MIC3263 Datasheet, PDF (11/32 Pages) Micrel Semiconductor – Six-Channel WLED Driver for Backlighting Applications with Flicker-Free Dimming

Micrel, Inc.
MIC3263
Functional Description
The MIC3263 is a six-channel LED driver. A constant
output current converter is the preferred method for driving
LEDs. The MIC3263 is specifically designed to operate as
a constant-current LED driver to keep the current in all six
channels constant. PWM dimming is employed in each
channel. Each channel of LED current is individually and
tightly regulated during each Duty Ratios (DR) on-time.
During the DR off-time the LED current is turned off. The
duty cycle of the DR pulse determines the brightness of
the LEDs. The MIC3263 is designed to operate as a boost
controller in which the output voltage is higher than the
input voltage. This configuration allows for the design of
multiple LEDs in series to help maintain color and
brightness. During each DR pulse off-time the boost
converter is turned off (not switching). The boost converter
is on (switching) during each DR pulse on-time.
The MIC3263 has a very-wide input voltage range of 6V
and 40V to help accommodate for a diverse range of input
voltage applications. In addition, the LED current can be
programmed through the use of an external resistor
(R_ISET). This provides design flexibility in adjusting the
current for a particular application. The MIC3263 can
control the brightness of the LEDs via its PWM dimming
capability. Applying a PWM dimming signal (up to 40kHz)
to the DRC pin allows for control of the brightness of the
LED. It has a boost stage that boosts the V_IN to a high
enough voltage to forward bias the LED channels. The
MIC3263 is a constant current controller. The controller
keeps the current in each of the six channels at a constant
value. Each channel has an independent current regulator
in series with each LED channel. The current in each
channel is within 3% of the others.
The MIC3263 uses three main control loops (Figure 1
control loops):
1) Current Amp loop (Fastest)
2) Booster loop (Fast)
3) Capacitor Reference Voltage (CRV) loop (Slow)
The current Amplifier Loop is faster than the Boost Loop
and CRV Loop. CRV is the reference voltage for the boost
error amp.
Figure 1. Constant-Current Control Loops
Figure 2. Simplified Control Loop
The objective of these loops is to keep the LED current
constant. The boost output voltage V_OUT will vary when
CRV changes. V_OUT will be what it needs to be to keep
ILEDs constant. The current amp loop is so fast the other
loops can be viewed as static DC values. On a pulse to
pulse basis the boost loop is fast enough that CRV is a
constant value.
The goal of the CRV loop is to keep the collector’s voltage
V(IO1–IO6) at or about 1.2V, thereby keeping the bipolar
transistor in the linear region and also keep the power loss
across the bipolar as low as possible. Keeping the bipolar
in the linear region allows the current amp loop to be able
to regulate the LED current.
January 2010
11
M9999-012110