mic3230 Micrel Semiconductor, mic3230 Datasheet - Page 10

• Manufacturer Part Number: mic3230
• Description: Constant Current Boost Controller For Driving High Power Leds
• Manufacturer: Micrel Semiconductor

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
resistance of 3.5Ω.
V
V
is the bias supply for the internal circuitry of the MIC323x.
A 10µF ceramic bypass capacitor is required at the V
for proper operation.
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
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.
Eq. (5)
Slope Compensation
The MIC323x is a peak current mode controller and
requires slope compensation.
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
through the use of a single external component results in
design flexibility. This slope compensation resistor, R
can be calculated using Equation 6:
where V
specifications.
Eq. (6)
Micrel, Inc.
Select R
January 2009
DD
DD
DD
SLC
is an internal linear regulator powered by V
. The ability to set the proper slope compensation
and have a source resistance of 2Ω and a sink
R
IN_MAX
CS
SLC
R
using Equation 5:
CS
=
=
and V
(
V
(
OUT
V
L
OUT
CS
×
MAX
OUT_MAX
250
. When 0.45V is present at the IS
MAX
This pin is for filtering only and
L
−
−
×
V
μ
A
IN
V
F
×
IN
SW
MIN
can be selected to system
F
MIN
. 0
SW
)
×
45
Slope compensation is
)
R
×
CS
D
+
I
L
PK
_
LIMIT
IN
and V
DD
SLC
pin
DD
,
10
Current Sense I
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
compensation.
generates a ramped voltage across R
in Figure 3. The signal at the IS pin is the sum of V
(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).
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
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
6µA current source charges the compensation capacitor
and the soft start time can be calculated in Equation 7:
Eq. (7)
V
be as high as 5V.
A 10nF ceramic capacitor will make this system stable at
all operating conditions.
Eq. (8)
COMP_STEADY_STATE
T
V
Where:
SOFTSTART
Figure 3. Slope compensation waveforms
COMP
OUT
_
STEADY
V
Ai = 1.4 V/V
D = Duty cycle (0 to1)
T = period
will increase from V
V
S
CS
A
This ramp of 250µA per period, T,
PK
is usually between 0.7V to 3V, but can
PK
≈
=
_
=
STATE
C
I
I
L
COMP
RAMP
_
T
PK
COMP
=
×
×
Ai
R
×
R
in Figure 1). For switching
CS
SLC
V
×
SLC
COMP_STEAD
(
V
IN
that is used for slope
6
×
A
SLC
to it’s final value. A
μA
PK
D
+
×
and is labeled V
M9999-011409-A
Vcs
T
MIC3230/1/2
Y_STATE
and
PK
)
CS
+ V
A
A