LM3407
Application Note 1763 LM3407 Evaluation Board Reference Design
Literature Number: SNVA308A
National Semiconductor
Application Note 1763
SH Wong
January 21, 2009
Introduction
with an external 1% thick film current setting resistor. The
converter features a DIM pin which accepts standard logic
pulses for controlling the brightness of the LED array, making
the LM3407 ideal for use as a precision power LED driver or
constant current source.
This application note introduces the design of a sample circuit
with the LM3407 providing 350 mA constant current to drive
an LED array of 6 high power LEDs connected in series. The
board can accept an input voltage ranging from 22V to 30V.
The schematic, PCB layout, bill of materials, and circuit design criteria are shown in detail. Typical performance and
operating waveforms are also provided for reference.
The LM3407 is a Pulse-Width-Modulation (PWM) floating
buck converter with an integrated N-channel power MOSFET
designed to provide precision constant current output for driving high power LEDs, such as the Lumileds Luxeon® power
LEDs and OSRAM Golden DRAGON® LEDs. The switching
frequency is selectable between 300 kHz and 1 MHz by
changing the value of the frequency setting resistor, allowing
for the use of small external components. The LM3407 features a Pulse Level Modulation (PLM) control scheme which
ensures the accuracy of the constant current output well within 10% over input voltage and operating temperature ranges
Evaluation Board Schematic
LM3407 Evaluation Board Reference Design
LM3407 Evaluation Board
Reference Design
30046701
FIGURE 1. LM3407 Evaluation Board Schematic
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© 2009 National Semiconductor Corporation
300467
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30046702
FIGURE 2. LM3407 Evaluation Board PCB Top Overlay
30046703
FIGURE 3. LM3407 Evaluation Board PCB Top View
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30046704
FIGURE 4. LM3407 Evaluation Board PCB Bottom View
Evaluation Board Quick Setup Procedures
Step
Procedure
1
Remove all jumpers on the evaluation board.
Notes
2
Connect the LED array of 6 power LEDs to J1.
3
Connect Power Supply output to the VIN terminal of the
evaluation board.
4
Set the power supply output voltage to 24V.
6
Check the voltage of the VCC terminal of the board.
VCC = 4.5V ± 8%
7
Short pin 1 and 2 of J3 by using a jumper.
LEDs fully turned ON
8
Check the LED current (IOUT) by using an ammeter.
IOUT = 350mA ± 6%
9
Short J2 by using a jumper to check the shutdown function.
IOUT = 0
VIN should not exceed 30V
Evaluation Board Performance Characteristic
Description
Symbol
Input Voltage
VIN
Output Current
IOUT
DIM pin connected to VCC
Output Current
Variation
|ΔIOUT|
All VIN and IOUT Conditions
Efficiency
Condition
Min
Typ
Max
22
24
30
Unit
V
330
350
370
mA
6
%
No. of LED = 6
93
96
%
No. of LED = 4
90
95
%
No. of LED = 2
85
92
%
3
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components, do not connect the LED array with incorrect polarity or alter the connections of the LED array when the
evaluation board is connected to power. It is highly recommended to attach the LED array to a heat sink for heat
dissipation and to apply force ventilation to the LED array as
necessary.
Design Procedure
CONNECTING TO LED ARRAY
The LM3407 evaluation board features a female 6-pin SIP
connector J1 for board-to-board connection of the LED array.
Figure 5 shows the pin-out of J1. To avoid damaging the
30046705
FIGURE 5. Connecting an LED Array to the LM3407 Evaluation Board
SETTING THE SWITCHING FREQUENCY
The switching frequency of the LM3407 evaluation board is
programmable by adjusting the value of the frequency setting
resistor RFS. The default value of the RFS pre-installed on
the evaluation board is 40.2 kΩ, at which the switching frequency is 1MHz. In order to guarantee good current regulation, it is suggested to set the switching frequency between
300KHz and 1MHz. The switching frequency is calculated by
the expression shown below:
SETTING THE LED CURRENT
The output current of the evaluation board is adjustable by
changing the current setting resistors RISNS1 and RISNS2.
By default, the value of both RISNS1 and RISNS2 is 1.13Ω
at 1% tolerance, which results in a resistance of 0.565Ω. This
value of RISNS sets the output current (IOUT) at 350 mA. The
value of RISNS can be calculated by using the equation:
When selecting the value of the current setting resistors
(RISNS1 and RISNS2), it is important to ensure the rated
powers of the resistors are not exceeded. For example, when
IOUT is set at 350mA, the total power dissipation on RISNS1
and RISNS2 in steady state is 0.35 mA2 x 0.565Ω, which
equals 69 mW, indicating a resistor of 1/8W power rating is
appropriate.
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for 40kΩ ≤ RFS ≤ 150 kΩ
For the convenience of selecting the value of RFS, a selection
chart of fSW against RFS is provided:
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Switching Frequency VS. RFS
(TA = 25°C)
Peak to Peak Inductor Ripple Current:
Peak Inductor Current:
where n is the number of LEDs in a string and VF is the forward
voltage of one LED.
The minimum inductance required for the specific application
can be calculated by:
30046707
The LM3407 is internally compensated and requires no external components for feedback compensation. The components of this evaluation board are optimized for driving 6
power LEDs with the input voltage between 22V and 30V. If
different conversions are required, such as changes to input
voltage and loading conditions, L1 and RFS may need to be
changed to ensure stable operation.
Since the evaluation board is designed to drive a LED array
of 6 LEDs, the default value of the inductor is 33µH to ensure
CCM operation for the input voltage between 22V and 30V
with 1MHz switching frequency. For the applications with different input voltage or number of LEDs, the inductance of the
inductor may have to be changed to maintain accurate output
current. Table 1 shows the suggested inductance of the inductor for 500kHz and 1MHz switching frequency.
The output diode of the evaluation board circuit is selected
depending on the output voltage and current. The diode must
have a rated reverse voltage higher than the input voltage of
the regulator and the peak current rating must be higher than
the expected maximum inductor current. Using a schottky
diode with low forward voltage will decrease power dissipation and increase conversion efficiency.
SELECTION OF INDUCTOR AND DIODE
In order to achieve accurate constant current output, the
LM3407 is required to operate in Continuous Conduction
Mode (CCM) under all operating conditions. In general, the
magnitude of the inductor ripple current should be kept as
small as possible. If the PCB size is not limited, higher inductance values result in better accuracy of the output current.
However, in order to minimize the physical size of the circuit,
an inductor with minimum physical outline should be selected
such that the converter always operates in CCM and the peak
inductor current does not exceed the saturation current limit
of the inductor. The ripple and peak current of the inductor
can be calculated as follows:
TABLE 1. Suggested Inductance Value of the Inductor
Inductor selection table for fSW = 500 kHz, COUT = 4.7 µF (1 µF for 1 LED)
VIN/V
Number of LED
1
2
3
4
5
22 µH
22 µH
22 µH
33 µH
22 µH
22 µH
22 µH
33 µH
33 µH
33 µH
22 µH
5
22 µH
10
22 µH
22 µH
15
22 µH
22 µH
22 µH
20
22 µH
33 µH
25
22 µH
33 µH
30
22 µH
47 µH
6
7
22 µH
Inductor selection table for fSW = 1 MHz, COUT = 4.7 µF (1 µF for 1 LED)
5
22 µH
10
22 µH
22 µH
15
22 µH
22 µH
22 µH
20
22 µH
22 µH
22 µH
22 µH
22 µH
25
22 µH
22 µH
22 µH
22 µH
22 µH
22 µH
30
22 µH
33 µH
22 µH
22 µH
22 µH
22 µH
5
22 µH
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should have a logic low of 1V maximum and logic high of 2V
minimum. The DIM terminal is internally pulled down to
ground by a 400 kΩ resistor, which should be connected to
either logic high or low and should not be left open. In steady
state, the expression of the average LED driving current is:
LED DIMMING
There are two ways to disable the current output (IOUT) of the
evaluation board circuit. The current output of the LM3407
evaluation board can be disabled by connecting either the
DIM or EN pin to ground. Connecting the EN pin to ground
will shutdown the internal linear regulator and maintain minimal power consumption. Connecting the DIM pin to ground
will only disable the current output of the LM3407, while the
internal oscillator and control circuits remain active to facilitate
fast wake up.
In general, dimming of the LED array can be achieved by applying a logic pulse chain to the DIM terminal of the evaluation
board to periodically enable and disable the LM3407 and
control the average IOUT of the LED array. Since the color
characteristics of a LED are closely related to the driving current, dimming by adjusting the current setting resistor causes
the color temperature to drift. To control the brightness of the
LED array effectively, PWM dimming should be used. PWM
dimming is a dimming method which controls the ON/OFF
time ratio of the LED(s) at fixed frequency.
The DIM terminal on the evaluation board is directly connected to the DIM pin of the LM3407, which provides a PWM
signal input for dimming of the LED array. In order to properly
enable and disable the LM3407, the PWM dimming signal
LIMITS OF PWM DIMMING
The maximum PWM dimming frequency, minimum duty cycle, and maximum duty cycle are shown in Figure 6. The
maximum dimming frequency should not exceed 1/50 of the
switching frequency fSW of the LM3407. To avoid visible flicker, dimming frequencies lower than 100 Hz are not recommended. In Figure 6, T is the period of the PWM dimming
signal. The interval tD represents the time delay from a logic
high of the dimming signal and the onset of the output current.
tSU and tSD are the time needed for the output current to slew
up from zero to steady state and slew down to zero respectively. In the figure, it can be seen that the minimum duty cycle
of the dimming signal should not be shorter than the sum of
tSU and tSD of the output current.
30046712
FIGURE 6. Limits of the PWM Dimming Signal
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6
Bill of Materials
Designation
Description
Package
Manufacture Part #
Vendor
U1
LED Driver IC, LM3407
eMSOP-8
LM3407
NSC
L1
Inductor 33µH 0.58A
4.0 x 4.0 x 1.8 (mm)
LPS4018-333ML
Coilcraft
* Inductor 33µH 0.56A
4.8 x 4.3 x 3.5 (mm)
CR43NP-330K
Sumida
D1
Schottky Diode 40V 1.0A
DO-214AC (SMA)
SS14
Vishay
CIN, COUT
Cap MLCC 50V 4.7µF X7R
1210
GRM32ER71H475K88L
Murata
CVCC
Cap MLCC 10V 1.0µF X5R
0805
GRM188R61A105KA61D
Murata
RISNS1, RISNS2
Chip Resistor 1.13Ω 1%
0805
CRCW08051R13F
Vishay
RFS
Chip Resistor 40.2kΩ 1%
0805
CRCW08054022F
Vishay
J1
6-pin Connector
DIP-12
535676-5
Tyco Electronics
J2
2-way Jumper System
2.54 (mm) Pitch
J3
3-way Jumper System
2.54 (mm) Pitch
VCC, GND, EN, DIM,
ISNS, LX
Terminal pin
2.29 (mm) Dia.
160-1026
Cambion
VIN, GND
Terminal pin
1.57 (mm) Dia.
160-1512
Cambion
PCB
LM3407 Evaluation Board
59 x 40 (mm)
J3
2-pin Jumper
NSC
*Alternative Supplier
7
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fier D1, inductor L1 and output capacitor COUT should be
kept as short as possible to reduce the voltage spikes at the
LX pin. CVCC is the output filter capacitor for the internal linear regulator of the LM3407, it is recommended to be placed
close to the pin VCC. The input filter capacitor CIN should be
located close to L1 and the cathode of D1. If CIN is connected
to the VIN pin by a long trace, a 0.1µF capacitor should be
added close to pin VIN for noise filtering. In normal operation,
heat will be generated inside the LM3407 and may damage
the device if no thermal management is applied. For more
detail on switching power supply layout considerations see
Application Note AN-1149: Layout Guidelines for Switching
Power Supplies.
PCB LAYOUT GUIDE
Since copper traces of PCBs carry resistance and parasitic
inductance, the longer the copper trace, the higher the resistance and inductance. These factors introduce voltage and
current spikes to the switching nodes and impair the performance of the whole circuit. To optimize the performance of
the LM3407, the rule of thumb is to keep the connections between components as short and direct as possible. Since true
average current regulation is achieved by detecting the average switch current, the current setting resistors RISNS1 and
RISNS2 must be located as close to the LM3407 as possible
to reduce the parasitic inductance of the copper trace and
avoid noise pick-up. The connections between LX pin, recti-
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Typical Performance and Waveforms
All curves and waveforms taken at TA = 25°C unless otherwise
specified.
Efficiency vs Input Voltage
(TA = -40°C)
Efficiency vs Input Voltage
(TA = 25°C)
30046713
30046714
Efficiency vs Input Voltage
(TA = 125°C)
Output Current vs Input Voltage
(TA = 25°C)
30046715
30046716
Inductor Current @ fSW = 1MHz
(VIN = 12V, 2LEDs, L = 33µH)
Inductor Current @ fSW = 500kHz
(VIN = 12V, 2LEDs, L = 33µH)
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30046719
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Inductor Current @ fSW = 1MHz
(VIN = 24V, 2LEDs, L = 33µH)
Inductor Current @ fSW = 500kHz
(VIN = 24V, 2LEDs, L = 33µH)
30046720
30046721
DIM Pin Enable
(VIN = 24V, 2LEDs, L = 33µH, fSW = 500kHz)
DIM Pin Disable
(VIN = 24V, 2LEDs, L = 33µH, fSW = 500kHz)
30046723
30046722
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LM3407 Evaluation Board Reference Design
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