Texas Instruments Incorporated AN-1839 User's Guide

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

User's Guide

SNVA342E–July 2008–Revised April 2013

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 θJAof 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.

All trademarks are the property of their respective owners.

SNVA342E–July 2008–Revised April 2013 AN-1839 LM3402/LM3404 Fast Dimming and True Constant LED Current

Submit Documentation Feedback

Copyright © 2008–2013, Texas Instruments Incorporated

Evaluation Board

1

90 mil

10 mil

35 mil

10 mil

35 mil

90 mil

2
1

3

4

C1

V

IN

V

OUT

7
8

6

5

SW

BOOT

GND

DIM

VIN

VCC

RON

CS

C3

R1A

Q1

Q4

L1

R3

LEDs on separate PCB

Optional

R5

Q3

2

CONN-1

C4

V

DIM

C6

JMP-1

U1

External Voltage

Source Optional

4V to 6V

Single package (SC70-6)

Complementary N+P Channel

D1

D2

C2

C5

LM3404

R2

Q3

1

R1B

R4

R6

V

DIM

1N4148

Dual

Thermal Performance

www.ti.com

Figure 1. LM3402 / 04 Schematic

Figure 2. LM3402/04 PSOP Thermal PAD and Via Layout

2

AN-1839 LM3402/LM3404 Fast Dimming and True Constant LED Current SNVA342E–July 2008–Revised April 2013
Evaluation Board

Copyright © 2008–2013, Texas Instruments Incorporated

Submit Documentation Feedback

L

T

S

DT

S

t

I

LED(t)

I

F

V

OUT

L

VIN - V

OUT

'i

L

D =

t

ON

t

ON

+ t

OFF

=

= tON x f

SW

t

ON

T

S

www.ti.com

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 (IQtypically 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.

Connecting to LED Array

The average inductor current equals the average LED current whether an output capacitor is used or not.

Figure 3. Buck Converter Inductor Current Waveform

A voltage signal, V
ground. V

is fed back to the CS pin, where it is compared against a 200 mV reference (V

SNS

comparator turns on the power MOSFET when V

, is created as the LED current flows through the current setting resistor, R

SNS

falls below V

SNS

. The power MOSFET conducts for a

REF

REF

). A

SNS

, to

controlled on-time, tON, set by an external resistor, RON.

(1)

SNVA342E–July 2008–Revised April 2013 AN-1839 LM3402/LM3404 Fast Dimming and True Constant LED Current

Submit Documentation Feedback

3

Copyright © 2008–2013, Texas Instruments Incorporated

Evaluation Board

I

LED-MIN

= IF -

'i

L

2

t

I

LED(t)

i

PEAK

t

ON

I

F

'i

D

'i

L

i

TARGET

i

LED-MIN

t

OFF

t

D

R

SNS

V

SNS

C

S

I

LED

+

-

Constant On Time Overview

www.ti.com

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.

Figure 5. I

Figure 4. V

Current Waveform

SENSE

SNS

Circuit

Using Figure 3 and Figure 5 and the equations of a line, calculate I

LED-MIN

.

(2)

4

AN-1839 LM3402/LM3404 Fast Dimming and True Constant LED Current SNVA342E–July 2008–Revised April 2013
Evaluation Board

Copyright © 2008–2013, Texas Instruments Incorporated

Submit Documentation Feedback

tON = k x

R

ON

V

IN

R

SNS

=

VIN - V

OUT

2L

x tON +

V

OUT

x t

D

L

(IF) -

0.20V

0.2V = R

SNS

VIN - V

OUT

2L

x tON +

V

OUT

L

x t

D

IF -

i

TARGET

= IF -

VIN - V

OUT

2L

x tON +

V

OUT

L

x t

D

V

OUT

L

'iD =

t

D

I

TARGET

= IF -

'i

L

2

+'i

D

=

VIN - V

OUT

2L

'i

2

x t

ON

www.ti.com

Where

The delta of the inductor current is given by:

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 i

ΔiDis the magnitude of current beyond the target current and equal to:

Therefore:

The point at which you want the current sense comparator to give the signal to turn on the FET equals:

Therefore:

IF= I

i

TARGET

LED-Average

x R

SNS

Standard On-Time Set Calculation

(3)

(4)

knowing there is a delay.

TARGET

(5)

(6)

(7)

= 0.20V (8)

Finally R

can be calculated.

SNS

7 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:

Where

k = 1.34 x 10

At the conclusion of tONthe control MOSFET turns off for a minimum OFF time (t
once t

OFF-MIN

The LM3402/04 have minimum ON and OFF time limitations. The minimum on time (tON) is 300 ns, and
the minimum allowed off time (t

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.

-10

is complete the CS comparator compares V

) is 300 ns.

OFF

SNS

and V

) of 300 ns, and

again, waiting to begin the next cycle.

REF

OFF-MIN

(9)

(10)

(11)

(12)

SNVA342E–July 2008–Revised April 2013 AN-1839 LM3402/LM3404 Fast Dimming and True Constant LED Current

Submit Documentation Feedback

Copyright © 2008–2013, Texas Instruments Incorporated

Evaluation Board

5

Application Circuit Calculations

8 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

• V

• IF= 500 mA (typical application)

• Estimated efficiency = 82%

• fSW= fast as possible

• Design for typical application within tONand t
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
I

PEAK

Calculate tON, t

From the datasheet there are minimum control MOSFET ON and OFF times that need to be met.
t

OFF

tONminimum = 300 ns
The minimum ON time will occur when VINis at its maximum value. Therefore calculate RONat 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.

= (3 x 3.4V +200 mV) = 10.4V

OUT

= 500 mA + 125 mA = 625 mA

and R

OFF

ON

minimum = 300 ns

limitations

OFF

www.ti.com

6

AN-1839 LM3402/LM3404 Fast Dimming and True Constant LED Current SNVA342E–July 2008–Revised April 2013
Evaluation Board

Copyright © 2008–2013, Texas Instruments Incorporated

Submit Documentation Feedback

V

OUT

V

IN

x K

t

OFF

= t

ON

- 1

D =

V

OUT

V

IN

x K

=

t

ON

t

ON

+ t

OFF

tON = k x

R

ON

V

IN

www.ti.com

Application Circuit Calculations

Figure 6. V

Figure 7. V

OUT-MAX

OUT-MIN

vs f

vs f

SW

SW

RON= 135 kΩ (use standard value of 137 kΩ)
tON= 306 ns
Check to see if t

minimum is satisfied. This occurs when VINis at its minimum value.

OFF

At VIN= 36V, and RON= 137 kΩ calculate tONfrom previous equation.
tON= 510 ns

(13)

We know that:

Rearranging the above equation and solving for t

t

= 938 ns (satisfied)

OFF

SNVA342E–July 2008–Revised April 2013 AN-1839 LM3402/LM3404 Fast Dimming and True Constant LED Current

Submit Documentation Feedback

Copyright © 2008–2013, Texas Instruments Incorporated

with tONset to 510 ns

OFF

(14)

(15)

7

Evaluation Board

I

LED(t)

i

PEAK

t

ON

I

F

'i

L

i

LED-MIN

t

OFF

t

Application Circuit Calculations

VIN(V) V

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

VIN(V) V

36 10.4 0.192
48 10.4 0.211
60 10.4 0.223

Table 1. Example 1 ON and OFF Times

(V) t

OUT

ON

Table 2. Example 1 Ripple Current

(V) ΔiL(A)

OUT

t

OFF

www.ti.com

Calculate R

Calculate R

SNS

at VINtypical (48V), and average LED current (IF) set to 500 mA.

SNS

Figure 8. Inductor Current Waveform

• IF= 500 mA

• VIN= 48V

• V

• L = 68 µH

• tD= 220 ns

• tON= 382 ns
Using equations from the COT Overview section, calculate R

8

AN-1839 LM3402/LM3404 Fast Dimming and True Constant LED Current SNVA342E–July 2008–Revised April 2013
Evaluation Board

OUT

= 10.4V

.

SNS

Copyright © 2008–2013, Texas Instruments Incorporated

Submit Documentation Feedback