January 18, 2008
LM3407
350 mA, Constant Current Output Floating Buck Switching
Converter for High Power LEDs
LM3407 350 mA, Constant Current Output Floating Buck Switching Converter for High Power
LEDs
General Description
The LM3407 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 N-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 low power shutdown. An exposed pad eMSOP-8 package provides excellent heat dissipation and thermal performance. Input UVLO and output
open-circuit protection ensure a robust LED driver solution.
Typical Application
Features
Input operating range 4.5V to 30V
■
Output voltage range: 0.1VIN to 0.9V
■
Accurate constant current output
■
Cycle-by-Cycle current limit
■
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 Under Voltage Lock Out (UVLO)
■
Thermal shutdown protection
■
eMSOP-8 Package
■
IN
Applications
LED Driver
■
Constant Current Source
■
Automotive Lighting
■
General Illumination
■
Industrial Lighting
■
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© 2008 National Semiconductor Corporation 300466 www.national.com
Connection Diagram
LM3407
8-Lead Plastic eMSOP-8 Package
Top View
Mini SOIC Exp Pad (MUY08A)
30046602
Ordering Information
Order Number Package Type NSC Package Drawing Supplied As
LM3407MY eMSOP-8 MUY08A 1000 Units on Tape and Reel
LM3407MYX 3500 Units on Tape and Reel
Pin Descriptions
Pin(s) Name Description Application Information
1 ISNS LED Current Sense pin Connect resistor R
sensing resistor should be placed close to this pin.
2 DIM PWM Dimming Input pin Applying logic level PWM signal to this pin controls the average brightness of the
LED string.
3 EN Device Enable pin Applying logic high to this pin or leaving this pin open enables the switcher. When
this pin is pulled low, the switcher is disabled and will enter low power shutdown
mode.
4 FS Switching Frequency
Setting pin
5 VIN Input Voltage pin The input voltage should be in the range of 4.5V to 30V.
6 VCC Internal Regulator
Output pin
7 GND Device Ground pin This pin should be connected to the system ground.
8 LX Drain of N-MOSFET
Switch
EP GND Thermal Pad The bottom pad should be connected to ground. For good thermal performance,
Connect resistor RFS from this pin to ground to set the switching frequency.
This output pin should be bypassed by a ceramic capacitor with a minimum value
of 1µF. High quality X5R or X7R ceramic capacitor is recommended.
Connect this pin to the output inductor and anode of the Schottky diode.
place 4 to 6 thermal vias from EP to bottom layer PCB ground plane.
from this pin to ground for LED current sensing. The current
ISNS
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LM3407
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
VIN to GND -0.3V to 36V
VIN to GND (Transient) 42V (500 ms)
LX to GND -0.3V to 36V
LX to GND (Transient) -3V (2 ns), 42V (500 ms)
ISNS, FS, DIM, EN to GND -0.3V to 7V
ESD Rating
Human Body Model (Note 2) 2kV
Electrical Characteristics V
= 12V unless otherwise indicated. Typical and limits appearing in plain type apply
IN
Junction Temperature 150°C
Storage Temperature −65°C to + 125°C
Soldering Information
Lead Temperature (Soldering,
10sec)
Infrared or Convection (20sec) 235°C
Operating Ratings
V
IN
Junction Temperature Range −40°C to + 125°C
Thermal Resistance (θJA) (Note 3)
4.5V to 30V
50°C/W
for TA = TJ = 25°C (Note 4). Limits appearing in boldface type apply over full Operating Temperature Range. Datasheet min/max
specification limits are guaranteed by design, test, or statistical analysis.
Symbol Parameter Conditions Min Typ Max Units
SYSTEM PARAMETERS
I
IN
I
Q
I
SHUT
V
UVLO
V
UVLO-HYS
V
EN_H
V
EN_L
V
DIM_H
V
DIM_L
f
SW
t
ON-MIN
T
SD
T
SD-HYS
Operating Input Current
Quiescent Input current
4.5V ≤ VIN ≤ 30V
VEN = 5V, V
PWM
4.5V ≤ VIN ≤ 30V
VEN = 5V, V
PWM
= 5V, LX = open
= 0V
0.58 0.78 0.98 mA
0.20 0.27 0.39 mA
Shutdown Input Current VEN = 0V 36 48 60 µA
Input Under Voltage Lock-out Threshold VIN Rising 3.6 4.5 V
UVLO Hysteresis VIN Falling 200 mV
EN pin HIGH Threshold VEN Rising 1.9 2.4 V
EN pin LOW Threshold VEN Falling 1.3 1.75 V
DIM pin HIGH Threshold V
DIM pin LOW Threshold V
Switching Frequency
Rising 1.9 2.4 V
DIM
Falling 1.3 1.75 V
DIM
RT = 80 kΩ
RT = 40 kΩ
500 kHz
1000
Minimum On-time 200 ns
Thermal Shutdown Threshold 165 °C
Thermal Shutdown Hysteresis 25 °C
INTERNAL VOLTAGE REGULATOR
V
CC
VCC Regulator Output Voltage (Note 5) VIN = 12V 4.5 V
N-MOSFET DRIVER
R
DS(ON)
Main Switch ON Resistance I
= 80mA 0.77 1.45
sink
CONTROL LOOP
A
EA
Error Amp Open Loop Gain 60 dB
260°C
Ω
Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions under which operation of the
device is intended to be functional. For guaranteed specifications and test conditions, see the Electrical Characteristics.
Note 2: The human body model is a 100pF capacitor discharged through a 1.5kΩ resistor into each pin.
Note 3: θJA of 50°C/W with thermal pad, EP soldered to a minimum of 2 square inches of 1 oz. Copper on the top or bottom PCB layer.
Note 4: Typical specification represent the most likely parametric norm at 25°C operation.
Note 5: 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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Typical Performance Characteristics Unless otherwise specified, all curves shown are taken in typical
application at VIN = 12V, TA = 25°C, and I
LM3407
Output Current vs Input Voltage
(TA = -40°C)
= 350 mA (driving two power LEDs).
LED
Output Current vs Input Voltage
(TA = 25°C)
Output Current vs Input Voltage
(TA = 125°C)
Efficiency vs Input Voltage
(TA = 25°C)
30046603
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Efficiency vs Input Voltage
(TA = 40°C)
30046604
Efficiency vs Input Voltage
(TA = 125°C)
30046605
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30046606
LM3407
Switch On Time vs Input Voltage
VCC Voltage vs Input Voltage
30046607
Operating Input Current vs Input Voltage
30046608
Output Current vs R
ISNS
Switching Frequency vs R
30046609
FS
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Continuous Mode Operation
(VIN = 12V, L = 33µH, fSW = 1MHz)
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LM3407
Continuous Mode Operation
(VIN = 12V, L = 33µH, fSW = 500kHz)
Continuous Mode Operation
(VIN = 24V, L = 33µH, fSW = 1MHz)
Continuous Mode Operation
(VIN = 24V, L = 33µH, fSW = 500kHz)
DIM Pin Disable Transient
(VIN = 12V, L = 33µH, fSW = 1MHz)
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DIM Pin Enable Transient
(VIN = 12V, L = 33µH, fSW = 1MHz)
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Simplified Functional Block Diagram
LM3407
Functional Description
OVERVIEW
The LM3407 is a constant current output floating buck switching converter with wide input voltage range and low feedback
current sense reference voltage. These characteristics make
the LM3407 an efficient solution to provide constant current
to high power LEDs. The device is ideal for automotive, industrial and general lighting applications where high power
LEDs are used as the lighting source. The LM3407 has an
integrated power N-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 low power shutdown.
An exposed pad eMSOP-8 package provides excellent heat
dissipation and thermal performance. Input UVLO and output
open-circuit protection ensure a robust LED driver solution.
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
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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
R
during the ON cycle is monitored by the Average Cur-
ISNS
rent Sensing block. The switch will remain ON until the average inductor current equals 198mV / R
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 1.
. When the switch
ISNS
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LM3407
30046619
FIGURE 1. Operating Waveforms of a Floating Buck Converter
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 2 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. V
sistor R
and is a reference pulse that is synchronized and has an
identical pulse width to V
is the voltage drop across the current sensing re-
ISNS
. V
ISNS
is the center of the inductor current ramp
MSL
.
ISNS
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FIGURE 2. LM3407 Switching Waveforms
LM3407
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The switching frequency and duty ratio of the converter equal:
By comparing the area of V
an error signal is generated. Such a comparison is functionally equivalent to comparing the middle level of I
during the ON-period of a switching cycle. The error signal is
and VRP over the ON period,
ISNS
SNS
to V
RP
fed to a PWM comparator circuit to produce the PWM control
pulse to drive the internal power N-MOSFET. Figure 3 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
3 shows the implementation of the PWM switching signal.
In closed loop operation, the difference between V
VRP is reflected in the changes of the switching duty cycle of
MSL
and
the power switch. This behavior is independent of the inductance of the inductor and input voltage because for the same
set of I
only one V
nals named V
and duty cycles but different shapes of trapezoidal wave-
* R
OUT
, ON time, and switching period, there exists
ISNS
. Figure 4 shows two sets of current sense sig-
MSL
ISNS1
and V
that have identical frequencies
ISNS2
forms, each generating identical PWM signals.
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FIGURE 3. Pulse-Level Transformation
When V
switching duty cycle of the power switch will be reduced to
lower V
the switching duty cycle of the power switch will be increased
to raise V
become lower than V
regulation, the switching duty cycle of the power switch will
be increased and eventually push up V
V
REF
to I
OUT
achieved by regulating V
of V
is higher than V
MSL
. When V
MSL
. For example, when I
MSL
is lower than the peak value of VRP,
MSL
REF
. Since in typical floating buck regulators V
* R
, true average output current regulation can be
ISNS
and VRP under closed loop operation.
ISNS
, the peak value of VRP, the
REF
is decreased, V
. In order to maintain output current
MSL
OUT
until V
MSL
. Figure 5 shows the waveforms
MSL
MSL
will
MSL
equals
is equal
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LM3407
30046622
FIGURE 4. Implementation of the PWM Switching Signal
FIGURE 5. Waveforms of V
ISNS
INTERNAL VCC REGULATOR
The LM3407 has an internal 4.5V 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 30V. The VCC pin provides voltage regulated at
4.5V for VIN ≤ 6V. For 4.5V ≤ VIN ≤ 6V, VIN pin will be connected to VCC pin directly by an internal bypassing switch.
For stability reason, an external capacitor C
680 nF (1 µF recommended) must be connected to the VCC
with at least
VCC
pin.
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.
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
30046624
and VRP Under Closed Loop Operation
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.
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 50µA. The
EN pin is internally pulled high by a 5µA 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.
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, 3.4V 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
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LM3407
pin voltage exceeds the UVLO threshold, 3.6V typical, the internal circuits are then enabled and normal operation begins.
Application Information
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 will allow the use of smaller external
components but will decrease 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. In order to guarantee 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 the expression below:
In the equation, fSW is the oscillator frequency and RFS is the
frequency setting resistance. The above equation 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.
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. Since 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.
C
is optional and can be omitted for applications where
OUT
small brightness variation is acceptable. Omitting C
helps reduce the cost and board size of the converter. With
the absence of C
ductor current. In order to ensure proper operation of the
, the LED forward current equals the in-
OUT
OUT
also
converter the peak inductor current must not exceed the rated
forward current of the LEDs. Otherwise the LEDs may be
damaged.
SELECTION OF INDUCTOR
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:
Inductor Peak to Peak Ripple Current:
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
R
, which is calculated from the following equation:
ISNS
To ensure the accuracy of the output current, a resistor with
1% tolerance should be used for R
the designer to ensure that the rated power of the resistor is
. It is also important for
ISNS
not exceeded with reasonable margin. For example, when
I
is set to 350 mA, the total power dissipation on R
OUT
steady state is (0.35A)2 x 0.565Ω, which equals 69 mW, in-
ISNS
in
dicating a resistor of 1/8W power rating is appropriate.
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
The output capacitor C
ple, filter noise, and smooth output voltage. This capacitor
is used to reduce LED current rip-
OUT
Peak Inductor Current:
where η 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:
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, I
physical size of the required inductor may be too large and
. However, in some situations the
OUT
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 C
the magnitude of the inductor ripple current can be relaxed to
OUT
in place,
80% of the output current. Figure 6 illustrates the relationship
between I
OUT
, I
L(peak)
, and I
L(ripple)
.
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LM3407
30046625
FIGURE 6. Relationship between I
OUT
, I
L(peak)
and I
L(ripple)
Table 1 provides the suggested inductance of the inductor for
500 kHz and 1 MHz switching frequency operation with
C
= 4.7µF and I
OUT
L(ripple)
= 0.8 x I
OUT
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, C
= 4.7µF (1µF for 1 LED)
OUT
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, C
= 4.7µF (1µF for 1 LED)
OUT
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
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 freewheeling diode depends on both the output voltage and cur-
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rent. 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.
LM3407
PRINTED CIRCUIT BOARD DESIGN
Since 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. Since true average current regulation is achieved by detecting the average
switch current, the current setting resistor R
cated as close as possible to the LM3407 to reduce the
must be lo-
ISNS
parasitic inductance of the copper trace and avoid noise pickup. The connections between the LX pin, rectifier D1, inductor
L1, and output capacitor C
possible to reduce the voltage spikes at the LX pin. It is recommended that C
nal linear regulator of the LM3407, be placed close to the VCC
, the output filter capacitor for the inter-
VCC
should be kept as short as
OUT
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 0.1µF capacitor should be added close to
VIN pin 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 details on
switching power supply layout considerations see Application
Note AN-1149: Layout Guidelines for Switching Power Supplies.
FIGURE 7. Typical Application Schematic for 6 LEDs
FIGURE 8. Typical Application Schematic for 1 LED
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30046627
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Physical Dimensions inches (millimeters) unless otherwise noted
LM3407
8-Lead Plastic eMSOP Package
NS Package Number MUY08A
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Notes
LM3407
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Notes
LEDs
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