MIC3263 Datasheet, PDF (16/32 Pages) Micrel Semiconductor – Six-Channel WLED Driver for Backlighting Applications with Flicker-Free Dimming

Micrel, Inc.
MIC3263
The input frequency to the DRC pin can be 100Hz to
40kHz and the internal dimming frequency DR will be
determined by R_DFS.
The duty cycle of the input frequency at DRC is converted
according to Table 2 for the actual dimming duty cycle.
For direct dimming control, connect DFS to V_DD and use
the MODE pin for the input dimming pulse. This method by
passes the internal dimming control and allows for
dimming control by the external PWM.
DFS Filter
In addition to the R_DFS resistor-to-ground at the DFS pin, a
series RC filter is required when operating at dimming
frequencies below 1kHz. The reason is that the DFS pin is
the output of a transconductance differential amplifier. The
differential amplifier has a high-frequency pole.
At low dimming frequencies of around 1kHz R_DFS is high
around 100kΩ and the differential amplifier pole produces
a phase shift that can cause instabilities in the DFS
control.
Therefore, a RC filter is required to compensate for the
lagging phase shift created by the pole by adding a zero
and therefore, a phase lead at the DFS pin. Use a 4kΩ
resistor in series with a 2.2nF ceramic capacitor. When
using a dimming frequency of 2 kHz or less. The filter has
no ill effect at higher dimming frequencies.
COMP
Connect a capacitor and resistor to ground to compensate
the boost stage.
DRC
Dimming Ratio Control (DRC) is an input PWM dimming
control. The MIC3263 converts this to one of sixteen
dimming ratios that is used to dim the LEDs. The dimming
ratio is built on a log scale.
CINT
C_INT integrates the DRC input pulse. For a PWM frequency
range of around 1kHz use 100nF. For a PWM frequency
range of around 20kHz pulse, use 10nF. For a PWM
frequency range of around 100Hz pulse use 1μF.
ISET
Set the LED current of all six channels by this resistor. Use
2kΩ for 30mA and 3kΩ for 20mA. The R_ISET is inversely
proportional to I_LED. Use the following equation to find
R_ISET:
RISET
=
60
I_LED
Ω
For the best current matching accuracy design for an I_LED
current of 15mA to 30mA.
CRV
Use a 2.2μF capacitor at the CRV pin. This is used as the
reference voltage of the boost stage. The CRV capacitor is
continually being charged or discharged in order to keep
V_OUT at the right level (refer to Functional Diagram
illustration). CRV will be charged to keep the IO’s at about
1.2V.
IO1─IO6
These are the connections to the linear-mode current
amplifier in each channel. Connect the cathode end of the
LED channels to these pins. The control loop will keep this
at about 1.2V. 1.2V insures that the current amplifier is in
the linear region and therefore can regulate the LED
current.
In cases where there are a different number of LEDs in a
channel, the V(IO) of the channel with the fewest LEDs will
have a higher V(IO). V(IO) can be as high as 7.5V before
the fault monitoring circuits will sense that channel as a
short to V_OUT.
When there are a different number of LEDs in a channel
the IO voltage will be higher in the channels that have less
LEDs in order to keep the LEDs biased correctly. A
difference of up to 7.5V between channels can occur
because of this. If the circuits that monitors the IO pins
sees a fault, that channel will turn off and that channel’s IO
pin will be at high impedance. An off channel’s IO pin will
be near or below the booster output voltage. On a channel
that has a shorted LED, that channel’s IO voltage will
increase to keep correct voltage drops on the other series
LEDs. It is best to use equal number of LEDs in each
channel but there will always be differences in the LEDs
voltage drops so all IOs will not have the exact same
voltage. Each channel has its own monitoring circuit
monitoring the IO1─IO6 pins. If any V(IO) drops below
0.5V (if an LED opens), that channel is turned off and the
other channels are unaffected. If any IO goes about 7.5V
(if several LEDs short to V_OUT), that channel is turned off
and the other channels are unaffected.
VSW
This is the boost-stage switch node, the collector of the
internal power switch.
EN
Connect EN high to enable the part, low to disable. Do not
leave the EN pin floating.
VIN
Supply voltage to the part (6V–40V).
January 2010
16
M9999-012110