TI1 LM3414HV 1a 65v led driver evaluation board Datasheet

LM3414HV
Application Note 2076 LM3414HV 1A 65V LED Driver Evaluation Board
Literature Number: SNVA451B
National Semiconductor
Application Note 2076
SH Wong
August 10, 2010
Introduction
Standard Settings of the LM3414HV
Evaluation Board
The LM3414HV is a 65V floating buck LED driver that designed to drive up to 18 pieces of serial High Brightness LEDs
(HBLEDs) with up to 1000mA LED forward current. With the
incorporation of the proprietary Pulse-Level-Modulation
(PLM) technology, the LM3414HV requires no external current sensing resistor to facilitate LED current regulation. The
LM3414HV features a dimming control input (DIM pin) that
allows PWM dimming control. The LM3414HV is available in
LLP-8 (3mm x 3mm outline) and ePSOP8 to fulfil the requirements of small solution size and high thermal performance
respectively. In order to demonstrate the performance of the
LM3414 family, the LM3414HV is selected for the evaluation
boards because of the wide input voltage range (4.5V to 65V)
providing the best flexibility to fit the requirements of different
applications. Two versions of evaluation board with identical
schematic are available with the LM3414HV in either LLP-8
or PSOP-8 package. The board with LLP-8 package demonstrates the high power density of the device. The board with
PSOP-8 package demonstrates the functionality of the
LM3414HV with enhanced thermal performance. The
schematic, bill of materials and PCB layout for the evaluation
boards are provided in this document. The evaluation boards
can be adapted to different application requirements by
changing the values of a few components only. This evaluation board is also suitable for the LM3414 with maximum
acceptable input voltage reduced to 42VDC.
Vin range: 4.5V to 65V
No. of LEDs: 1 - 18
LED current: 1A
Switching frequency: 500 kHz
LM3414HV 1A 65V LED Driver Evaluation Board
LM3414HV 1A 65V LED
Driver Evaluation Board
30128701
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FIGURE 1. Standard Schematic for the LM3414HV Evaluation Board
© 2010 National Semiconductor Corporation
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Board Connectors and Test Pins
30128702
FIGURE 2. Connection Diagram
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Terminal Designation
Description
VIN
Power supply positive (+ve) connection
GND
Power supply negative (-ve) connection
LED+
Connect to cathode of the serial LED string
LED-
Connect to anode of the serial LED string
DIM
PWM dimming signal input (TTL signal compatible)
THM+
Connect to PTC thermal sensor for thermal foldback control
THM-
Connect to PTC thermal sensor for thermal foldback control
2
Design Example
The LM3414HV evaluation board can be powered by a DC
voltage source in the range of 4.5V to 65V through the banana-plug type connectors (VIN and GND) on the board as
shown in figure 2. This evaluation board is designed to provide 1A (ILED)output current to a LED string containing up to
18 pieces of serial HBLEDs. The anode and cathode of the
LED string should connect to the LED+ and LED- bananaplug type connectors on the board respectively. By default,
the LM3414HV on the evaluation board is enabled. The LEDs
will light up as long as appropriate input voltage is applied to
the evaluation board.
Assuming a LED string containing six serial HBLEDs is being
driven by the board with 700mA (ILED). The forward voltages
of one HBLED with 700mA driving current under different operation temperatures are:
Vf(60C) @700mA = 3.0V
Vf(25C) @700mA = 3.2V
Vf(-10C) @700mA = 3.5V
Step 1. Defining input voltage range
Because the LM3414HV is a floating buck LED driver, the input voltage to the LED driver must be higher than the total
forward voltage of the LEDs under all conditions. As the forward voltage of a common HBLED could increase as the
driving current increases or the operation temperature decreases, it is essential to ensure the minimum supply voltage
is at least 10% higher than the possible highest forward voltage of the LED string. For example, assuming the forward
voltage of a HBLED is 3.2V at TA = 25°C and 3.5V at TA =
-10°C at 700mA driving current. When 6 pieces of LED are
connected in series, the total forward voltage of the LED string
at 25°C and -10°C are 19.2V and 21V respectively. In order
to secure current regulation under -10°C, the input voltage
should not be lower than 23.1V. In this example, a standard
24V DC power supply with no more than +/– 3% output voltage variation can be used.
Adjusting the Output Current
The resistor RIADJ defines the output current of the
LM3414HV evaluation board. The default value of RIADJ is
3.09kΩ, which sets the LED driving current to 1A. The LED
current can be changed by adjusting the value of RIADJ with
equation (1):
(1)
Table 1 shows the suggested value of RIADJ for common
output current settings:
ILED (mA)
RIADJ (kΩ)
350
8.93
400
7.81
500
6.25
600
5.21
700
4.46
800
3.91
900
3.47
1000
3.13
Step 2. Defining switching frequency fSW
When the maximum LED forward voltage and minimum input
voltage are identified, the switching frequency of the
LM3414HV can be defined. The switching frequency of the
LM3414HV must be set in the range of 250kHz to 1MHz. Because the LM3414HV is designed to operate in continuous
conduction mode (CCM) with 400ns minimum switch ON time
limit, the maximum allowable switching frequency is restricted
by the minimum input voltage, VIN(MIN) and maximum LED
forward voltage, Vf(MAX) according to equation (3):
TABLE 1. Examples for RIADJ Setting
Adjusting the Switching Frequency
The resistor RFS defines the switching frequency of the
LM3414HV evaluation board. The default value of the RIADJ
is 40kΩ that sets the switching frequency to 500kHz. The LED
current is adjustable by altering the resistance of RFS according to the equation (2):
(3)
In this example, because a 24V DC power supply with +/- 3%
output voltage variation is used, VIN(MAX) is 24.72V. The minimum forward voltage of the LED string Vf(MIN) is 18V because
the forward voltage of the LED string will be at the lowest level
when the operation temperature rises to 60°C. According to
equation (3), with VIN(MAX)=24.72V and Vf(MIN)=18V, the
switching frequency, fSW should not set higher than 1.82MHz.
However, because the switching frequency of the LM3414HV
must set in the range of 250kHz to 1MHz, 1MHz switching
frequency is selected.
(2)
Table 2 shows the suggested value of RFS for different
switching frequencies:
fSW (kHz)
RFS (kΩ)
250
8.93
500
7.81
1000
6.25
Step 3. Inductor Selection
The inductance of the inductor, L1 can be decided according
to the switching frequency and output current settings determined in step 1 and step 2. The inductance must be adequate
to maintain the LM3414HV to operate in CCM. The minimum
inductance can be calculated by following equation (4):
TABLE 2. Examples for RFS Setting
When setting the switching frequency, it is necessary to ensure the on time of the internal switch is no shorter than
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400ns; otherwise the driving current to the LEDs will increase
and may eventually damage the LEDs.
Connecting to LEDs and Power
Supply
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PWM Dimming Control
The average LED current can be controlled by applying PWM
dimming signal across the DIM and GND terminals of the
LM3414HV evaluation board. The board accepts standard
TTL level dimming signal. The output of the board is enabled
when the DIM terminal is pulled high. The average LED current is adjustable according to the ON duty ratio of the PWM
dimming signal by equation (6):
(4)
In equation (4), ILED is the average output current of the
LM3414HV circuit to drive the LED string. IRIP(P-P) is the peakto-peak value of the inductor current ripple. Assuming that the
required LED current is 700mA, 50% inductor current ripple
and 1MHz switching frequency, the inductance should be no
less than 14uH. Because common power inductor carries +/20% inductance tolerance, a standard 18uH inductor with
+/-20% tolerance can be used.
Other than deciding a suitable inductance value, it is essential
to ensure the peak inductor current is not exceeding the rated
saturation current of the inductor. The peak inductor current
is governed by the following equation:
(6)
In equation (6), ILED(AVG) is the average current flows through
the LED string and DDIM is the ON duty ratio of the PWM dimming signal being applied to the DIM pin of the LM3414HV.
Analog Dimming Control
As the output current of the LM3414HV is defined by the current being drawn to GND through RIADJ proportionally, analog dimming control (true output current control) can be
accommodated by applying external current to RIADJ of the
LM3414HV evaluation board. Figure 3 shows an example circuit for analog dimming control. With analog dimming control.
Injecting additional current through the RIADJ to GND can
effectively reduce the LED current (ILED). The relationship of
ILED and IEXT is governed by equation (7).
(5)
In equation (5), IL(PEAK) is the peak inductor current. As a 18uH
with +/- 20% variation is used, the minimum inductance L
(MIN) is 14.4uH. With 700mA LED current, the peak inductor
current is 836mA, thus a standard 18uH power inductor with
1A saturation current (ISAT) can be used.
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FIGURE 3. Reducing LED current with external current to the IADJ pin
on end application, the components required in this thermal
foldback control circuitry are not included in the LM3414HV
evaluation board. Physical pads and connections for R1, R2
and Q1 have been reserved on the board for component
mounting. In order to detect the temperature of the LED string,
a Positive Temperature Coefficient (PTC) thermistor, RPTC
should be connected across the THM+ and THM- terminals
of the LM3414HV evaluation board. In figure 4, the bipolar
transistor, Q1 is biased by a potential divider composes of R1
and RPTC. When the temperature of the LEDs rises, the volt-
(7)
In equation (7), IEXT is the external current being injected into
RIADJ. As IEXT increases, ILED decreases.
Figure 4 shows a practical thermal foldback control circuit
which reduces the LED current when the temperature of the
LED sting is exceeding certain preset threshold. Because the
temperature threshold for thermal foldback control depends
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4
mal foldback control is activated and the LED current reduces
according to IEXT.
30128721
FIGURE 4. Thermal Foldback Control with PTC thermistor
RPTC(25C) = 330Ω
RPTC(80C) = 1.2kΩ
RPTC(100C) = 10kΩ
Design Example
The LM3414HV evaluation board is used to drive a LED string
at 700mA and thermal foldback control is needed to take
place when the temperature of the LED strings exceeds 80°
C as presented in Figure 5.
In Figure 5, the LED current with the LED temperature below
80°C (ILED(normal)) is 700mA. As the temperature of the LED
goes up to 80°C, thermal foldback begins and reduces the
LED driving current with respect to the increase of resistance
of RPTC. As the temperature of the LEDs reaches 100°C, the
LED current reduces to zero. Provided that the resistance of
the thermistor RPTC under 80C and 100°C are 1.2kΩ and
10kΩ respectively, the values of R1 and R2 can be calculated
following the steps listed below.
At 80°C:
30128722
FIGURE 5. Reduction of LED current with thermal
foldback control
(8)
Assume the resistance of the PTC thermistor under 25°C, 80°
C and 100°C are:
5
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age drop across RPTC increases as the resistance of RPTC
increases. As the emitter voltage of Q1 reaches 1.255V, ther-
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At 100°C:
Tiny Board Outline
The tiny packages of the LM3414 family are exceptionally
suitable for the applications that require high output power in
limited space. In order to demonstrate the high power density
of the LM3414HV, the core circuitry of this evaluation boards
are completed in compact form factors: 22mm x 19mm for
LLP-8 package, 26mm x 19mm for PSOP-8 package. The
schematic of the core circuitry is as shown in Figure 6. The
core circuitry can be extracted by cutting out from the PCB
frame of the board as shown in Figure 7.
(9)
30128725
FIGURE 6. Core Circuitry of the LM3414HV Evaluation Boards
30128726
FIGURE 7. Extracting the core circuitry from the LM3414HV evaluation boards
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30128727
FIGURE 8. Connecting to the core circuitry
The board of the core circuitry features four connection pads
for connections to DC power supply and LED string, as shown
in Figure 8. To ensure thermal performance of the board, a
heatsink attaches to the bottom layer of the board may be
required depending on actual operation environment.
Bill of Materials
Designation
Description
Package
Manufacturer Part #
U1
LED Driver IC, LM3414HV
LLP8 / PSOP8
LM3414MH
Vendor
NSC
D1
Schottky Diode 100V 2A
SS2PH10-M3/84A
Vishay
L1
Power Inductor 47 µH
MMD-08EZ-470M-S1
MAG.Layers
CIN
Cap MLCC 100V 2.2 µF X7R 1210
1210
GRM32ER72A225KA35L
Murata
CVCC
Cap MLCC 10V 1 µF X5R 0603
603
GRM185R61A105KE36D
Murata
RIADJ
Chip Resistor 3.09 kΩ 1% 0603
603
CRCW06033K09FKEA
Vishay
RFS
Chip Resistor 40.2 kΩ 1% 0603
603
CRCW060340K2FKEA
Vishay
VIN, GND,
LED+, LED-
Banana Jack 5.3(mm) Dia
5.3 (mm) Dia.
575-8
KEYSTONE
VIN, GND,
LED+,LED-,
THM+, THM-,
DIM,
Turret 2.35(mm) Dia
2.35 (mm) Dia.
1502-2
KEYSTONE
PCB
LM3414EVAL PCB 85 X 54 (mm)
85 X 54 (mm)
Q1
NPN Bipolar Transistor
SOT23
R1,R2,RFS_1,
RFS_2,RIADJ_1
NA
603
JP1,JP2,JP3
NA
603
7
NSC
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Typical Performance Characteristics
All curves taken at VIN = 48V with configuration in typical application for driving twelve power LEDs with four output channels active
and output current per channel = 350 mA. TA = 25°C, unless otherwise specified.
Output Current (A)
Efficiency (%)
30128730
30128731
ILED(A) vs RIADJ(kΩ)
fSW(kHz) vs RFS(kΩ)
30128732
30128733
ILED with VDIM rising
ILED with VDIM falling
30128708
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30128709
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Evaluation Board Layout (LLP-8 Package)
30128712
Top Layer and Top Overlay
30128713
Bottom Layer and Bottom Overlay
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Evaluation Board Layout (PSOP-8 Package)
30128714
Top Layer and Top Overlay
30128715
Bottom Layer and Bottom Overlay
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Notes
11
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LM3414HV 1A 65V LED Driver Evaluation Board
Notes
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