lt3743 Linear Technology Corporation, lt3743 Datasheet - Page 12

• Manufacturer Part Number: lt3743
• Description: High Current Synchronous Step-down Led Driver With Three-state Control
• Manufacturer: Linear Technology Corporation

LT3743
OPERATION
The LT3743 utilizes fi xed frequency, average current
mode control to accurately regulate the inductor current,
independently from the output voltage. This is an ideal
solution for applications requiring a regulated current
source including driving high current LEDs where the
forward junction voltage can range from 2V to 6V with a
dynamic resistance of 20mΩ to 40mΩ. The control loop
will regulate the current in the inductor at an accuracy
of 6%. For additional operation information, refer to the
Block Diagram in Figure 1.
The control loop has two independent reference inputs,
determined by the analog control pins, CTRL_H and
CTRL_L. When the CTRL_SEL pin is low, the control loop
uses the reference determined by the CTRL_L pin and
when high, the loop uses the reference determined by
the CTRL_H pin. The analog voltage at the CTRL_L and
CTRL_H pins is buffered and produces a reference volt-
age across an internal resistor. The internal buffers have
a 1.5V clamp on the output, limiting the analog control
range of the CTRL_L and CTRL_H pins from 0V to 1.5V.
The average current mode control loop uses the internal
reference voltage to regulate the inductor current, as a
voltage drop across the external sense resistor, R
In many applications, a rapid transition between the two
regulated current states is desirable to provide background
LED color mixing for pure colors in an RGB projector or
display. For this purpose, pulse width modulation dimming
can be achieved with both the PWM and CTRL_SEL pins.
When the PWM pin is low, the regulated current in the
inductor is zero and both output capacitors are discon-
nected. When the PWM pin is high, and the CTRL_SEL pin
is low, the regulated current in the inductor is determined
by the analog voltage at the CTRL_L pin. When the PWM
and CTRL_SEL pins are both high, the regulated current
in the inductor is determined by the analog voltage at the
CTRL_H pin.
The LT3743 uses a unique switched output capacitor
topology and two independent compensation networks
to transition between the two regulated current states in
less than 2μs. When the CTRL_SEL pin is low and the
PWM pin is high, the PWMGL output pin is high, switch-
ing in the output capacitor for the CTRL_L current level.
The CTRL_L output capacitor stores the LED forward
12
S
.
voltage drop when the control loop regulates the low cur-
rent level. When the CTRL_SEL pin changes to the high
state, a 150ns delay ensures that the output capacitors
are not connected at the same time. After this delay, the
output capacitor for the CTRL_H level is switched in when
PWMGH goes high and immediately delivers current to the
LED. The CTRL_H output capacitor has the voltage drop
of the LED with the regulated current determined by the
analog voltage at the CTRL_H pin. To achieve minimum
transition delay, the inductor is precharged to 70% of the
regulation current level just after the PWMGH pin goes
high. Conversely, when the PWM pin goes low, the induc-
tor is discharged to 70% of the low current level before
normal switching at the low current level commences.
The error amplifi er for the average current mode control
loop also has a common mode lockout that regulates the
inductor current so that the error amplifi er is never oper-
ated out of the common mode range. The common mode
range is with an output voltage from 0V to 2V below the
V
The overcurrent set point is equal to the high level regulated
current level set by the CTRL_H pin with an additional
23mV offset between the SENSE
overcurrent is limited on a cycle-by-cycle basis; shutting
switching down once the overcurrent level is reached.
Overcurrent is not soft started.
The output voltage may be limited with a resistor divider
from the output back to the FB pin. The reference at the
FB pin is 1.0V. If the output voltage level is high enough
to engage the voltage loop, the regulated inductor current
will be reduced so that the output voltage is limited. If the
voltage at the FB pin reaches 1.3V (30% higher than the
regulation level), an internal open-LED fl ag is set, shutting
down switching for 13μs and switching in both output
capacitors to fully drain the inductor’s current.
During start-up, the SS pin is held low until the fi rst time
the PWM pin goes high. Once the PWM signal goes high,
the capacitor at the SS pin is charged with a 10μA current
source. The internal buffers for the CTRL_L and CTRL_H
signals are limited by the voltage at the SS pin, slowly
ramping the regulated inductor current to the current de-
termined by the voltage at the CTRL_H or CTRL_L pins.
IN
supply rail.
+
and SENSE
–
pins. The
3743f