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LM3407
SNVS553C – JANUARY 2008 – REVISED NOVEMBER 2016
LM3407 350-mA, Constant Current Output Floating Buck Switching Converter
for High-Power LEDs
1 Features
3 Description
•
•
•
•
The LM3407 device is a constant current output
floating buck switching converter designed to provide
constant current to high-power LEDs. The device is
ideal for automotive, industrial, and general lighting
applications. The LM3407 has an integrated power Nchannel MOSFET that makes the application solution
compact and simple to implement. An external 1%
thick-film resistor allows the converter output voltage
to adjust as needed to deliver constant current within
10% accuracy to a serially connected LED string of
varying number and type. Converter switching
frequency is adjustable from 300 kHz to 1 MHz. The
LM3407 features a dimming input to enable LED
brightness control by Pulse Width Modulation (PWM).
Additionally, a separate enable pin allows for lowpower shutdown. An exposed pad MSOP-8 package
provides excellent heat dissipation and thermal
performance. Input UVLO and output open-circuit
protection ensure a robust LED driver solution.
1
•
•
•
•
•
•
Input Operating Range 4.5 V to 30 V
Output Voltage Range: 0.1 VIN to 0.9 VIN
Accurate Constant Current Output
Independent Device Enable (CMOS Compatible)
and PWM Dimming Control
Converter Switching Frequency Adjustable From
300 kHz to 1 MHz
No External Control Loop Compensation Required
Supports Ceramic and Low ESR Output
Capacitors
Input Undervoltage Lockout (UVLO)
Thermal Shutdown Protection
MSOP-8 PowerPAD Package
2 Applications
•
•
•
•
•
LED Drivers
Constant Current Sources
Automotive Lighting
General Illumination
Industrial Lighting
Device Information(1)
PART NUMBER
LM3407
PACKAGE
BODY SIZE (NOM)
MSOP-PowerPAD (8) 3.00 mm × 3.00 mm
(1) For all available packages, see the orderable addendum at
the end of the data sheet.
Simplified Application Schematic
1
An IMPORTANT NOTICE at the end of this data sheet addresses availability, warranty, changes, use in safety-critical applications,
intellectual property matters and other important disclaimers. PRODUCTION DATA.
LM3407
SNVS553C – JANUARY 2008 – REVISED NOVEMBER 2016
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Table of Contents
1
2
3
4
5
6
7
Features ..................................................................
Applications ...........................................................
Description .............................................................
Revision History.....................................................
Pin Configuration and Functions .........................
Specifications.........................................................
1
1
1
2
3
4
6.1
6.2
6.3
6.4
6.5
6.6
4
4
4
4
5
6
Absolute Maximum Ratings ......................................
ESD Ratings..............................................................
Recommended Operating Conditions.......................
Thermal Information ..................................................
Electrical Characteristics...........................................
Typical Characteristics ..............................................
Detailed Description .............................................. 9
7.1 Overview ................................................................... 9
7.2 Functional Block Diagram ......................................... 9
7.3 Feature Description................................................. 10
7.4 Device Functional Modes........................................ 13
8
Application and Implementation ........................ 14
8.1 Application Information............................................ 14
8.2 Typical Applications ................................................ 17
9 Power Supply Recommendations...................... 20
10 Layout................................................................... 20
10.1 Layout Guidelines ................................................. 20
10.2 Layout Example .................................................... 20
11 Device and Documentation Support ................. 21
11.1
11.2
11.3
11.4
11.5
11.6
Documentation Support .......................................
Receiving Notification of Documentation Updates
Community Resources..........................................
Trademarks ...........................................................
Electrostatic Discharge Caution ............................
Glossary ................................................................
21
21
21
21
21
21
12 Mechanical, Packaging, and Orderable
Information ........................................................... 21
4 Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Revision B (May 2013) to Revision C
Page
•
Added ESD Ratings table, Thermal Information table, Feature Description section, Device Functional Modes,
Application and Implementation section, Power Supply Recommendations section, Layout section, Device and
Documentation Support section, and Mechanical, Packaging, and Orderable Information section....................................... 1
•
Changed RθJA for DGN package from 50°C/W to 55.6°C/W .................................................................................................. 4
Changes from Revision A (January 2009) to Revision B
•
2
Page
Changed layout of National Semiconductor Data Sheet to TI format. ................................................................................ 20
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5 Pin Configuration and Functions
DGN Package
8-Pin MSOP
Top View
8
1
ISNS
LX
DIM
GND
EN
VCC
2
7
3
6
EP
4
5
FS
VIN
Pin Functions
PIN
NO.
NAME
I/O
DESCRIPTION
1
ISNS
I
Connect resistor RISNS from this pin to ground for LED current sensing. The current sensing
resistor should be placed close to this pin.
2
DIM
I
PWM Dimming Control Pin. Applying a logic level PWM signal to this pin controls the
intended brightness of the LED string.
3
EN
I
Applying logic high to this pin or leaving it open enables the switcher. When pulled low the
switcher is disabled and will enter low power shutdown mode.
4
FS
I
Switching Frequency Setting Pin. Connect resistor RFS from this pin to ground to set the
switching frequency.
5
VIN
I
Input Voltage Pin. The input voltage should be in the range of 4.5 V to 30 V
6
VCC
O
Internal Regulator Output Pin. This pin should be bypassed to ground by a ceramic capacitor
with a minimum value of 1 µF.
7
GND
—
This pin should be connected to the system ground.
8
LX
O
Drain of N-MOSFET Switch. Connect this pin to the output inductor and anode of the
Schottky diode.
EP
EP
—
Thermal Pad (Power Ground). Used to dissipate heat from the package during operation.
Must be electrically connected to GND external to the package.
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6 Specifications
6.1 Absolute Maximum Ratings
over operating free-air temperature range (unless otherwise noted) (1)
VIN to GND
MIN
MAX
UNIT
–0.3
36
V
VIN to GND (transient)
42 (500 ms)
V
–0.3
36
V
–3 (2 ns)
42 (500 ms)
V
LX to GND
LX to GND (transient)
FS, ISNS, DIM, EN to GND
–0.3
7
V
Storage temperature, Tstg
–65
125
°C
(1)
Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings
only, which do not imply functional operation of the device at these or any other conditions beyond those indicated under Recommended
Operating Conditions. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
6.2 ESD Ratings
VALUE
V(ESD)
(1)
(2)
Electrostatic discharge
Human-body model (HBM), per ANSI/ESDA/JEDEC JS-001 (1)
±2000
Charged-device model (CDM), per JEDEC specification JESD22C101 (2)
±750
UNIT
V
JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process.
JEDEC document JEP157 states that 250-V CDM allows safe manufacturing with a standard ESD control process.
6.3 Recommended Operating Conditions
over operating free-air temperature range (unless otherwise noted)
MIN
MAX
VIN
4.5
30
UNIT
V
Junction temperature
–40
125
°C
6.4 Thermal Information
LM3407
THERMAL METRIC (1)
DGN (MSOP)
UNIT
8 PINS
RθJA
Junction-to-ambient thermal resistance
55.6
°C/W
RθJC(top)
Junction-to-case (top) thermal resistance
50.7
°C/W
RθJB
Junction-to-board thermal resistance
28.8
°C/W
ψJT
Junction-to-top characterization parameter
1.6
°C/W
ψJB
Junction-to-board characterization parameter
28.6
°C/W
RθJC(bot)
Junction-to-case (bottom) thermal resistance
4.9
°C/W
(1)
4
For more information about traditional and new thermal metrics, see the Semiconductor and IC Package Thermal Metrics application
report.
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6.5 Electrical Characteristics
MIN and MAX limits apply for TJ = –40°C to +125°C unless specified otherwise. VIN = 12 V unless otherwise indicated.
PARAMETER
TEST CONDITIONS
MIN (1)
TYP (2)
MAX (1)
0.58
0.78
0.98
mA
UNIT
SYSTEM PARAMETERS
IIN
Operating input current
4.5 V ≤ VIN ≤ 30 V, LX = open,
VPWM = VEN = 5 V
IQ
Quiescent Input current
4.5 V ≤ VIN ≤ 30 V,
VPWM = 0 V, VEN = 5 V
0.2
0.27
0.39
mA
ISHUT
Shutdown input current
VEN = 0 V
36
48
60
µA
VUVLO
Input undervoltage lockout
threshold
VIN Rising
3.6
4.5
V
VUVLO-HYS
UVLO hysteresis
VIN Falling
200
VEN_H
EN Pin HIGH threshold
VEN Rising
1.9
VEN_L
EN Pin LOW threshold
VEN Falling
VDIM_H
DIM Pin HIGH threshold
VDIM Rising
VDIM_L
DIM Pin LOW threshold
VDIM Falling
1.3
1.75
1.3
1.75
1.9
RT = 80 kΩ
500
RT = 40 kΩ
1000
fSW
Switching frequency
tON-MIN
Minimum on-time
200
TSD
Thermal shutdown threshold
165
TSD-HYS
Thermal shutdown hysteresis
25
mV
2.4
V
V
2.4
V
V
kHz
ns
°C
INTERNAL VOLTAGE REGULATOR
VCC
VCC regulator output voltage (3)
VIN = 12 V
Main switch ON resistance
ISINK = 80 mA
4.5
V
MAIN SWITCH
RDS(ON)
0.77
1.45
Ω
CONTROL LOOP
AEA
(1)
(2)
(3)
Error amp open loop gain
60
dB
All limits specified at room temperature (TYP) and at temperature extremes (MIN/MAX). All room temperature limits are 100%
production tested. All limits at temperature extremes are specified through correlation using standard Statistical Quality Control (SQC)
methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).
Typical specification represent the most likely parametric norm at 25°C operation.
VCC provides self bias for the internal gate drive and control circuits. Device thermal limitations limit external loading to the pin.
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6.6 Typical Characteristics
All curves taken at VIN = 12 V with configuration in typical application for driving two power LEDs with ILED = 0.35 A shown in
this data sheet and TA = 25°C, unless otherwise specified.
TA = -40°C
TA = 25°C
Figure 1. Output Current vs Input Voltage
TA = 125°C
Figure 2. Output Current vs Input Voltage
TA = -40°C
Figure 4. Efficiency vs Input Voltage
Figure 3. Output Current vs Input Voltage
TA = 25°C
TA = 125°C
Figure 5. Efficiency vs Input Voltage
6
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Figure 6. Efficiency vs Input Voltage
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Typical Characteristics (continued)
All curves taken at VIN = 12 V with configuration in typical application for driving two power LEDs with ILED = 0.35 A shown in
this data sheet and TA = 25°C, unless otherwise specified.
Figure 7. Switch On Time vs Input Voltage
Figure 8. Operating Input Current vs Input Voltage
Figure 9. VCC Voltage vs Input Voltage
Figure 10. Output Current vs RISNS
VIN = 12 V
Figure 11. Switching Frequency vs RFS
L = 33 µH
fSW = 1 MHz
Figure 12. Continuous Mode Operation
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Typical Characteristics (continued)
All curves taken at VIN = 12 V with configuration in typical application for driving two power LEDs with ILED = 0.35 A shown in
this data sheet and TA = 25°C, unless otherwise specified.
VIN = 12 V
L = 33 µH
fSW = 500 kHz
VIN = 24 V
Figure 13. Continuous Mode Operation
VIN = 24 V
L = 33 µH
fSW = 1 MHz
Figure 14. Continuous Mode Operation
fSW = 500 kHz
VIN = 12 V
Figure 15. Continuous Mode Operation
VIN = 12 V
L = 33 µH
L = 33 µH
fSW = 1 MHz
Figure 16. DIM Pin Enable Transient
L = 33 µH
fSW = 1 MHz
Figure 17. DIM Pin Disable Transient
8
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7 Detailed Description
7.1 Overview
The LM3407 is a high power floating buck LED driver with a wide input voltage range. The device requires no
loop compensation network. The integrated power N-MOSFET enables high-output power with up to 350-mA
output current. The combination of Pulse Width Modulation (PWM), control architecture, and the proprietary
Pulse Level Modulation (PLM) ensures accurate current regulation, good EMI performance, and provides high
flexibility on inductor selection. High-speed dimming control input allows precision and high resolution brightness
control for applications require fine brightness adjustment.
7.2 Functional Block Diagram
FS
VIN
VIN
3.6V
VCC
regulator
Clock
Generator
+
UVLO
VCC
VCC
LX
SD
6
DIM
Set
DIM
SWITCH
CONTROL
Slope Compensation
DIM
+
PWM
Comparator
400 k:
3.6V
EA
gm
3.6V
5 PA
EN
+
Q1
ISNS
Reset
Waveform shaping and
Average
Current Sense
SD
198mV
+
-
GND
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7.3 Feature Description
7.3.1 Floating Buck Switching Converter
The LM3407 is designed for floating buck configuration. Different from conventional buck converters, a low-side
power N-MOSFET is used. The floating buck configuration simplifies the driver stage design and reduces the die
size of the power MOSFET. Additionally, the connections of the power diode, inductor and output capacitor are
switched to ground with a ground referenced power switch, Q1. The extraction of inductor current information can
be easily realized by a simple current sensing resistor. These benefits combine to provide a high efficiency, low
cost, and reliable solution for LED lighting applications.
The operation of the LM3407 constant current output floating buck converter is explained below. With the internal
switch Q1 turned ON, current flows through the inductor L1 and the LED array. Energy is also stored in the
magnetic field of the inductor during the ON cycle. The current flowing through RISNS during the ON cycle is
monitored by the Average Current Sensing block. The switch will remain ON until the average inductor current
equals 198 mV / RISNS. When the switch is turned OFF, the magnetic field starts to collapse and the polarity of
the inductor voltage reverses. At the same time, the diode is forward biased and current flows through the LED,
releasing the energy stored in the inductor to the output. True average output current is achieved as the
switching cycle continuously repeats and the Average Current Sensing block controls the ON duty cycle. A
constant current output floating buck converter only works in Continuous Conduction Mode (CCM); if the
converter enters Discontinuous Conduction Mode (DCM) operation, the current regulation will deteriorate and the
accuracy of LED current cannot be maintained. The operating waveforms for the typical application circuit are
shown in Figure 18.
VLX
time
ILX
time
ID1
time
VISNS
ILED x RISNS = 198 mV
time
ILED, IL1
ILED = 198 mV / RISNS
time
tON
tOFF
T
T = tON + tOFF
Figure 18. Operating Waveforms of a Floating Buck Converter
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Feature Description (continued)
7.3.2 Pulse Level Modulation (PLM)
The LM3407 incorporates the innovative Pulse Level Modulation technique. With an external 1% thick film
resistor connected to the ISNS pin, the converter output voltage can adjust automatically as needed to deliver
constant current within 10% accuracy to a serially connected LED string of different number and type. Pulse
Level Modulation is a novel method to provide precise constant current control with high efficiency. It allows the
use of low side current sensing and facilitates true average output current regulation regardless of the input
voltage and inductor value. Pulse Level Modulation can be treated as a process that transforms a trapezoidal
pulse chain into a square pulse chain with an amplitude equal to the center of inductor current ramp. Figure 19
shows the waveform of the converter in steady state. In the figure, IL1 is the inductor current and ILX is the switch
current into the LX pin. VISNS is the voltage drop across the current sensing resistor RISNS. VMSL is the center of
the inductor current ramp and is a reference pulse that is synchronized and has an identical pulse width to VISNS.
IL1
IOUT = IL1(AVG)
IOUT
time
ILX
time
VISNS
VMSL
time
VRP
VREF
time
tON
tOFF
T
Figure 19. LM3407 Switching Waveforms
The switching frequency and duty ratio of the converter equal:
tON
D=
tON + tOFF
and
fSW =
1
tON + tOFF
(1)
By comparing the area of VISNS and VRP over the ON period, an error signal is generated. Such a comparison is
functionally equivalent to comparing the middle level of ISNS to VRP during the ON-period of a switching cycle.
The error signal is fed to a PWM comparator circuit to produce the PWM control pulse to drive the internal power
N-MOSFET. Figure 20 shows the implementation of the PWM switching signal. The error signal is fed to a PWM
comparator circuit to produce the PWM control pulse to drive the internal power N-MOSFET. Figure 20 shows
the implementation of the PWM switching signal.
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Feature Description (continued)
In closed-loop operation, the difference between VMSL and VRP is reflected in the changes of the switching duty
cycle of the power switch. This behavior is independent of the inductance of the inductor and input voltage
because for the same set of IOUT * RISNS, ON time, and switching period, there exists only one VMSL. Figure 21
shows two sets of current sense signals named VISNS1 and VISNS2 that have identical frequencies and duty cycles
but different shapes of trapezoidal waveforms, each generating identical PWM signals.
VISNS1
VMSL
0
VPWM
0
VISNS2
VMSL
0
Figure 20. Pulse-Level Transformation
When VMSL is higher than VREF, the peak value of VRP, the switching duty cycle of the power switch will be
reduced to lower VMSL. When VMSL is lower than the peak value of VRP, the switching duty cycle of the power
switch will be increased to raise VMSL. For example, when IOUT is decreased, VMSL will become lower than VREF.
In order to maintain output current regulation, the switching duty cycle of the power switch will be increased and
eventually push up VMSL until VMSL equals VREF. Because in typical floating buck regulators VMSL is equal to IOUT
× RISNS, true average output current regulation can be achieved by regulating VMSL. Figure 22 shows the
waveforms of VISNS and VRP under closed loop operation.
1/fSW
VRP
VREF
D/fSW
VPWM
0
D/fSW
+
1/fSW
Error
Amplifier
-
0
PWM signal
Generator
To power switch
VISNS
VMSL
0
D/fSW
1/fSW
PWM
sawtooth
T
Figure 21. Implementation of the PWM Switching Signal
12
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Feature Description (continued)
T
tON
VRP
IOUT * RISNS
0
VISNS
VMSL < VREF
VMSL = VREF
Figure 22. Waveforms of VISNS and VRP Under Closed-Loop Operation
7.3.3 Internal VCC Regulator
The LM3407 has an internal 4.5 V linear regulator. This regulated voltage is used for powering the internal
circuitry only and any external loading at the VCC pin is not recommended. The supply input (VIN) can be
connected directly to an input voltage up to 30 V. The VCC pin provides voltage regulated at 4.5 V for VIN ≤ 6 V.
For 4.5 V ≤ VIN ≤ 6 V, VIN pin will be connected to VCC pin directly by an internal bypassing switch. For stability
reason, an external capacitor CVCC with at least 680 nF (1 µF recommended) must be connected to the VCC pin.
7.3.4 Clock Generator
The LM3407 features an integrated clock generator to control the switching frequency of the converter, fSW. An
external resistor RFS, connected to the FS pin and ground, determines the switching frequency. The oscillator
frequency can be set in the range of 300 kHz to 1 MHz. The relationship between the frequency setting
resistance and the oscillator frequency is described in the Application Information Section.
7.3.5 PWM Dimming of LED String
Dimming of LED brightness is achieved by Pulse Width Modulation (PWM) control of the LED current. Pulse
Width Modulation control allows LED brightness to be adjusted while still maintaining accurate LED color
temperature. The LM3407 accepts an external PWM dimming signal at the DIM pin. The signal is buffered before
being applied to the internal switch control block responsible for controlling the ON/OFF of the power switch, Q1.
The DIM pin is internally pulled low by a resistor and no LED current will be available when the DIM pin is
floating or shorted to ground. Functionally, the DIM pin can also be used as an external device disable control.
Device switching will be disabled if the DIM pin is not connected or tied to ground.
7.3.6 Input Under-Voltage Lock-Out (UVLO)
The LM3407 incorporates an input Under-Voltage Lock-Out (UVLO) circuit with hysteresis to keep the device
disabled when the input voltage (VIN) falls below the Lock-Out Low threshold, 3.4 V typical. During the device
power-up, internal circuits are held inactive and the UVLO comparator monitors the voltage level at the VIN pin
continuously. When the VIN pin voltage exceeds the UVLO threshold, 3.6 V typical, the internal circuits are then
enabled and normal operation begins.
7.4 Device Functional Modes
7.4.1 Low-Power Shutdown Mode
The LM3407 comes with a dedicated device enable pin, EN, for low-power shutdown of the device. By putting
the device in shutdown mode, most of the internal circuits will be disabled and the input current will reduced to
below typically 50 µA. The EN pin is internally pulled high by a 5-µA current source. Connecting the EN pin to
ground will force the device to enter low power shutdown mode. To resume normal operation, leave the EN pin
open or drive with a logic high voltage.
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8 Application and Implementation
NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
8.1 Application Information
8.1.1 Switching Frequency Selection
The selection of switching frequency is based on the consideration of the conversion efficiency, size of the
passive components, and the total solution cost. In general, increasing the switching frequency allows the use of
smaller external components but decreases the conversion efficiency. Thus, the selection of switching frequency
is a compromise between the system requirements and may vary from design to design. The LM3407 switching
frequency can be set in the range from 300 kHz to 1 MHz by adjusting the value of RFS. The switching frequency
is inversely proportional to the value of RFS. To ensure good operation stability, a resistor with 1% tolerance
between 40 kΩ and 96 kΩ and with good thermal stability is suggested.
The switching frequency is estimated by Equation 2:
fSW =
40 Meg
RFS
+ 40 in kHz
where
•
•
fSW is the oscillator frequency
RFS is the frequency setting resistance
(2)
Equation 2 is only valid for oscillator frequencies in the range of 300 kHz to 1 MHz, so the frequency setting
resistance will be in the range of about 40 kΩ to 150 kΩ.
8.1.2 LED Current Setting
The LED current setting is important to the lifetime, reliability, and color temperature of the LED string. The LED
current should be properly selected according to the characteristics of the LED used. Over-driving the LED array
can cause the color temperature to shift and will shorten the lifetime of the LEDs. The output current of the
LM3407 can be set by RISNS, which is calculated from Equation 3:
0.198V
RISNS =
IOUT
(3)
To ensure the accuracy of the output current, a resistor with 1% tolerance should be used for RISNS. It is also
important for the designer to ensure that the rated power of the resistor is not exceeded with reasonable margin.
For example, when IOUT is set to 350 mA, the total power dissipation on RISNS in steady state is (0.35 A)2 × 0.565
Ω, which equals 69 mW, indicating a resistor of 1/8W power rating is appropriate.
8.1.3 Input and Output Capacitors
The input capacitor supplies instantaneous current to the LM3407 converter when the internal power switch Q1
turns ON. The input capacitor filters the noise and transient voltage from the input power source. Using low ESR
capacitors such as ceramic and tantalum capacitors is recommended. Similar to the selection criteria for the
output capacitor, ceramic capacitors are the best choice for the input to the LM3407 due to their high ripple
current rating, low ESR, and relatively small size compared to other types. A 4.7-µF X7R ceramic capacitor for
the input capacitor is recommended
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Application Information (continued)
The output capacitor COUT is used to reduce LED current ripple, filter noise, and smooth output voltage. This
capacitor should have low ESR and adequate capacitance. Excessively large output capacitances create long
enable and disable times, which is particularly significant when a high dimming frequency is used. Because the
loading and input conditions differ from design to design, a 2.2-µF X7R ceramic capacitor is a good initial
selection. A DC voltage rating equal to or higher than twice the forward voltage of the LED string is
recommended.
COUT is optional and can be omitted for applications where small brightness variation is acceptable. Omitting
COUT also helps reduce the cost and board size of the converter. With the absence of COUT, the LED forward
current equals the inductor current. To ensure proper operation of the converter, the peak inductor current must
not exceed the rated forward current of the LEDs. Otherwise the LEDs may be damaged.
8.1.4 Selection of Inductor
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, 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:
Inductor Peak to Peak Ripple Current:
IL(ripple) = VIN - (n x VF) - 0.198 1 +
1
RISNS
x (n x VF)
L x VIN x fSW
(4)
Peak Inductor Current:
IL(peak) =
0.198 IL(ripple)
+
2
RISNS
where
•
•
n is the number of LEDs in a string
VF is the forward voltage of one LED.
(5)
The minimum inductance required for the specific application can be calculated by Equation 6:
Lmin = VIN - (n x VF) - 0.198 x 1 +
1
RISNS
x (RISNS x n x VF)
0.197 x VIN x fSW
(6)
For applications with no output capacitor in place, the magnitude of the inductor ripple current should not be
more than 20% of the average inductor current, which is equivalent to the output current, IOUT. However, in some
situations the physical size of the required inductor may be too large and thus not allowed. The output capacitor
can help absorb this current ripple to significantly reduce the ripple component along the LED string. With an
output capacitor COUT in place, the magnitude of the inductor ripple current can be relaxed to 80% of the output
current. Figure 23 illustrates the relationship between IOUT, IL(peak), and IL(ripple).
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Application Information (continued)
IL1
IL (peak)
IOUT
IL (ripple)
time
tON
T
tOFF
Figure 23. Relationship Between IOUT, IL(peak) and IL(ripple)
Table 1 provides the suggested inductance of the inductor for 500 kHz and 1 MHz switching frequency operation
with COUT = 4.7 µF and IL(ripple) = 0.8 × IOUT
Table 1. Suggested Inductance Value of the Inductor
VIN / V
Number of LED
1
2
3
4
5
6
7
Inductor selection table for FSW = 500 kHz, 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
33 µH
22 µH
22 µH
22 µH
25
22 µH
33 µH
33 µH
22 µH
22 µH
22 µH
30
22 µH
47 µH
33 µH
33 µH
33 µH
22 µH
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
33 µH
22 µH
22 µH
22 µH
22 µH
30
22 µH
33 µH
33 µH
33 µH
22 µH
22 µH
22 µH
8.1.5 Free-Wheeling Diode
The LM3407 is a non-synchronous floating buck converter that requires an external free-wheeling diode to
provide a path for recirculating current from the inductor to the LED array when the power switch is turned OFF.
Selecting the free-wheeling diode depends on both the output voltage and current. The diode must have a rated
reverse voltage higher than the input voltage of the converter and a peak current rating higher than the expected
maximum inductor current. Using a schottky diode with a low forward voltage drop can reduce power dissipation
and enhance conversion efficiency.
16
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8.2 Typical Applications
8.2.1 LM3407 Design Example
Figure 24. LM3407 Design Example Schematic
8.2.1.1 Design Requirements
• Input Voltage: VIN = 12 V ±10%
• LED String Voltage: VLED = 6.4 V (2 series white LEDs)
• LED Current: ILED = 350 mA
• Switching Frequency: fSW = 1 MHz
8.2.1.2 Detailed Design Procedure
This design is intended to be a small size, low-cost solution. An output capacitor will not be used to save on size
and cost so a high switching frequency will be used and a higher value inductor than recommended in Table 1
will be used to keep LED current ripple lower.
8.2.1.2.1 Calculate RISNS
For 350 mA LED current calculate the value for RISNS using Equation 7.
RISNS =
0.198V 0.198V
=
IOUT
0.35A
(7)
Choose a standard value of RISNS = 0.565 Ω.
8.2.1.2.2 Calculate RFS
Calculate the value of RFS for 1-MHz switching frequency using Equation 8.
RFS
40 × 106
40 × 106
=
=
= 41.6k
fSW - 40
1000 - 40
(8)
Choose a standard value of RFS = 40.2 kΩ.
8.2.1.2.3 Choose L
Referring to Table 1 the recommended inductor value for 12 V input and 2 LED output is 22 µH.
Choose a higher standard value of L = 33 µH to reduce ripple since an output capacitor will not be used for this
design.
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Typical Applications (continued)
8.2.1.2.4 Choose CIN and CVCC
Choose the recommended values of CIN = 4.7 µF and CVCC = 1 µF. CIN should be a 16 V or greater ceramic
capacitor and CVCC should be a 10 V or greater ceramic capacitor. Both should use an X5R or X7R dielectric.
8.2.1.3 Application Curve
Figure 25. LED Current and Switch Voltage Waveforms
8.2.2 Typical Application for Driving 6 LEDs
Figure 26 shows a high voltage, 6-W application for driving 6 LEDs. The switching frequency is set at 1 MHz and
the LED current is set at 350 mA.
Figure 26. LM3407 6 LED Example Schematic
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Typical Applications (continued)
8.2.3 Typical Application for Driving 1 LED
Figure 27 shows a low voltage, 1-W application for driving 1 LED. The switching frequency is set at 1 MHz and
the LED current is set at 350 mA.
Figure 27. LM3407 1 LED Example Schematic
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9 Power Supply Recommendations
Use any DC output power supply with a maximum voltage high enough for the application. The power supply
should have a minimum current limit of at least 1 A.
10 Layout
10.1 Layout Guidelines
Because the 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 may impair circuit performance. To optimize the performance of the LM3407, the rule of thumb is to keep the
connections between components as short and direct as possible. Because true average current regulation is
achieved by detecting the average switch current, the current setting resistor RISNS must be located as close as
possible to the LM3407 to reduce the parasitic inductance of the copper trace and avoid noise pick-up. The
connections between the LX pin, rectifier D1, inductor L1, and output capacitor COUT should be kept as short as
possible to reduce the voltage spikes at the LX pin. TI recommends that CVCC, the output filter capacitor for the
internal linear regulator of the LM3407, be placed close to the VCC pin. 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 0.1-µF capacitor
should be added close to VIN pin for noise filtering.
CAUTION
In normal operation, heat will be generated inside the LM3407 and may damage the
device if no thermal management is applied. For more details on switching power
supply layout considerations see AN-1149 Layout Guidelines for Switching Power
Supplies (SNVA021).
10.2 Layout Example
GND
RISNS
L1
LED-
G
GND
+
LX
-
ISNS
VIN/LED+
D1
DIM
GND
CVCC
RFS
EN
VCC
FS
VIN
CIN
THERMAL/POWER VIA
Figure 28. Layout Recommendation
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11 Device and Documentation Support
11.1 Documentation Support
11.1.1 Related Documentation
For related documentation see the following:
AN-1149 Layout Guidelines for Switching Power Supplies (SNVA021).
11.2 Receiving Notification of Documentation Updates
To receive notification of documentation updates, navigate to the device product folder on ti.com. In the upper
right corner, click on Alert me to register and receive a weekly digest of any product information that has
changed. For change details, review the revision history included in any revised document.
11.3 Community Resources
The following links connect to TI community resources. Linked contents are provided "AS IS" by the respective
contributors. They do not constitute TI specifications and do not necessarily reflect TI's views; see TI's Terms of
Use.
TI E2E™ Online Community TI's Engineer-to-Engineer (E2E) Community. Created to foster collaboration
among engineers. At e2e.ti.com, you can ask questions, share knowledge, explore ideas and help
solve problems with fellow engineers.
Design Support TI's Design Support Quickly find helpful E2E forums along with design support tools and
contact information for technical support.
11.4 Trademarks
E2E is a trademark of Texas Instruments.
All other trademarks are the property of their respective owners.
11.5 Electrostatic Discharge Caution
These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
11.6 Glossary
SLYZ022 — TI Glossary.
This glossary lists and explains terms, acronyms, and definitions.
12 Mechanical, Packaging, and Orderable Information
The following pages include mechanical, packaging, and orderable information. This information is the most
current data available for the designated devices. This data is subject to change without notice and revision of
this document. For browser-based versions of this data sheet, refer to the left-hand navigation.
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PACKAGE OPTION ADDENDUM
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10-Dec-2020
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
Eco Plan
(2)
Lead finish/
Ball material
MSL Peak Temp
Op Temp (°C)
Device Marking
(3)
(4/5)
(6)
LM3407MY/NOPB
ACTIVE
HVSSOP
DGN
8
1000
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
STZB
LM3407MYX/NOPB
ACTIVE
HVSSOP
DGN
8
3500
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
STZB
(1)
The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2)
RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance
do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may
reference these types of products as "Pb-Free".
RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.
Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of
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