QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 850A
3V-14.4VIN, BOOST LED DRIVER
LTC3783
DESCRIPTION
Demonstration circuit 850A is a 3V-14.4Vin, Boost
LED Driver featuring the LTC3783 – a high PWM
dimming ratio capable, LED driver controller. This
circuit was designed to attain a highly efficient, out-
put current regulating power supply from a wide in-
put voltage range while achieving an unprecedented
dimming ratio of greater than 1000:1. This circuit
also incorporates an output over voltage protection
feature to limit output voltage to less than 27V if the
LED string is inadvertently disconnected.
Design files for this circuit board are available.
Call the LTC factory.
,
LTC and LT are registered trademarks of Linear Technology Corporation.
Table 1. Performance Summary (T
A
= 25°C)
PARAMETER
Minimum Input Voltage
Maximum Input Voltage
Output Voltage V
OUT
Typical Output Ripple V
OUT
Maximum Output Current
Output Current Regulation
Nominal Switching Frequency
Efficiency
Run Control
V
IN
= 8V, V
OUT
= 20V
Falling Voltage-Off
Rising Voltage-On
Logic Low Voltage-Off
Logic High Voltage-On
Logic Low Voltage-Off
Logic High Voltage-On
Over line and load
V
IN
= 3V to 14.4V
V
IN
= 8V
CONDITION
VALUE
3V
14.4V
V
IN
< V
OUT
< 25V
65mV
P–P
360mA
±2.0%
290kHz
91.5% Typical
1.223V Min
1.348V Typical
0.5V MAX
1.2V Min
0.8V Typical
1.6V Typical
1000:1
SYNC Control
PWM Dimming Control
PWM Dimming Ratio
OPERATING PRINCIPLES
The LTC3783 LED controller will operate when an in-
put voltage of 3V to 14.4V is applied. The exception
is when the RUN terminal, E6, is tied to GND, E7.
This inhibits the LTC3783’s switching and puts it into
shutdown. (This can be accomplished via an open
collector/drain configuration with a transistor or a
mechanical switch.) When the LTC3783 is taken out
of shutdown, it begins a controlled soft-start increase
of the output current.
If the PWMIN pin, E4, is high or left floating, R9 will
conduct the LED current. This current develops a
voltage across R9 that is fed to the FBN pin. This
voltage is compared to the voltage on the FBP pin
which is developed from the resistor divider, R5 and
1
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 850A
3V-14.4VIN, BOOST LED DRIVER
R6, from the reference voltage, VREF. When the volt-
ages at the FBP and FBN pins are equal, the controller
is in output current regulation.
If for any reason the impedance at the LED terminals
gets higher than design limits (i.e. disconnected
LEDs), the LTC3783 will limit the output voltage to its
over voltage point (OVP) of 27V. This ensures no
damage to the circuit components.
PWM dimming is carried out by applying a PWM sig-
nal to the PWMIN terminal, E4. This signal is then
buffered and used to drive the PWM FET, Q3. This
FET interrupts the current through the LEDs, thus ef-
fecting PWM dimming of the LED string. High dim-
ming ratios are achievable due to the output current
state being saved by the controller each time that the
PWM FET is turned off.
The LTC3783 can be synchronized to an external
clock that is faster than the free-running frequency
via the SYNC pin, E5. As always, see the LTC3783
datasheet for more detailed information on all of the
above features.
Depending on system ripple requirements,
fewer/lower value input and output capacitors can be
used. This demo board is shipped with a mere
65mVpp of output and 85mVpp of input ripple!
QUICK START PROCEDURE
Demonstration circuit 850A is easy to set up to evalu-
ate the performance of the LTC3783. Refer to Figure 1
for proper measurement equipment setup and follow
the procedure below:
measuring the input or output voltage
ripple, care must be taken to avoid a long ground lead
on the oscilloscope probe. Measure the output (or
input) voltage ripple by touching the probe tip and
probe ground directly across the VOUT/LED+ (E3)
and GND (E9) terminals. See Figure 2 for proper
scope probe measurement technique.
1.
NOTE:
When
nals in order to get an accurate input voltage
measurement.
3.
Connect a string of LEDs that will yield < 25V at
360mA between the output terminals VOUT/LED+
(E3) to LED- (E8).
Turn on the power to the input power supply.
NOTE:
Make
4.
sure that the input voltage never ex-
ceeds 15V – electrical overstresses can occur.
5.
Set an input power supply that is capable of 3V to
15V at 3A to a voltage of 8V. Then turn off the
supply.
With power off, connect the supply to the input
terminals VIN (E1) and GND (E2).
a.
Check for the proper output current of 360mA ±
2%. If the output current is < 360mA ± 2% and the
output voltage is 25V, the OVP may be activated.
Check connections and LED string voltage and
make adjustments as necessary.
a.
2.
Input voltages lower than 3V can result in too
much power dissipation due to the larger cur-
rents needed to sustain regulation.
If efficiency measurements are desired, an am-
meter capable of measuring at last 3Adc can be
put in series with the input supply in order to
measure the DC850’s input current.
A voltmeter with a capability of measuring at
least 15V can be placed across the input termi-
If efficiency measurements are desired, an am-
meter that is capable of handling 500mA can be
put in series with the output LED string in order
to measure the DC850’s output current.
A voltmeter with a capability of measuring at
least 30V can be placed across the output termi-
nals VOUT/LED+ (E3) and GND (E9) in order to
get an accurate output voltage measurement.
b.
b.
c.
2
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