NSC LM3407MY 350 ma, constant current output floating buck switching converter for high power led Datasheet

LM3407
350 mA, Constant Current Output Floating Buck Switching
Converter for High Power LEDs
General Description
Features
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.
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Input operating range 4.5V to 30V
Output voltage range: 0.1VIN to 0.9VIN
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 Under Voltage Lock Out (UVLO)
Thermal shutdown protection
eMSOP-8 Package
Applications
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LED Driver
Constant Current Source
Automotive Lighting
General Illumination
Industrial Lighting
Typical Application
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© 2009 National Semiconductor Corporation
300466
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LM3407 350 mA, Constant Current Output Floating Buck Switching Converter
for High Power LEDs
January 21, 2009
LM3407
Connection Diagram
30046602
Top View
8-Lead Plastic eMSOP-8 Package
Mini SOIC Exp Pad (MUY08A)
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
1
ISNS
LED Current Sense pin Connect resistor RISNS from this pin to ground for LED current sensing. The current
sensing resistor should be placed close to this pin.
Description
Application Information
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
4
FS
Switching Frequency
Setting 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.
Connect resistor RFS from this pin to ground to set the switching frequency.
5
VIN
Input Voltage pin
The input voltage should be in the range of 4.5V to 30V.
6
VCC
Internal Regulator
Output pin
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.
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
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Connect this pin to the output inductor and anode of the Schottky diode.
The bottom pad should be connected to ground. For good thermal performance,
place 4 to 6 thermal vias from EP to bottom layer PCB ground plane.
2
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
VIN to GND
VIN to GND (Transient)
LX to GND
LX to GND (Transient)
ISNS, FS, DIM, EN to GND
ESD Rating
Human Body Model (Note 2)
-0.3V to 36V
42V (500 ms)
-0.3V to 36V
-3V (2 ns), 42V (500 ms)
-0.3V to 7V
150°C
−65°C to + 125°C
260°C
235°C
Operating Ratings
VIN
Junction Temperature Range
4.5V to 30V
−40°C to + 125°C
Thermal Resistance (θJA) (Note 3)
2kV
50°C/W
Electrical Characteristics
VIN = 12V unless otherwise indicated. Typical and limits appearing in plain type apply
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
4.5V ≤ VIN ≤ 30V
0.58
0.78
0.98
mA
0.20
0.27
0.39
mA
36
48
60
µA
4.5
SYSTEM PARAMETERS
IIN
Operating Input Current
IQ
Quiescent Input current
ISHUT
Shutdown Input Current
VUVLO
Input Under Voltage Lock-out Threshold VIN Rising
3.6
UVLO Hysteresis
VIN Falling
200
VEN_H
EN pin HIGH Threshold
VEN Rising
VEN_L
EN pin LOW Threshold
VEN Falling
VDIM_H
DIM pin HIGH Threshold
VDIM Rising
VDIM_L
DIM pin LOW Threshold
VDIM Falling
Switching Frequency
VEN = 5V, VPWM = 5V, LX = open
4.5V ≤ VIN ≤ 30V
VEN = 5V, VPWM = 0V
VUVLO-HYS
fSW
tON-MIN
VEN = 0V
1.9
V
mV
2.4
V
1.3
1.75
1.3
1.75
V
RT = 80 kΩ
500
kHz
RT = 40 kΩ
1000
1.9
V
2.4
V
Minimum On-time
200
ns
TSD
Thermal Shutdown Threshold
165
°C
TSD-HYS
Thermal Shutdown Hysteresis
25
°C
4.5
V
INTERNAL VOLTAGE REGULATOR
VCC
VCC Regulator Output Voltage (Note 5) VIN = 12V
N-MOSFET DRIVER
RDS(ON)
Main Switch ON Resistance
Isink = 80mA
0.77
1.45
Ω
CONTROL LOOP
AEA
Error Amp Open Loop Gain
60
dB
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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LM3407
Junction Temperature
Storage Temperature
Soldering Information
Lead Temperature (Soldering,
10sec)
Infrared or Convection (20sec)
Absolute Maximum Ratings (Note 1)
LM3407
Typical Performance Characteristics
Unless otherwise specified, all curves shown are taken in typical
application at VIN = 12V, TA = 25°C, and ILED = 350 mA (driving two power LEDs).
Output Current vs Input Voltage
(TA = -40°C)
Output Current vs Input Voltage
(TA = 25°C)
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30046632
Output Current vs Input Voltage
(TA = 125°C)
Efficiency vs Input Voltage
(TA = -40°C)
30046633
30046604
Efficiency vs Input Voltage
(TA = 25°C)
Efficiency vs Input Voltage
(TA = 125°C)
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30046606
4
Operating Input Current vs Input Voltage
30046607
30046608
VCC Voltage vs Input Voltage
Output Current vs RISNS
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Switching Frequency vs RFS
Continuous Mode Operation
(VIN = 12V, L = 33µH, fSW = 1MHz)
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30046611
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LM3407
Switch On Time vs Input Voltage
LM3407
Continuous Mode Operation
(VIN = 12V, L = 33µH, fSW = 500kHz)
Continuous Mode Operation
(VIN = 24V, L = 33µH, fSW = 1MHz)
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Continuous Mode Operation
(VIN = 24V, L = 33µH, fSW = 500kHz)
DIM Pin Enable Transient
(VIN = 12V, L = 33µH, fSW = 1MHz)
30046615
30046616
DIM Pin Disable Transient
(VIN = 12V, L = 33µH, fSW = 1MHz)
30046617
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LM3407
Simplified Functional Block Diagram
30046618
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 198mV / 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 1.
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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LM3407
30046619
FIGURE 1. Operating Waveforms of a Floating Buck Converter
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. 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.
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
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LM3407
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FIGURE 2. LM3407 Switching Waveforms
The switching frequency and duty ratio of the converter equal:
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 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 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 4 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.
30046623
FIGURE 3. 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. Since in typical floating buck regulators VMSL is equal
to IOUT * RISNS, true average output current regulation can be
achieved by regulating VMSL. Figure 5 shows the waveforms
of VISNS and VRP under closed loop operation.
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LM3407
30046622
FIGURE 4. Implementation of the PWM Switching Signal
30046624
FIGURE 5. Waveforms of VISNS and VRP Under Closed Loop Operation
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 CVCC with at least
680 nF (1 µF recommended) must be connected to the VCC
pin.
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.
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.
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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
10
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 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. 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.
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 C OUT, the LED forward current equals the inductor current. In order 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.
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
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:
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:
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.
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 the following equation:
Peak Inductor Current:
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 (0.35A)2 x 0.565Ω, which equals 69 mW, indicating a resistor of 1/8W power rating is appropriate.
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:
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LM3407
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.
LM3407
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 6 illustrates the relationship
between IOUT, IL(peak), and IL(ripple).
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
30046625
FIGURE 6. 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 = 4.7µF and IL(ripple) = 0.8 x 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 = 4.7µF (1µF 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 = 4.7µF (1µF 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
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.
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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22 µH
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30046626
FIGURE 7. Typical Application Schematic for 6 LEDs
30046627
FIGURE 8. Typical Application Schematic for 1 LED
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LM3407
L1, and output capacitor COUT should be kept as short as
possible to reduce the voltage spikes at the LX pin. It is recommended 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 0.1µF 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.
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 RISNS must be located as close as possible to the LM3407 to reduce the
parasitic inductance of the copper trace and avoid noise pickup. The connections between the LX pin, rectifier D1, inductor
LM3407
Physical Dimensions inches (millimeters) unless otherwise noted
8-Lead Plastic eMSOP Package
NS Package Number MUY08A
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14
LM3407
Notes
15
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LM3407 350 mA, Constant Current Output Floating Buck Switching Converter
for High Power LEDs
Notes
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LIFE SUPPORT POLICY
NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR
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