User's Guide
SNVA342E – July 2008 – Revised April 2013
AN-1839 LM3402/LM3404 Fast Dimming and True
Constant LED Current Evaluation Board
1
Introduction
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.
2
Circuit Performance with LM3404
This evaluation board (see Figure 1) uses the LM3404 to provide a constant forward current of 700 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.
3
Thermal Performance
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 SO8 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.
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Thermal Performance
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VDIM
1 N4148
Dual
VIN
VDIM
JMP-1
External Voltage
Source Optional
C6
D2
R2
R3
4V to 6V
Q1
5 CS
LM3404
GND 4
6 RON
DIM 3
VOUT
C3
7 VCC
BOOT 2
8 VIN
SW
C2
Q32
L1
1
U1
C1
R6
R4
C4
D1
C5
Q4 R5
Optional
CONN-1
Q31
LEDs on separate PCB
R1B
R1A
Single package (SC70-6)
Complementary N+P Channel
Figure 1. LM3402 / 04 Schematic
90 mil
10 mil
10 mil
90 mil
35 mil
35 mil
Figure 2. LM3402/04 PSOP Thermal PAD and Via Layout
2
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Connecting to LED Array
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4
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.
5
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.
6
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 ontimer 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.
D=
tON
tON
=
= tON x fSW
tON + tOFF
TS
(1)
The average inductor current equals the average LED current whether an output capacitor is used or not.
'i
IF
ILED(t)
L
VIN - VOUT
L
VOUT
L
t
DTS
TS
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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Constant On Time Overview
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ILED
CS
+
RSNS
VSNS
-
Figure 4. VSNS Circuit
6.1
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.
iPEAK
'i
L
IF
'iD
iTARGET
iLED-MIN
ILED(t)
t
tON
tOFF
tD
Figure 5. ISENSE Current Waveform
Using Figure 3 and Figure 5 and the equations of a line, calculate ILED-MIN.
ILED-MIN = IF -
4
'iL
2
(2)
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Standard On-Time Set Calculation
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Where
IF = ILED-Average
(3)
The delta of the inductor current is given by:
VIN - VOUT
'i
x tON
=
2L
2
(4)
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.
ITARGET = IF -
'iL
+ 'iD
2
(5)
ΔiD is the magnitude of current beyond the target current and equal to:
'iD =
VOUT
tD
L
(6)
Therefore:
iTARGET = IF -
VOUT
VIN - VOUT
x tD
x tON +
2L
L
(7)
The point at which you want the current sense comparator to give the signal to turn on the FET equals:
iTARGET x RSNS = 0.20V
(8)
Therefore:
0.2V = RSNS IF -
VIN - VOUT
V
x tON + OUT x tD
L
2L
(9)
Finally RSNS can be calculated.
RSNS =
7
0.20V
V - VOUT
VOUT x tD
(IF) - IN
x tON +
2L
L
(10)
Standard On-Time Set Calculation
The control MOSFET on-time is variable, and is set with an external resistor RON (R2 from Figure 1). Ontime is governed by the following equation:
tON = k x
RON
VIN
(11)
Where
k = 1.34 x 10-10
(12)
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.
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Application Circuit Calculations
8
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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.
ΔiL = 500 mA x 0.50 = 250 mA
IPEAK = 500 mA + 125 mA = 625 mA
Calculate tON, tOFF and 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 in
Figure 6 and Figure 7.
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Figure 6. VOUT-MAX vs fSW
Figure 7. VOUT-MIN vs fSW
tON = k x
RON
VIN
(13)
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:
D=
VOUT
VIN x K
=
tON
tON + tOFF
(14)
Rearranging the above equation and solving for tOFF with tON set to 510 ns
tOFF = tON
VIN x K
VOUT
-1
(15)
tOFF = 938 ns (satisfied)
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Application Circuit Calculations
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Table 1. Example 1 ON and 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, and 60V)
Calculate Inductor Value
With 50% ripple at VIN = 48V
• IF = 500 mA
• ΔiL = 250 mA (target)
• L = 57 µH (68 µH standard value)
Calculate Δi for VIN = 36V, 48V, and 60V with L = 68 µH
Table 2. Example 1 Ripple Current
VIN (V)
VOUT (V)
ΔiL (A)
36
10.4
0.192
48
10.4
0.211
60
10.4
0.223
Calculate RSNS
Calculate RSNS at VIN typical (48V), and average LED current (IF) set to 500 mA.
iPEAK
'i
IF
L
iLED-MIN
ILED(t)
t
tON
tOFF
Figure 8. Inductor Current Waveform
•
•
•
•
•
•
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.
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RSNS =
0.20V
V - VOUT
VOUT x tD
(IF) - IN
x tON +
2L
L
Or:
RSNS =
0.20V
VIN - VOUT
(IF) 2L
k x RON
VOUT x tD
+
VIN
L
(16)
Therefore: RSNS = 467 mΩ
Calculate Average LED current (IF)
Calculate average current through the LEDs for VIN = 36V and 60V.
VIN - VOUT
VOUT x tD
0.20V
+
(tON) IF = R
2L
L
SNS
(17)
Table 3. Example 1 Average LED Current
VIN (V)
VOUT (V)
IF (A)
36
10.4
0.490
48
10.4
0.500
60
10.4
0.506
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 and 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:
tON = k x
RON
VIN
(18)
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 RON = 137 kΩ calculate tON from the above equation:
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Application Circuit Calculations
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tON = 510 ns
VIN x K
tON
=
VOUT
tON + tOFF
(19)
Rearrange the above equation and solve for tOFF with tON set to 510 ns
tOFF = tON
VIN x K
VOUT
-1
(20)
tOFF = 365 ns (satisfied)
Table 4. Example 2 On and Off Time
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
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
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
Four Series LEDs
Five Series LEDs
Calculate Switching Frequency
The switching frequency will only change with output voltage.
fSW =
VOUT
VIN x K x tON
(21)
Substituting equations:
fSW =
VOUT
K x k x RON
(22)
1
tON + tOFF
(23)
Or:
fSW =
• fSW = 691 kHz (VOUT = 10.4V)
• fSW = 916 kHz (VOUT = 13.8V)
• fSW = 1.14 MHz (VOUT = 17.2V)
Calculate Inductor Value
L=
VIN - VOUT
'i
x tON
(24)
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, and 60V with L = 68 µH.
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Table 5. Example 2 Ripple Current
VOUT (V)
ΔiL (A)
36
10.4
0.192
48
10.4
0.211
60
10.4
0.223
36
13.8
0.166
48
13.8
0.192
60
13.8
0.208
36
17.2
0.141
48
17.2
0.173
60
17.2
0.193
VIN (V)
Three Series LEDs
Four Series LEDs
Four Series LEDs
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
RSNS =
0.20V
(IF) - VIN - VOUT
2L
x tON +
VOUT x tD
L
(25)
RSNS = 446 mΩ
Calculate Average Current through LED
All combinations of VIN, VOUT with RSNS = 446 mΩ
VIN - VOUT
VOUT x tD
0.20V
+
(tON) IF = R
2L
L
SNS
(26)
Table 6. Example 2 Average LED Current
VIN (V)
VOUT (V)
IF (A)
36
10.4
0.511
48
10.4
0.521
60
10.4
0.526
36
13.8
0.487
48
13.8
0.500
60
13.8
0.508
36
17.2
0.463
48
17.2
0.479
60
17.2
0.489
Three Series LEDs
Four Series LEDs
Five Series LEDs
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Modified COT Application Circuit
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In this application you can see that there is a difference of 63 mA between the low and high of the
average LED current.
9
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:
tON = k x
RON
VIN - VOUT
(27)
Ignore the PNP transistor’s VBE voltage drop.
Design to the same criteria as the previous example with the improved application and compare results.
10
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
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
Calculate tON, tOFF and 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:
RON = tON
VIN - VOUT
k
(28)
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
tOFF = tON
VIN x K
VOUT
-1
(29)
tOFF = 577 ns (satisfied)
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Modified Application Circuit Design Example 3
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VIN
Improved Average
Current Circuit
R2
Q1
R3
LM3404
5 CS
6 RON
GND
4
DIM
3
VOUT
C3
7
VCC
8
VIN
BOOT
2
SW
1
L1
U1
D1
C1
C2
C4
Optional
C5
LEDs on separate PCB
R1
Figure 9. Improved Average LED Current Application Circuit
Table 7. Example 3 On and Off Times
Three Series LEDs
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
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
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
Four Series LEDs
Five Series LEDs
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Modified Application Circuit Design Example 3
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Calculate Switching Frequency
VOUT
VIN x K x tON
fSW =
Or:
1
tON + tOFF
fSW =
(30)
Table 8. 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
Four Series LEDs
Five Series LEDs
Calculate Inductor Value
L=
VIN - VOUT
'i
tON = k x
x tON
RON
VIN - VOUT
(31)
Therefore:
L=
RON
'i
xk
(32)
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:
RSNS =
0.20V
(IF) - VIN - VOUT
2L
x tON +
VOUT x tD
L
(33)
Substitute improved circuit tON calculation:
RSNS =
0.20V
V - VOUT k x RON
V
xt
(IF) - IN
+ OUT D
2L
VIN - VOUT
L
(34)
Simplified:
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Modified Application Circuit Design Example 4
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0.20V
RSNS =
(IF) -
VOUT x tD
k x RON
+
L
2L
(35)
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.
IF =
VOUT x tD
VIN - VOUT k x RON
0.20V
+
2L
RSNS
VIN - VOUT
L
(36)
Simplified:
IF =
k x RON
VOUT x tD
0.20V
+
L
2L
RSNS
(37)
Table 9. Example 3 Average LED Current
VIN (V)
VOUT (V)
IF (A)
36
10.4
0.511
48
10.4
0.511
60
10.4
0.511
36
13.8
0.500
48
13.8
0.500
60
13.8
0.500
36
17.2
0.489
48
17.2
0.489
60
17.2
0.489
Three Series LEDs
Four Series LEDs
Five Series LEDs
In this application you can see that there is a difference of 22 mA between the low and high of the
average LED current.
11
Modified Application Circuit Design Example 4
•
•
•
•
•
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
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Modified Application Circuit Design Example 4
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• 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 and RON
•
•
•
•
•
•
1
fSW
VOUT
VIN x K
tON =
(38)
VOUT = 13.8V
VIN = 48V
IF = 500 mA
tD = 220 ns
η = 0.85
fSW = 500 kHz
tON ≊ 705 ns
RON =
tON
(VIN - VOUT)
k
(39)
RON ≊ 179 kΩ (use standard value of 182 kΩ)
Calculate Inductor Value
L=
•
•
•
•
RON
'i
xk
(40)
IF = 500 mA
ΔiL = 250 mA (target)
RON = 182 kΩ
L = 100 µH
Calculate ΔiL with L = 100 µH (VIN = 48V, VOUT = 13.8V)
ΔiL = 241 mA (all combinations)
Calculate Switching Frequency
fSW =
VOUT
VIN x K x tON
Or:
fSW =
1
tON + tOFF
(41)
Table 10. Example 4 Switching Frequency
VIN (V)
VOUT (V)
fSW (kHz)
36
10.4
374
48
10.4
412
60
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
Four Series LEDs
Five Series LEDs
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Dimming
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Calculate RSNS
0.20V
RSNS =
VOUT x tD
k x RON
(IF) +
L
2L
•
•
•
•
•
•
(42)
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
IF =
k x RON
VOUT x tD
0.20V
+
L
2L
RSNS
(43)
Table 11. 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
Four Series LEDs
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.
12
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.
13
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 should be at least one order of magnitude lower than the steady
state switching frequency in order to prevent aliasing.
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Contrast Ratio Definition
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For illustrations, see Figure 10. 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.
T
T
T
DIM
D
tD
DMIN
tSD
tSU
tD
tSU
DMAX
tSD
tD
tSU
tSD
IF
T=
1
fPWM
DMIN =
T - tSD
tD + tSU
T
DMAX =
T
Figure 10. Contrast Ratio Definitions
14
Contrast Ratio Definition
Contrast Ratio (CR) = 1/DMIN
DMIN = (tD + tSU) x fDIM
18
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External MOSFET Dimming and Contrast Ratio
www.ti.com
DIM
5V/Div
200 mA/Div
IF
2 Ps/DIV
Figure 11. tD and tSU (DIM Pin)
15
External MOSFET Dimming and Contrast Ratio
MOSFET Q4 and its drive circuitry are provided on the demonstration PCB (see Figure 12). When
MOSFET Q4 is turned on, it shorts LED+ to LED-, therefore redirecting the 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. For typical
delays and rise time for external MOSFET dimming, see Figure 13 - Figure 15.
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External MOSFET Dimming and Contrast Ratio
www.ti.com
VDIM
1 N4148
Dual
From VCC
LM3402/04
VDIM
JMP-1
D2
External Voltage
Source Optional
C5
4V to 6V
R6
Q32
L1
R4
Optional
C4
Q4 R5
CONN-1
Q31
LEDs on separate PCB
Single package (SC70-6)
Complementary N+P Channel
R1A
R1B
11.0
1.1
0.8
5.0
0.5
ILED
2.0
ILED (A)
VDIM (V)
VDIM
8.0
0.2
-0.1
-1.0
8.0
0.0
8.4
16.6
24.8
33.0
TIME (Ps)
Figure 12. VIN = 24V, 3 series LEDs @ 400mA
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Fast Dimming + Improved Average Current Circuit
www.ti.com
11
1.1
VDIM
0.8
ILED
0.5
ILED (A)
VDIM (V)
7
3
0.2
40 ns
-1
-100
-60
-20
20
-0.1
60
100
TIME (ns)
Figure 13. tD + tSU Graph
12.0
1.00
VDIM
0.60
4.0
0.20
VDIM (V)
ILED (A)
8.0
ILED
36 ns
-0.20
0.0
-100
-60
-20
20
60
100
TIME (ns)
Figure 14. tD + tSD Graph
16
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:
tON = k x
RON
VIN - VOUT
(44)
tON equation becomes:
tON = k x
RON
VIN - 0.2V
(45)
when Q4 shunt MOSFET is on during fast dimming.
tOFF equation during normal operation is:
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Linearity with Fast Dimming
tOFF = tON
VIN x K
VOUT
www.ti.com
-1
(46)
tOFF equation then becomes:
tOFF = tON
VIN x K
0.2V
-1
(47)
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.
0.5
34.0
fSW = 650 kHz
ILED (A)
28.0
VSW (V)
fSW = 75 kHz
16.0
0.1
ILED (A)
0.2
22.0
VSW (V)
0.4
-0.1
10.0
VDIM (V)
-0.3
4.0
-2.0
0.5
-6.0
7.0
13.5
-0.4
20.0
TIME (Ps)
Figure 15. Improved Avg ILED Circuit + Fast Dimming
17
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).
350
300
ILED (A)
250
200
fDIM = 500 Hz
fDIM = 25 kHz
150
100
fDIM = 5 kHz
50
0
0
10 20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
Figure 16. Linearity With Fast Dimming
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LM3404 Improved ILED Average and Fast Dimming Demonstration Board
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18
LM3404 Improved ILED Average and Fast Dimming Demonstration Board
VDIM
1 N4148
Dual
VIN
VDIM
JMP-1
External Voltage
Source Optional
C6
D2
R2
R3
4V to 6V
Q1
5 CS
LM3404
GND 4
6 RON
DIM 3
VOUT
C3
7 VCC
BOOT 2
8 VIN
SW
U1
C1
C2
C5
R6
Q32
L1
1
R4
D1
C4
Q4 R5
Optional
CONN-1
Q31
LEDs on separate PCB
R1B
R1A
Single package (SC70-6)
Complementary N+P Channel
Figure 17. VIN = 9V to 18V, ILED = 700 mA, 3 x 3.4V White LED Strings (fSW ≊ 500 kHz)
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Bill of Materials
19
www.ti.com
Bill of Materials
Part ID
Part Value
Mfg
Part Number
U1
1A Buck LED Driver
SO PowerPAD 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
L1
33 µH
CoilCraft
D01813H-333
R1A, R1B
0.62Ω 1% 0.25W 1206
ROHM
MCR18EZHFLR620
R2
47.5 kΩ 1%
Vishay
CRCW08054752F
R3
1.0 kΩ, 1%
Vishay
CRCW08051001F
R4, R5
1Ω, 1%
Vishay
CRCW08051R00F
CRCW08051002F
R6
10 kΩ, 1%
Vishay
Q1
SOT23 PNP
Diodes INC
MMBT3906
Q4
SOT23-6 N-CH 2.4A, 20V
ZETEX
ZXMN2A01E6
Q3
SC70-6, P + N Channel
Vishay
Si1539DL
1502-2
Test Points
Connector
Keystone
VIN, GND, LED+, LED-
Connector
Keystone
575-8
JMP-1
Jumper
Molex
22-28-4023
J15
50Ω BNC
Amphenol
112538
20
Layout
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