MIC3230 Datasheet, PDF (10/20 Pages) Micrel Semiconductor – Constant Current Boost Controller for Driving High Power LEDs

Micrel, Inc.
MIC3230/1/2
Dithering (MIC3231 Only)
The MIC3231 has a feature which dithers the switching
frequency by ±12%. The purpose of this dithering is to
help achieve a spread spectrum of the conducted EMI
noise. This can allow for an overall reduction in noise
emission by approximately 10dB.
Internal Gate Driver
External FETs are driven by the MIC323x’s internal low
impedance gate drivers. These drivers are biased from the
V_DD and have a source resistance of 2Ω and a sink
resistance of 3.5Ω.
V_DD
V_DD is an internal linear regulator powered by V_IN and V_DD
is the bias supply for the internal circuitry of the MIC323x.
A 10µF ceramic bypass capacitor is required at the V_DD pin
for proper operation. This pin is for filtering only and
should not be utilized for operation.
Current Limit
The MIC323x family features a current limit protection
feature to prevent any current runaway conditions. The
current limit circuitry monitors current on a pulse by pulse
basis. It limits the current through the inductor by sensing
the voltage across R_CS. When 0.45V is present at the IS
pin, the pulse is truncated. The next pulse continues as
normally until the IS pin reaches 0.45V and it is truncated
once again. This will continue until the output load is
decreased.
Select R_CS using Equation 5:
Eq. (5)
( ) ^RCS
=
0.45
V − V _OUTMAX
IN _MIN
L × F_SW
×D
+ I _{LPK _ LIMIT}
Slope Compensation
The MIC323x is a peak current mode controller and
requires slope compensation. Slope compensation is
required to maintain internal stability across all duty cycles
and prevent any unstable oscillations. The MIC323x uses
slope compensation that is set by an external resistor,
R_SLC. The ability to set the proper slope compensation
through the use of a single external component results in
design flexibility. This slope compensation resistor, R_SLC,
can be calculated using Equation 6:
( ) _{Eq. (6)}
R_SLC
=
V_OUTMAX −V_INMIN × R_CS
L × 250μA × F_SW
where V_{IN_MAX} and V_{OUT_MAX} can be selected to system
specifications.
Current Sense I_S
The IS pin monitors the rising slope of the inductor current
(m1 in Figure 5) and also sources a ramp current
(250µA/T) that flows through R_SLC that is used for slope
compensation. This ramp of 250µA per period, T,
generates a ramped voltage across R_SLC and is labeled V_A
in Figure 3. The signal at the IS pin is the sum of V_CS + V_A
(as shown in Figure 3). The current sense circuitry and
block diagram is displayed in Figure 4. The IS pin is also
used as the current limit (see the previous section on
Current Limit).
Figure 3. Slope compensation waveforms
Soft Start
The boost switching convertor features a soft start in order
to power up in a controlled manner, thereby limiting the
inrush current from the line supply. Without this soft start,
the inrush current could be too high for the supply. To
prevent this, a soft start delay can be set using the
compensation capacitor (C_COMP in Figure 1). For switching
to begin, the voltage on the compensation cap must reach
about 0.7V. Switching starts with the minimum duty cycle
and increases to the final duty cycle. As the duty cycle
increases, V_OUT will increase from V_IN to it’s final value. A
6µA current source charges the compensation capacitor
and the soft start time can be calculated in Equation 7:
Eq. (7) T_SOFTSTART
≈ ^{CCOMP × VCOMP_STEADY_STATE}
6μA
V_{COMP_STEADY_STATE} is usually between 0.7V to 3V, but can
be as high as 5V.
( ) Eq. (8) V_{COMP _ STEADY _ STATE} = Ai × V_APK + Vcs_PK
Where: V_APK
=
I _RAMP
T
×
R_SLC
×
D×T
and
V_CSPK = I_{L_ PK} × R_CS
Ai = 1.4 V/V
D = Duty cycle (0 to1)
T = period
A 10nF ceramic capacitor will make this system stable at
all operating conditions.
January 2009
10
M9999-011409-A