DC1521A - Demo Manual

DEMO CIRCUIT 1521A
LT3956
QUICK START GUIDE
LT3956
High Voltage Monolithic LED Driver
DESCRIPTION
Demonstration circuit 1521A is a high voltage monolithic LED driver with an integrated 3.3A, 84V power
switch. The demonstration circuit is assembled as a
boost topology from 6V-80V* input and up to 67V
LED output at 370mA (*VIN must not exceed VLED
for proper operation). DC1521A features PWM dimming, analog dimming, shutdown, undervoltage lockout, an open LED status pin, and open LED overvoltage protection. Efficiency at 12VIN is as high as 94%
for 67VLED at 370mA output.
The switching frequency is set at 375kHz although the
range for the IC is 100kHz to 1MHz. The VMODE
pin/terminal provides an output flag in case the LEDs
are detached from the circuit. EN/UVLO provides undervoltage lockout with hysteresis and a shutdown
function that is used by simply tying the pin/terminal
to GND. CTRL provides analog dimming by simply
pulling the pin/terminal low or by reducing VIN below
11V. Analog dimming with low VIN is changed or
removed by changing the values of the EN/UVLO resistors that are used. PWM dimming changes the
brightness of the LEDs without changing the LED current. A PWM dimming pulse can be tied to the PWM
pin/terminal. If PWM is not used, the pin must still be
pulled high and it can be attached to INTVCC or another source.
DC1521A PWM dimming is simple. The boost topology LED string returns to the LED- terminal and the
PWM dimming MOSFET between LED- and PGND
turns the LED string on and off given an input to the
PWM dimming terminal. For the highest pwm dimming ratio, it is recommended to use 100Hz as a pwm
dimming frequency. Information regarding PWM
dimming ratios and performance can be found in the
LT3956 datasheet in the applications section.
Soft-start is provided with a simple and small capacitor to maintain control of the switch current during
turn on for high power designs.
Modifications can be made to the DC1521A in order
to convert the board from a boost topology LED driver
to any other topology. Buck-boost mode, buck mode,
and SEPIC schematics are provided in the datasheet.
Please consult the factory or LT3956 datasheet for
details regarding how to customize the DC1521A or
how to design different topologies for custom specifications.
The LT3956 datasheet gives a complete description of
the part, operation and applications information. The
datasheet must be read in conjunction with this Quick
Start Guide for demonstration circuit 1521A. The
LT3956 is assembled in a 36-lead plastic UHE package with two thermally enhanced pads for SW and
GND. Proper board layout is essential for both proper
operation and maximum thermal performance. See the
datasheet section ‘Layout Considerations’.
Design files for this circuit board are available. Call
the LTC factory.
LT is a trademark of Linear Technology Corporation
1
LT3956
PERFORMANCE SUMMARY SpecificationsareatTA= 25°C
PARAMETER
Input Voltage VIN Range
LED String Voltage Range
Switching Frequency
ILED
Maximum Output Voltage - Open LED Voltage
Efficiency
VMODE
Undervoltage Lockout
Peak Switch Current Limit
UVLO
CONDITION
Operating
Operating
R1 = 28.7k
11V < VIN < VLED
VIN=12V VLED=67V ILED =370mA
VIN=12V VLED=67V ILED =370mA
VIN=12V OPEN LED VOUT=78V
Minimum IC Rating
Undervoltage Lockout
VALUE TYPICAL)
6V to VLED (see text)
VIN < VLED < 67V
375kHz
370mA
78.4V
94%
7.1V
0V
5.5V UVLO w/ 6V rising turn-on
3.3A
VIN Rising, 0°C < TA < 85°C
VIN Rising
VIN Falling
QUICK START PROCEDURE
Demonstration circuit 1521A is easy to set up to
evaluate the performance of the LT3956. Follow the
procedure below:
NOTE. Make sure that the VIN DC input voltage does not exceed 80V
or the LED string voltage at 370mA. Start with all power supplies
turned off.
1. Connect a string of LEDs with forward voltage less
than 68V (when running at 370mA) to the LED+
and LED- terminals on the PCB as shown in Figure 1.
2. Connect the EN/UVLO terminal to GND.
3. With power off, connect the VIN power supply to
the PVIN (or VIN) and GND terminals. Make sure
that the power setting on the supply is not set
above 80V or above the expected LED string voltage before the supply is tied to VIN and turned on.
2
4. Connect 5V or a 5V PWM waveform to the PWM
terminal. If PWM is not used, connect PWM to a
5V source or to IntVcc by stuffing a zero ohm resistor in R14 on the PCB. PWM must be pulled
high for the LEDs to run.
5. Turn the VIN power supply on.
6. Release the connection between EN/UVLO and
GND.
7. Observe the LED string running at the programmed LED current.
8. For PWM dimming, connect a PWM 100Hz or
higher signal to the PWM terminal and observe
the reduction of brightness in the LED string related to the PWM dimming duty cycle.
LT3956
Figure 1. Proper Measurement Equipment Setup
TERMINAL OPTIONS
DC1521A has both PVIN and VIN terminals. The schematic shows that the PVIN and VIN terminals are
shorted together with a resistor R12. With R12 in
place, power can be supplied to either PVIN or VIN in
order to run the circuit as a boost. The VIN range of
6V to 80V is the operational range of the IC. However,
for proper operation, the input voltage should not exceed the LED string voltage since this would cause
excessive load currents in a boost. If the board is reconfigured as a different topology, R12 can be removed and the VIN terminal and pin can be powered
from a separate source or tied to a different input.
The OUT terminal is available for the customer to
monitor the state of the output capacitor directly. The
OUT terminal can also be used as an input for buck
mode topology. In general, OUT is 250mV higher than
the VLED string voltage when the LEDs are running at
full power.
The PWM terminal must be pulled high in order for
the board to operate. It is preferred that the PWM terminal/pin is tied to IntVcc of the LT3956 by placing a
zero ohm resistor in R14 if PWM dimming is not
used. For board evaluation it can be tied to an external
5V source as well.
3
LT3956
VMODE is pulled up to INTVCC as assembled, but can
be pulled up to another source if desired. When used
as a constant voltage regulator, VMODE can indicate
that the output is in voltage regulation as opposed to
constant current regulation.
The CTRL terminal is used for analog dimming. The
CTRL terminal and pin voltage directly determine the
amount of LED current that is driven. If CTRL is pulled
high, the LED string runs at full 370mA current. However, if the CTRL pin voltage is pulled low, the LED
current is reduced. Resistors R5 and R6 reduce the
LED current if PVIN drops below 11V. This keeps the
peak switch current from reaching the IC’s internally
programmed limit.
There are both GND and PGND terminals on the circuit. The GND terminals are considered to be local
signal ground and should not be connected to power
input. The PGND terminals can be connected to power
inputs and outputs if needed. If diagnostics are connected to GND terminals to measure signal waveforms
or voltages make sure that long ground loops are not
introduced that will allow PGND currents to run
through the GND plane. The layout of the PCB with
separate GND and PGND planes is very important to
proper operation of the IC. Note that pin 12 (PGND) is
not connected to the other PGND pins externally, but
that it is tied to GND to create a star ground for the
circuit. All of the PGND pins are connected together
internally in the IC.
The EN/UVLO terminal is used to shut down the demonstration circuit. Tie the terminal to GND and the
converter stops switching, the PWMOUT pin goes low
and the PWM dimming MOSFET is turned off. As long
as the PWM dimming MOSFET is used properly, the
LEDs do not turn on during shutdown. Resistors R3
and R4 set the undervoltage lockout level and the rising turn-on level of the LT3956. If used in buck mode,
the OUT terminal can be connected to VIN through a
resistive short and can become the power input terminal for that topology.
VMODE indicates that the circuit is running and that
the LED string has been removed from the terminals.
ILED (mA)
100%
95%
90%
85%
80%
75%
70%
65%
60%
55%
50%
0
10
20
30
40
50
60
VIN (V)
Figure 2. DC1521A Efficiency with 67V LEDs at 370mA
4
70
LT3956
400
350
ILED (mA)
300
250
200
150
100
6V Turn On
50
5.5V UVLO
0
5
10
15
20
25
VIN (V)
Figure 3. DC1521A LED Current Drops at Low VIN (CTRL) and Turns Off at Lower VIN (UVLO)
Figure 4. DC1521A 11VIN 67VLED 370mA 100Hz PWM Dimming waveforms 200:1 (left) 100:1 (right). ILED (violet), IL1 (green),
PWM (red)
Figure 5. DC1521A 24VIN 67VLED 370mA 100Hz PWM Dimming waveforms 200:1 (left) 100:1 (right). ILED (violet), IL1 (green),
PWM (red)
5
LT3956
6
Similar pages