_SW
× D_max
+ I_{L _ pkLimit}
Therefore;
R_CS
=
(28v
−
0.45
8v )× (0.50)
+ 1.9A
= 179mΩ
47μH × 500kHz
Using a standard value 150mΩ resistor for R_CS, we obtain
the following for R_SLC:
R_SLC
=
(28 − 8)× 150mΩ
47μH × 250μA × 500kHz
=
511Ω
Use the next higher standard value if this not a standard
value. In this example 511Ω is a standard value.
Check: Because we must use a standard value for Rcs and
R I _{SLC; L _pkLimit} may be set at a different level (if the calculated
value isn’t a standard value) and we must calculate the
actual I_{L _pkLimit} value (remember I_{L _pkLimit} is the same as
I_{in _ pkLimit} ).
Rearranging Equation 14a to solve for I_{L _ pkLimit} :
I _{in _ pkLimit}
= ^{(0.45 − IRAMP × RSLC × D)}
R_CS
I_{in _ actualLimit}
=
(0.45 − 250ua × 511× 0.75)
.150
= 2.34A
This is higher than the initial 1.2 × I_{L _PK _ max} = 1.9A limit
because we have to use standard values for R_CS and for
R_SLC. If I_{in _ actualLimit} is too high than use a higher value for
R_CS. The calculated value of R_CS for a 1.9A current limit was
179mΩ. In this example, we have chosen a lower value
which results in a higher current limit. If we use a higher
standard value the current limit will have a lower value. The
designer does not have the same choices for small valued
resistors as with larger valued resistors. The choices differ
from resistor manufacturers. If too large a current sense
resistor is selected, the maximum output power may not be
able to be achieved at low input line voltage levels. Make
sure the inductor will not saturate at the actual current limit
I_{in _ actualLimit} .
Perform a check at I_IN=2.34Apk.
V_{IS _ PIN} = 250μA × (0.78)× 511Ω + 2.34A × 150mΩ = 0.45V
March 2011
14
M9999-030311-D