QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 994
LT3003
DESCRIPTION
Demonstration circuit 994 features the LT3003 three
channel LED ballaster with PWM in a 10-lead MSOP
thermally enhanced MSE package. The demonstration
circuit is designed to ballast three channels of LED
current from 100mA to 350mA per channel. The
PWM dimming function of the LT3003 eliminates the
need for discrete MOSFETs. The LT3003 internally
performs the level-shift required for non-ground
referred PWM dimming in the buck mode and buck-
boost mode topologies allowing PWM dimming input
requirements to remain at logic-level. PWM dimming
capabilities reach 3000:1 at 100Hz. Over temperature
protection provides two open-collector pull-down
outputs for direct interface with the compensation pin
of most LT LED drivers and for use with a
microcontroller. Additional thermal shutdown
prevents the LT3003 from getting too hot.
The LT3003 can be configured in all LED driver
topologies as long as there are three strings of LEDs
and the pin voltage requirements are met. The boost,
buck mode, buck-boost mode, SEPIC, and buck LED
driver topologies can all use the LT3003 three
channel ballaster with PWM. The shutdown pin can
be tied to the switcher shutdown pin or in buck-boost
mode it must be tied to the PWM pin.
When a single LED channel is open and has no
current, the other two channels cut back their current
to zero or almost zero in order to protect themselves.
The LED current per string rating is between 100mA
and 350mA. The maximum voltage on VMAX pin is
48V and the maximum voltage on VIN pin is 40V.
For more details, please read the datasheet.
Design files for this circuit board are available. Call
the LTC factory.
LT is a registered trademark of Linear Technology Corporation
THREE CHANNEL LED BALLASTER WITH PWM
QUICK START PROCEDURE
Follow the test procedure outlined below and see
Figure1 to setup and test the LT3003 in the boost LED
driver topology. The LT3003 needs to run with three
strings of LEDs and an LED driver. The LEDs and LED
driver do not come with DC994A, it is a standalone
demonstration circuit that is meant to be used with
different LED drivers in different topologies for
evaluation.
1. Connect the three strings of LEDs from the
LED+ terminal of the boost LED driver to the
three LED terminals of the DC994A.
2. Connect the VEE terminal of the DC994A to
the LED- (GND) terminal of the boost LED
driver.
3. Connect the shutdown terminal of the
DC994A to the shutdown terminal of the LED
driver or to the VIN terminal of the DC994A if
the shutdown function is not being used.
4. Connect the VMAX terminal of the DC994A to
the output voltage at the output capacitor of
the boost LED driver. The DC841A shown in
Figure 1 does not have a terminal for this
node, so a wire has to be soldered to the
output capacitor.
5. Connect a 3.3V or 5V bench supply with
power OFF to the VIN and GND terminals of
DC994A.
6. Connect a 3V to 10V bench supply with power
OFF to the VIN and GND terminals of DC841A.
7. Turn on the input power supply for the
DC994A.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 994
8. Turn on the input power supply for the boost
LED driver and set the voltage.
9. Observe the LEDs working properly at 100mA
to 350mA per string without looking straight
at the LEDs. The extreme brightness may be
harmful to the eye.
10. For PWM dimming, tie the PWM terminal of
the DC994A to the PWM dimming input of the
boost LED driver if the driver has a PWM dim
input. Attach a logic-level PWM dimming
pulse waveform to the PWM terminal of the
DC994A at 100Hz with as high as 3000:1
PWM dimming ratio determined by the boost
LED driver. Observe the PWM dimming
brightness of the LEDs without looking
straight at them.
THREE CHANNEL LED BALLASTER WITH PWM
Figure 1 Proper Measurement Equipment Setup with
DC841A in Boost Mode JP1=JP2=open
DC994A OPTIONS
VIN PIN VOLTAGE RANGE
The operating pin voltage range for VIN pin on the
LT3003 is 3V to 40V. In most cases, it is not desirable to
tie VIN to the highest rail available due to both excessive
IC power consumption and to LED string current match-
ing accuracy. It is most optimal to tie the VIN pin to a
voltage that is between 3V and 7V higher than the VEE
pin voltage. In the cases of boost LED driver and buck
LED driver (LT3474 / LT3475), when the VEE pin is tied
to ground, a 3.3V or 5V supply on the VIN pin of the
LT3003 is optimal for best LED string current matching.
In buck-boost mode and buck mode LED topologies, the
VIN pin of the LT3003 should be tied to the VMAX pin
rail (Vout and Vin of the LED driver respectively) through
a zener diode that has a breakdown voltage of Vf =
VfLED – 5V approximately. This makes the VIN pin volt-
age about 4V above the VEE pin voltage that is not
ground-referred.
PWM DIMMING
DC994A is designed to provide LED PWM dimming in
every LED driver topology with a logic-level PWM dim-
ming input directly tied into the PWM pin of the LT3003.
The necessary level-shifting of the PWM dimming signal
is designed into the IC, eliminating extra level-shifting
discrete components from the circuit in certain topolo-
gies and replacing the PWM dimming MOSFET in all to-
pologies.
Some LED drivers have a PWM pin that can be tied to
the PWM pin of the LT3003 and to the same PWM signal
as shown in Figures 2, 3, and 4. The LT3003 PWM func-
tion also works with LED drivers that do not have a
PWM pin. Please refer to the datasheet for details.
When not in use, the PWM pin of the LT3003 can be left
floating. There is an internal pull-up on the PWM dim-
ming pin that eliminates the need to tie this pin high. In
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 994
buck-boost mode, the LT3003 PWM pin is shown to be
tied to the shutdown pin of the LT3003. This is needed
when PWM dimming is being used so that the VIN pin
current does not drain the output capacitor during PWM
off time by putting the IC in shutdown. If this is not an
issue, the shutdown pin can be released from the PWM
pin and should be tied high or to the system shutdown
signal instead. When the two pins PWM and /SHDN are
not tied together and PWM dimming is being used, the
maximum PWM dimming ratio is reduced when com-
pared with the case where the two pins are tied together.
BOOST, BUCK, BUCK MODE, AND BUCK-BOOST
MODE TOPOLOGIES
The LT3003 can be configured to work with any LED
driver topology. The different topologies such as boost,
buck, buck mode, and buck-boost mode are shown in
the LT3003 datasheet and below. The different topolo-
gies accommodate the different relationships between
the input voltage range and the LED string voltage range.
The LT3003 datasheet covers the different topologies as
shown in the figure below. Although these topology
schematics are shown in the LT3003 datasheet, the VIN
pin should not generally be tied directly to VIN or VOUT
of the LED driver circuit. A zener diode or other supply
should be used with the VIN pin to keep the VIN – VEE
pin voltage between 3V and 7V for minimal IC heating
and for optimal LED string current matching.
Figure 2 Boost LED driver topology
THREE CHANNEL LED BALLASTER WITH PWM
Figure 3 Buck mode LED driver topology
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 994
THREE CHANNEL LED BALLASTER WITH PWM
Figure 4 Buck-boost mode LED driver topology
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 994
THREE CHANNEL LED BALLASTER WITH PWM
Figure 5 DC994A schematic with features
Tie LED1/2/3 to
three LED strings
LED3
LED2
LED1
1
2
3
4
5
U1
Tie VEE to boost
LED driver ground
VEE
VMAX
3V-48V
VIN
3.3V or 5V
Tie VIN to 3.3V
or 5V source
VIN
C3
R1
1M
11
Tie VMAX to boost
LED driver VOUT
GND
LT3003EMSE
10
LED1
VEE
SHDN
9
LED2
SHDN
8
OT2
LED3
OT2
Tie /OT2 to LED drive r gm output (Vc)
7
VMAX
OT1
6
OT1
VIN
PWM
Tie /SHDN to LED
driver /SHDN
*
PWM
PWM logic level input may
also be tied to LED driver
GND
1uF 6.3V
*
/SHDN of
*
Ifxte rnally, LT3003 is pulle d-upde d.
e
R1 m ay not be nee
Figure 6 DC994A
boost
LED driver setup - /OT1 & /OT2 terminals, /SHDN terminal*, PWM terminal float is okay
(JP1, JP2 = OPEN)
5
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