NSC LM3404

National Semiconductor
Application Note 1839
Matthew Reynolds
December 10, 2008
Introduction
Thermal Performance
The LM3402/02HV and LM3404/04HV are buck regulator derived controlled current sources designed to drive a series
string of high power, high brightness LEDs (HBLEDs) at forward currents of up to 0.5A (LM3402/02HV) or 1.0A
(LM3404/04HV). This evaluation board demonstrates the enhanced thermal performance, fast dimming, and true constant
LED current capabilities of the LM3402 and LM3404 devices.
The PSOP-8 package is pin-for-pin compatible with the SO-8
package with the exception of the thermal pad, or exposed
die attach pad (DAP). The DAP is electrically connected to
system ground. When the DAP is properly soldered to an area
of copper on the top layer, bottom layer, internal planes, or
combinations of various layers, the θJA of the LM3404/04HV
can be significantly lower than that of the SO-8 package. The
PSOP-8 evaluation board is two layers of 1oz copper each,
and measures 1.25" x 1.95". The DAP is soldered to approximately 1/2 square inch of top and two square inches of
bottom layer copper. Three thermal vias connect the DAP to
the bottom layer of the PCB. A recommended DAP/via layout
is shown in figure 2.
Circuit Performance with LM3404
This evaluation board (figure 1) uses the LM3404 to provide
a constant forward current of 750 mA ±10% to a string of up
to five series-connected HBLEDs with a forward voltage of
approximately 3.4V each from an input of 18V to 36V.
LM3402/LM3404 Fast Dimming and True Constant LED Current Evaluation Board
LM3402/LM3404 Fast
Dimming and True Constant
LED Current Evaluation
Board
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FIGURE 1. LM3402 / 04 Schematic
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© 2008 National Semiconductor Corporation
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FIGURE 3. Buck Converter Inductor Current Waveform
A voltage signal, VSNS, is created as the LED current flows
through the current setting resistor, RSNS, to ground. VSNS is
fed back to the CS pin, where it is compared against a 200
mV reference (VREF). A comparator turns on the power MOSFET when VSNS falls below VREF. The power MOSFET conducts for a controlled on-time, tON, set by an external resistor,
RON.
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FIGURE 2. LM3402/04 PSOP Thermal PAD and Via Layout
Connecting to LED Array
The LM3402 / 04 evaluation board includes two standard 94
mil turret connectors for the cathode and anode connections
to a LED array.
Low Power Shutdown
The LM3402/04 can be placed into a low power shutdown
state (IQ typically 90 µA) by grounding the DIM terminal. During normal operation this terminal should be left open-circuit.
Constant On Time Overview
The LM3402 and LM3404 are buck regulators with a wide input voltage range and a low voltage reference. The controlled
on-time (COT) architecture is a combination of hysteretic
mode control and a one-shot on-timer that varies inversely
with input voltage. With the addition of a PNP transistor, the
on-timer can be made to be inversely proportional to the input
voltage minus the output voltage. This is one of the application
improvements made to this demonstration board that will be
discussed later (improved average LED current circuit).
The LM3402 / 04 were designed with a focus of controlling
the current through the load, not the voltage across it. A constant current regulator is free of load current transients, and
has no need for output capacitance to supply the load and
maintain output voltage. Therefore, in this demonstration
board in order to demonstrate the fast transient capabilities, I
have chosen to omit the output capacitor. With any Buck regulator, duty cycle (D) can be calculated with the following
equations.
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FIGURE 4. VSNS Circuit
SETTING THE AVERAGE LED CURRENT
Knowing the average LED current desired and the input and
output voltages, the slopes of the currents within the inductor
can be calculated. The first step is to calculate the minimum
inductor current (LED current) point. This minimum level
needs to be determined so that the average LED current can
be determined.
The average inductor current equals the average LED current
whether an output capacitor is used or not.
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iTARGET x RSNS = 0.20V
Therefore:
Finally RSNS can be calculated.
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FIGURE 5. ISENSE Current Waveform
Standard On-Time Set Calculation
The control MOSFET on-time is variable, and is set with an
external resistor RON (R2 from Figure1). On-time is governed
by the following equation:
Using figures 3 and 5 and the equations of a line, calculate
ILED-MIN.
Where
Where
IF = ILED-Average
k = 1.34 x 10-10
The delta of the inductor current is given by:
At the conclusion of tON the control MOSFET turns off for a
minimum OFF time (tOFF-MIN) of 300 ns, and once tOFF-MIN is
complete the CS comparator compares VSNS and VREF again,
waiting to begin the next cycle.
The LM3402 / 04 have minimum ON and OFF time limitations.
The minimum on time (tON) is 300 ns, and the minimum allowed off time (tOFF) is 300 ns.
Designing for the highest switching frequency possible
means that you will need to know when minimum ON and OFF
times are observed.
Minimum OFF time will be seen when the input voltage is at
its lowest allowed voltage, and the output voltage is at its
maximum voltage (greatest number of series LEDs).
The opposite condition needs to be considered when designing for minimum ON time. Minimum ON time is the point at
which the input voltage is at its maximum allowed voltage, and
the output voltage is at its lowest value.
There is a 220 ns delay (tD) from the time that the current
sense comparator trips to the time at which the control MOSFET actually turns on. We can solve for iTARGET knowing there
is a delay.
ΔiD is the magnitude of current beyond the target current and
equal to:
Therefore:
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The point at which you want the current sense comparator to
give the signal to turn on the FET equals:
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Application Circuit Calculations
To better explain the improvements made to the COT
LM3402 / 04 demonstration board, a comparison is shown
between the unmodified average output LED current circuit to
the improved circuit. Design examples 1 and 2 use two original LM3402 / 04 circuits. The switching frequencies will be
maximized to provide a small solution size.
Design example 3 is an improved average current application.
Example 3 will be compared against example 2 to illustrate
the improvements.
Example 4 will use the same conditions and circuit as example 3, but the switching frequency will be reduced to improve
efficiency. The reduced switching frequency can further reduce any variations in average LED current with a wide
operating range of series LEDs and input voltages.
Design Example 1
• VIN = 48V (±20%)
• Driving three HB LEDs with VF = 3.4V
• VOUT = (3 x 3.4V +200 mV) = 10.4V
• IF = 500 mA (typical application)
• Estimated efficiency = 82%
• fSW = fast as possible
• Design for typical application within tON and tOFF limitations
LED (inductor) ripple current of 10% to 60% is acceptable
when driving LEDs. With this much allowed ripple current, you
can see that there is no need for an output capacitor. Eliminating the output capacitor is actually desirable. An LED
connected to an inductor without a capacitor creates a near
perfect current source, and this is what we are trying to create.
In this design we will choose 50% ripple current.
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FIGURE 6. VOUT-MAX vs fSW
ΔiL = 500 mA x 0.50 = 250 mA
IPEAK = 500 mA + 125 mA = 625 mA
Calculate tON, tOFF & RON
From the datasheet there are minimum control MOSFET ON
and OFF times that need to be met.
tOFF minimum = 300 ns
tON minimum = 300 ns
The minimum ON time will occur when VIN is at its maximum
value. Therefore calculate RON at VIN = 60V, and set tON = 300
ns.
A quick guideline for maximum switching frequency allowed
versus input and output voltages are shown below in the two
graphs (figures 6 & 7).
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FIGURE 7. VOUT-MIN vs fSW
RON = 135 kΩ (use standard value of 137 kΩ)
tON = 306 ns
Check to see if tOFF minimum is satisfied. This occurs when
VIN is at its minimum value.
At VIN = 36V, and RON = 137 kΩ calculate tON from previous
equation.
tON = 510 ns
We know that:
4
tOFF = 938 ns (satisfied)
Example 1 ON & OFF Times
VIN (V)
VOUT (V)
tON
tOFF
36
10.4
5.10E-07
9.38E-07
48
10.4
3.82E-07
1.06E-06
60
10.4
3.06E-07
1.14E-06
Calculate Switching Frequency
VIN = 36V, 48 and 60V.
Substituting equations:
fSW = 691kHz (VIN = 36V, 48V, & 60V)
Calculate Inductor Value
With 50% ripple at VIN = 48V
• IF = 500 mA
Therefore: RSNS = 467 mΩ
Calculate Average LED current (IF)
Calculate average current through the LEDs for VIN = 36V and
60V.
• ΔiL = 250 mA (target)
• L = 57 µH (68 µH standard value)
Calculate Δi for VIN = 36V, 48V, and 60V with L = 68 µH
Example 1 Ripple Current
VIN (V)
VOUT (V)
ΔiL (A)
36
10.4
0.192
48
10.4
0.211
VIN (V)
VOUT (V)
IF (A)
60
10.4
0.223
36
10.4
0.490
48
10.4
0.500
60
10.4
0.506
Example 1 Average LED Current
Calculate RSNS
Calculate RSNS at VIN typical (48V), and average LED current
(IF) set to 500 mA.
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FIGURE 8. Inductor Current Waveform
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• IF = 500 mA
• VIN = 48V
• VOUT = 10.4V
• L = 68 µH
• tD = 220 ns
• tON = 382 ns
Using equations from the COT Overview section, calculate
RSNS.
Rearranging the above equation and solving for tOFF with
tON set to 510 ns
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Example 2 On & Off Time
Design Example 2
Design example 2 demonstrates a design if a single Bill of
Materials (Bom) is desired over many different applications
(number of series LEDs, VIN, VOUT etc).
• VIN = 48V (±20%)
• Driving 3, 4, or 5 HB LEDs with VF = 3.4V
• IF = 500 mA (typical application)
• Estimated efficiency = 82%
• fSW = fast as possible
• Design for typical application within tON and tOFF limitations
The inductor, RON resistor, and the RSNS resistor is calculated
for a typical or average design.
• VOUT = 3 x 3.4V + 200 mV = 10.4V
• VOUT = 4 x 3.4V + 200 mV = 13.8V
• VOUT = 5 x 3.4V + 200 mV = 17.2V
Calculate tON, tOFF & RON
In this design we will maximize the switching frequency so
that we can reduce the overall size of the design. In a later
design, a slower switching frequency is utilized to maximize
efficiency. If the design is to use the highest possible switching frequency, you must ensure that the minimum on and off
times are adhered to.
Minimum on time occurs when VIN is at its maximum value,
and VOUT is at its lowest value.
Calculate RON at VIN = 60V, VOUT = 10.4V, and set tON = 300
ns:
Three Series LEDs
VIN (V)
VOUT (V)
RON
tON
tOFF
36
10.4
137 kΩ
5.10E-07
9.38E-07
48
10.4
137 kΩ
3.82E-07
1.06E-06
60
10.4
137 kΩ
3.06E-07
1.14E-06
Four Series LEDs
36
13.8
137 kΩ
5.10E-07
5.81E-07
48
13.8
137 kΩ
3.82E-07
7.08E-07
60
13.8
137 kΩ
3.06E-07
7.85E-07
Five Series LEDs
36
17.2
137 kΩ
5.10E-07
3.65E-07
48
17.2
137 kΩ
3.82E-07
4.93E-07
60
17.2
137 kΩ
3.06E-07
5.69E-07
Calculate Switching Frequency
The switching frequency will only change with output voltage.
Substituting equations:
Or:
RON = 137 kΩ, tON = 306 ns
Check to see if tOFF minimum is satisfied:
tOFF minimum occurs when VIN is at its lowest value, and
VOUT is at its maximum value.
At VIN = 36V, VOUT = 17.2V, and R ON = 137 kΩ calculate tON
from the above equation:
tON = 510 ns
• fSW = 691 kHz (VOUT = 10.4V)
• fSW = 916 kHz (VOUT = 13.8V)
• fSW = 1.14 MHz (VOUT = 17.2V)
Calculate Inductor Value
Rearrange the above equation and solve for tOFF with tON set
to 510 ns
With 50% ripple at VIN = 48V, and VOUT = 10.4V
• IAVG = 500 mA
• ΔiL = 250 mA (target)
• L = 53 µH (68 uH standard value)
Calculate Δi for VIN = 36V, 48V, & 60V with L = 68 µH.
tOFF = 365 ns (satisfied)
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Example 2 Average LED Current
VOUT (V)
ΔiL (A)
10.4
0.192
VIN (V)
IF (A)
36
10.4
0.511
48
10.4
0.521
60
10.4
0.526
Three Series LEDs
Three Series LEDs
36
VOUT (V)
48
10.4
0.211
60
10.4
0.223
Four Series LEDs
Four Series LEDs
36
13.8
0.166
36
13.8
0.487
48
13.8
0.192
48
13.8
0.500
60
13.8
0.208
60
13.8
0.508
Five Series LEDs
Four Series LEDs
36
17.2
0.141
36
17.2
0.463
48
17.2
0.173
48
17.2
0.479
0.193
60
17.2
0.489
60
17.2
In this application you can see that there is a difference of 63
mA between the low and high of the average LED current.
Calculate RSNS
Calculate RSNS at VIN typical (48V), with four series LEDs
(13.8V = VOUT), and average LED current (IF) set to 500 mA.
• IF = 500 mA
• VIN = 48V
• VOUT = 13.8V
• L = 68 µH
• tD = 220 ns
• tON = 382 ns
Modified COT Application Circuit
With the addition of one pnp transistor and one resistor (Q1
and R3) the average current through the LEDs can be made
to be more constant over input and output voltage variations.
Refer to page one figure 1. Resistor RON (R2) and Q1 turn the
tON equation into:
Ignore the PNP transistor’s VBE voltage drop.
Design to the same criteria as the previous example with the
improved application and compare results.
RSNS = 446 mΩ
Calculate Average Current through LED
All combinations of VIN, VOUT with RSNS = 446 mΩ
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Example 2 Ripple Current
VIN (V)
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Calculate tON, tOFF & RON
Minimum ON time occurs when VIN is at its maximum value,
and VOUT is at its lowest value.
Calculate RON at VIN = 60V, VOUT = 10.4V, and set tON = 300
ns:
Modified Application Circuit Design
Example 3
Design Example 1
• VIN = 48V (±20%)
• Driving 3, 4, or 5 HB LEDs with VF = 3.4V
• IF = 500 mA (typical application)
• Estimated efficiency = 82%
• fSW = fast as possible
• Design for typical application within tON and tOFF limitations
RON = 111 kΩ (113 kΩ) tON = 306 ns
Check to see if tOFF minimum is satisfied.
At VIN = 36V, VOUT = 17.2V, and RON = 113 kΩ calculate
tON:.
tON = 806 ns
The inductor, RON resistor, and the RSNS resistor are calculated for a typical or average design.
• VOUT = 3 x 3.4V + 200 mV = 10.4V
• VOUT = 4 x 3.4V + 200 mV = 13.8V
• VOUT = 5 x 3.4V + 200 mV = 17.2V
tOFF = 577 ns (satisfied)
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FIGURE 9. Improved Average LED Current Application Circuit
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VIN (V)
VOUT (V)
RON
tON
tOFF
36
10.4
113 kΩ
5.92E-07
1.09E-07
48
10.4
113 kΩ
4.03E-07
1.12E-06
60
10.4
113 kΩ
3.06E-07
1.14E-06
Four Series LEDs
36
13.8
113 kΩ
6.83E-07
7.78E-07
48
13.8
113 kΩ
4.43E-07
8.21E-07
60
13.8
113 kΩ
3.28E-07
8.41E-07
Five Series LEDs
36
17.2
113 kΩ
8.06E-07
5.77E-07
48
17.2
113 kΩ
4.92E-07
6.34E-07
60
17.2
113 kΩ
3.54E-07
6.59E-07
Substitute improved circuit tON calculation:
Calculate Switching Frequency
Simplified:
Example 3 Switching Frequency
VIN (V)
VOUT (V)
fSW (kHz)
36
10.4
595
48
10.4
656
60
10.4
692
36
13.8
685
48
13.8
791
60
13.8
855
36
17.2
723
48
17.2
888
60
17.2
987
Three Series LEDs
Typical Application:
• VOUT = 13.8V
• IF = 500 mA
• RON= 113 kΩ
• L = 68 µH
• tD = 220 ns
RSNS = 462 mΩ
This equation shows that only variations in VOUT will affect the
average current over the entire application range. These variations should be very minor even with large variations in
output voltage.
Calculate Average Current through LED
Modified application circuit average forward current equation.
Four Series LEDs
Five Series LEDs
Calculate Inductor Value
Simplified:
Therefore:
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You can quickly see one benefit of the modified circuit. The
improved circuit eliminates the input and output voltage variation on RMS current.
• IF = 500 mA (typical application)
• ΔiL = 250 mA (target)
• RON= 113 kΩ
• L = 59 µH (68 µH standard value)
• ΔiL = 223 mA (L = 68 µH all combinations)
Calculate RSNS
Original RSNS equation:
Example 3 On & Off Times
Three Series LEDs
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Example 3 Average LED Current
VIN (V)
VIN (V)
VOUT (V)
IF (A)
13.8
0.500
36
17.2
0.489
48
17.2
0.489
60
17.2
0.489
VOUT (V)
IF (A)
Three Series LEDs
36
10.4
0.511
Five Series LEDs
48
10.4
0.511
60
10.4
0.511
Three Series LEDs
60
Four Series LEDs
36
13.8
0.500
48
13.8
0.500
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In this application you can see that there is a difference of 22
mA between the low and high of the average LED current.
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VIN (V)
VOUT (V)
fSW (kHz)
10.4
435
36
13.8
430
48
13.8
497
60
13.8
537
36
17.2
454
48
17.2
558
60
17.2
620
Three Series LEDs
60
• VIN = 48V (±20%)
• Driving 3, 4, or 5 HB LEDs with VF = 3.4V
• IF = 500 mA (typical application)
• Estimated efficiency = 82%
• fSW = 500 kHz (typ app)
The inductor, RON resistor, and the RSNS resistor are calculated for a typical or average design.
• VOUT = 3 x 3.4V + 200 mV = 10.4V
• VOUT = 4 x 3.4V + 200 mV = 13.8V
• VOUT = 5 x 3.4V + 200 mV = 17.2V
Reduce switching frequency for the typical application to
about 500 kHz to increase efficiency.
Calculate tON, tOFF & RON
Four Series LEDs
Five Series LEDs
Calculate RSNS
• VOUT = 13.8V
• VIN = 48V
• IF = 500 mA
• tD = 220 ns
• η = 0.85
• L = 100 µH
RSNS = 488 mΩ
Calculate Average Current through LED
• VOUT = 13.8V
• VIN = 48V
• IF = 500 mA
• tD = 220 ns
• η = 0.85
• fSW = 500 kHz
tON ≊ 705 ns
RON ≊ 179 kΩ (use standard value of 182 kΩ)
Calculate Inductor Value
Example 4 Average LED Current
VIN (V)
VOUT (V)
IF (A)
36
10.4
0.507
48
10.4
0.507
60
10.4
0.507
36
13.8
0.500
48
13.8
0.500
60
13.8
0.500
36
17.2
0.493
48
17.2
0.493
60
17.2
0.493
Three Series LEDs
• IF = 500 mA
• ΔiL = 250 mA (target)
• RON = 182 kΩ
• L = 100 µH
Calculate ΔiL with L = 100 µH (VIN = 48V, VOUT = 13.8V)
Four Series LEDs
ΔiL = 241 mA (all combinations)
Calculate Switching Frequency
Five Series LEDs
In the reduced frequency application you can see that there
is a difference of 14 mA between the low and high of the average current.
If the original tON circuit was used (no PNP transistor) with the
switching frequency centered around 500 kHz the difference
between the high and low values would be about 67 mA.
Example 4 Switching Frequency
VIN (V)
VOUT (V)
fSW (kHz)
36
10.4
374
48
10.4
412
Three Series LEDs
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Modified Application Circuit Design
Example 4
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should be at least one order of magnitude lower than the
steady state switching frequency in order to prevent aliasing.
Refer to figure 10 for illustrations. The interval tD represents
the delay from a logic high at the DIM pin to the onset of the
output current. The quantities tSU and tSD represent the time
needed for the LED current to slew up to steady state and
slew down to zero, respectively.
As an example, assume a DIM duty cycle DDIM equal to 100%
(always on) and the circuit delivers 500mA of current through
the LED string. At DDIM equal to 50% you would like exactly
½ of 500 mA of current through your LED string (250 mA).
This could only be possible if there were no delays (tD) between the on/off DIM signal and the on/off of the LED current.
The rise and fall times (tSU and tSD) of the LED current would
also need to be eliminated. If we can reduce these times, the
linearity between the PWM signal and the average current will
be realized.
Dimming
The DIM pin of the LM3402/04 is a TTL compatible input for
low frequency pulse width modulation (PWM) dimming of the
LED current. Depending on the application, a contrast ratio
greater than what the LM3402/04 internal DIM circuitry can
provide might be needed. This demonstration board comes
with external circuitry that allows for dimming contrast ratios
greater than 50k:1
LM3402 / 04 DIM Pin Operation
To fully enable and disable the LM3402 / 04, the PWM signal
should have a maximum logic low level of 0.8V and a minimum logic high level of 2.2V. Dimming frequency, fDIM, and
duty cycle, DDIM, are limited by the LED current rise time and
fall time and the delay from activation of the DIM pin to the
response of the internal power MOSFET. In general, fDIM
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FIGURE 10. Contrast Ratio Definitions
Contrast Ratio Definition
Contrast Ratio (CR) = 1/DMIN
DMIN = (tD + tSU) x fDIM
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FIGURE 11. tD & tSU (DIM Pin)
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Refer to figure 12. MOSFET Q4 and its drive circuitry are provided on the demonstration PCB. When MOSFET Q4 is
turned on, it shorts LED+ to LED-, therefore redirecting the
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FIGURE 12.
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FIGURE 14. tD + tSU Graph
FIGURE 13. VIN = 24V, 3 series LEDs @ 400mA
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inductor current from the LED string to the shunt MOSFET.
The LM3402 / 04 is never turned off, and therefore become a
perfect current source by providing continuous current to the
output through the inductor (L1). A buck converter with an
external shunt MOSFET is the ideal circuit for delivering the
highest possible contrast ratio. Refer to figures 13-15 for typical delays and rise time for external MOSFET dimming.
External MOSFET Dimming and
Contrast Ratio
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when Q2 shunt MOSFET is OFF during fast dimming.
This is an added benefit due to the fact that tOFF is greatly
increased, and therefore the switching frequency is decreased, which leads to improved efficiency (see figure 16).
Inductor L1 still remains charged, and as soon as Q4 turns off
current flows through the LED string.
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FIGURE 15. tD + tSD Graph
Fast Dimming + Improved Average
Current Circuit
Using both the Improved Average LED current circuit and the
external MOSFET fast dimming circuit together has additional
benefits. If RON and the converter's switching frequency
(fSW) is determined and set with the improved average LED
current circuit, the switching frequency will decrease once
VOUT is shorted during fast dimming. With MOSFET Q4 on,
VOUT is equal to VFB (200 mV). The tON equation then becomes almost identical to the original unmodified circuit equation.
Setting tON and RON:
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FIGURE 16. Improved Avg ILED Circuit + Fast Dimming
Linearity with Fast Dimming
Once the delays and rise/fall times have been greatly reduced, linear average current vs, duty cycle (DDIM) can be
achieved at very high dimming frequencies (fDIM) (see figure
17).
tON equation becomes:
when Q4 shunt MOSFET is on during fast dimming.
tOFF equation during normal operation is:
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tOFF equation then becomes:
FIGURE 17. Linearity with Fast Dimming
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VIN = 9V to 18V, ILED = 750 mA, 3 x 3.4V White LED Strings (fSW ≊ 500 kHz)
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LM3404 Improved ILED Average & Fast Dimming Demonstration Board
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Bill of Materials
Part ID
Part Value
Mfg
Part Number
U1
1A Buck LED Driver PSOP pkg
NSC
LM3404
C1, Input Cap
10 µF, 25V, X5R
TDK
C3225X5R1E106M
C2, C6 Cap
1 µF, 16V, X5R
TDK
C1608X5R1C105M
C3, VBOOST Cap
0.1 µF, X5R
TDK
C1608X5R1H104M
C4 Output Cap
10 µF, 25V, X5R (Optional)
TDK
C3225X5R1E106M
C5, VRON Cap
0.01 µF, X5R
TDK
C1608X5R1H103M
D1, Catch Diode
0.5Vf Schottky 2A, 30VR
Diodes INC
B230
D2
Dual SMT small signal
Diodes INC
BAV199
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112538
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16
AN-1839
Layout
30061665
17
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