Texas Instruments AN-1993 LM3406HV User Manual
AN-1993 LM3406HV Evaluation Board with 2 Wire
1 Introduction
The LM3406HV is a buck regulator controlled current source designed to drive a series string of high
power, high brightness LEDs (HBLEDs) such as the Luxeon™ K2 Emitter at forward currents of up to
1.5A. The converter's output voltage adjusts as needed to maintain a constant current through the LED
array.
User's Guide
SNVA407B–August 2009–Revised May 2013
Dimming
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SNVA407B–August 2009–Revised May 2013 AN-1993 LM3406HV Evaluation Board with 2 Wire Dimming
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Figure 1. Complete Circuit Schematic
1
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350 mA 700 mA
1000 mA
1500 mA
Circuit Performance
2 Circuit Performance
The LM3406HV circuit and BOM that come pre-installed on the evaluation board are optimized to run from
an input voltage of 24V, but the circuit can operate from a wide input voltage range of 6.0V to 75V. The
current output ranges from 0.35A to 1.5A. Figure 2 shows the program jumper settings used to program
currents of 0.35A, 0.7A, 1A, and 1.5A.
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Figure 2. Setting Output Current, J2
The LM3406HV is a step-down regulator with an output voltage range extending from a V
(the reference voltage) to a V
switching frequency. The regulator can maintain the output current through any number of LEDs as long
as the combined forward voltage of the array does not exceed V
following formula:
V
O-MAX
= V
x (1 - fSWx t
IN-MIN
OFF-MIN
For example, if VINis 24V ±10%, then V
output voltage for the converter is 21.6 x [1 - (5 x 105) x (230 x 10-9) = 19.1V. Output voltage is calculated
with the following formula:
VO= n x VF+ 0.2V
where
• n is the number of series-connected LEDs
• VFis the forward voltage of each LED
• 0.2V represents the voltage across the current sense resistor (2)
For InGaN LEDs (white, blue, blue-green) VFis typically 3.5V, and with a limit of (19.1 - 0.2) = 18.9V the
LM3406HV could drive as many as five in series. For AlInGaP LEDs (red, orange, amber) VFis typically
2.5V, so a V
of 18.9V would allow as many as seven to be driven in series.
O-MAX
3 Connecting the LED Array
The LM3406HV Evaluation Board includes test posts for connecting the LED/LED Array. Connect the
open anode of the array to LED+ and the cathode of the array to CS/LED-. Keep the leads from the board
to the LED(s) as short as possible to minimize inductance.
of 200 mV
determined by the ratio of the minimum off time (typically 230 ns) to the
O-MAX
O-MAX
. V
can be calculated with the
O-MAX
O-MIN
) (1)
is 21.6V. For a switching frequency of 500 kHz the maximum
IN-MIN
4 Setting the LED Current
The default forward current IFdelivered to the LED array when no program jumper is installed on J2 is
0.35A, set by resistor R6. The higher LED currents are set when the program jumper puts resistors R4,
R5 or R7 in parallel with R6. For users that wish to program a current other than one of the four default
levels, or for users who want the best accuracy at a given current, the program jumper J2 should be
removed, and R6 changed according to the following equation:
2
AN-1993 LM3406HV Evaluation Board with 2 Wire Dimming SNVA407B–August 2009–Revised May 2013
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T
D
tDt
SU
t
SD
tDt
SU
t
SD
tDt
SU
t
SD
D
MIN
D
MAX
T T
DIM
I
F
T
t+t
=
D
SUD
MIN
1
=T
PWM
f
T
=
tT -
SD
D
MAX
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Pulse Width Modulation (PWM) Dimming
R.35 = 0.2 / I
F
This resistor should be rated to handle the power dissipation of the LED current. For example, the closest
5% tolerance resistor to set an LED current of 0.35A is 0.56Ω. In steady state this resistor will dissipate
(0.352x 0.56) = 69 mW, indicating that a resistor with a 1/8W rating is more than capable of dissipating
the power.
5 Pulse Width Modulation (PWM) Dimming
The DIM1 terminal on the PCB provides an input for a logic-level pulse width modulation signal for
dimming of the LED array. In order to fully enable and disable the LM3406HV the PWM signal should
have a maximum logic low level of 0.8V and a minimum logic high level of 2.2V. Graphical representations
of minimum and maximum PWM duty cycle are illustrated in Figure 3. The interval tDrepresents the delay
from a logic high at the DIM pin to the rise in output current. The quantities tSUand tSDrepresent the time
needed for the output current to slew up to steady state and slew down to zero, respectively. It is
important to note that tDis a property of the LM3406HV and remains fixed in all applications. The slew
rates tSUand tSDare a function of the external circuit parameters VIN, VO, IF, inductance (L) and the
LM3406HV parameter t
shown in the Typical Performance Characteristics section, but the user should test every new circuit to
determine the actual PWM dimming response.
. Response times for a circuit driving three white LEDs at 1A from 24V are
OFF-MIN
(3)
The logic of DIM1 is active low, hence the LM3406HV will deliver regulated output current when the
voltage at DIM1 is high, and the current output is disabled when the voltage at DIM1 is low. Connecting a
constant logic low will disable the output. Note that an internal pullup esnures that the LM3406HV is
enabled if the DIM pin is open-circuited. The DIM1 function disables only the power MOSFET, leaving all
other circuit blocks functioning to minimize the converter response time, tD.
The DIM2 terminal provides a second method for PWM dimming by connecting to the gate of MOSFET
Q1 through the driver U5. Q1 provides a parallel path for the LED current. Shunting the output current
through a parallel MOSFET reduces the PWM dimming delays because the inductor current remains
continuous, providing faster response time for higher frequency and/or greater resolution in the PWM
dimming signal. The trade-off in this method is that the full current flows through Q1 while the LED is off,
resulting in lower efficiency. The LM3406HV evaluation board includes an output capacitor to reduce
output current ripple which is not initially populated, but the drawback of this output capacitor if used is
that it causes significant delays when using parallel MOSFET dimming. The output capacitor should be
SNVA407B–August 2009–Revised May 2013 AN-1993 LM3406HV Evaluation Board with 2 Wire Dimming
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removed to take full advantage of parallel MOSFET dimming.
Figure 3. PWM Dimming Limits
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3
GND
DIM
BOOT SW
VOUT
RON
LM3406HV
VIN
D1
L1
C
B
R
SNS
C
F
R
ON
C
IN
V
IN
VCC
CS
COMP
C
C
D2
V
O
2 Wire Input Dimming
The logic of DIM2 is active low, hence the regulated output current flows through the LED array when the
voltage at DIM2 is high, and the current flows through the shunt FET when the voltage at DIM2 is low.
Connecting a constant logic low to the DIM2 will turn off the LED but will not shut down the LM3406HV. A
voltage of up to 30V must be applied to the DPWR pin to operate U5.
6 2 Wire Input Dimming
The LM3406HV evaluation board has been designed for 2 wire dimming for systems that present a square
wave input voltage for dimming purposes. A diode, D2, separates the VIN pins from the VINS pin. When
the input voltage at VINS falls to 70% or less of the voltage at VIN the device stops switching and enters
dim mode. The capacitors C1 and C2 hold up the voltage at the VIN pins during this time so that the
LM3406HV is enabled and responds quickly when the voltage at VINS again exceeds 70% of the voltage
at VIN.
7 Low Power Shutdown
The LM3406HV can be placed into a low power shutdown (typically 240 µA) by grounding the OFF*
terminal. During normal operation this terminal should be left open-circuit.
8 Output Open Circuit
The LM3406HV will begin to operate as soon as VIN is greater than 6V and the DIM and RON pins are
not grounded. If the regulator is powered and enabled but no LED array is connected, the output voltage
will rise to VIN. The output of the circuit is rated to 50V (beyond the maximum input voltage) and will not
suffer damage, however care should be taken not to connect an LED array if the output voltage is higher
than the target forward voltage of the LED array in steady state.
If the LEDs are disconnected or one of the LEDs fails open-circuit while the LM3406HV is operating, the
output voltage will experience a surge as the current in the output inductor seeks a discharge path. The
output capacitor (if present) can absorb some of this energy, however circuits with little or no output
capacitance can experience a voltage spike that exceeds the rating of the VOUT pin. The evaluation
board uses a 10 kΩ resistor in series with the VOUT pin to limit current flowing into the pin. Alternatively, a
diode connected from VINto VOas shown in Figure 4 will clamp the spike to VINplus a diode drop and is
included on the evaluation board.
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4
AN-1993 LM3406HV Evaluation Board with 2 Wire Dimming SNVA407B–August 2009–Revised May 2013
Figure 4. Schottky Diode Protection for Open-Circuit
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