AN2823
Application note
Current source for LED driving based on the L5973AD
Introduction
LEDs are devices where the light intensity (brightness), measured in millicandelas or in
lumens, is proportional to the forward current flowing through them. Two main classes can
be identified given the typical forward current of the final application:
■
■
Low current LEDs, from 15 mA to 50 mA, mainly used in the portable market for
backlighting and signaling applications
High current LEDs, with forward current between 350 mA to 1000 mA, typically used in
lighting applications
The typical voltage drop across the LEDs when forward-biased depends on the color of the
emitted light, and the spread is high even when considering LEDs from the same production
lot, as shown in
Figure 1.
Figure 1.
V-I characteristic of 8 white LEDs of the same type
The spread of the forward voltage and the exponential trend of the I-V characteristic makes
current mode driving more convenient than voltage driving. As a consequence, the current
source regulates the LED current and the output voltage is fixed by the forward voltage
across each LED.
August 2009
Doc ID 15015 Rev 1
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Contents
AN2823
Contents
1
2
Basic principles of LED driving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
The L5973AD in LED driving applications . . . . . . . . . . . . . . . . . . . . . . . 6
2.1
Designing a high efficiency current source (topology 1) . . . . . . . . . . . . . . . 7
2.1.1
2.1.2
2.1.3
2.1.4
DC GAIN design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
The dimming operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Frequency compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
LED current accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.2
Designing a high efficiency current source (topology 2) . . . . . . . . . . . . . . 18
2.2.1
2.2.2
2.2.3
2.2.4
DC GAIN design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
The dimming operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Frequency compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
LED current accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.3
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
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Doc ID 15015 Rev 1
AN2823
List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.
Figure 20.
Figure 21.
Figure 22.
Figure 23.
Figure 24.
Figure 25.
Figure 26.
Figure 27.
Figure 28.
Figure 29.
Figure 30.
Figure 31.
Figure 32.
Figure 33.
V-I characteristic of 8 white LEDs of the same type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Basic schematic of series-driven LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Basic schematic of parallel-driven LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Basic schematic of a DC-DC converter driving LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Improved schematic of the current generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Block diagram of L5973AD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Schematic of the dimmable current source using L5973AD . . . . . . . . . . . . . . . . . . . . . . . . 10
Voltage mode architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Q1 disables the switching activity clamping the COMP signal . . . . . . . . . . . . . . . . . . . . . . 11
Time delay in the LED current leading edge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Dimming signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Load equivalent circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
aLED(s) plot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Power stage equivalent circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
OTA equivalent circuit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Module plot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Phase plot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Basic schematic of the current source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Schematic of the dimmable current source using L5973AD . . . . . . . . . . . . . . . . . . . . . . . . 20
Q1 disables the switching activity by clamping the COMP signal. . . . . . . . . . . . . . . . . . . . 22
Time delay in the LED current leading edge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Dimming signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
aLED(s) plot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Equivalent circuit of block K(s) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Module plot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Phase plot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Maximum efficiency vs. voltage drop (I
LED
=350 mA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Power losses across the sense resistor (I
LED
=350 mA) . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Spread of the LED current vs. voltage drop (I
LED
=350 mA) . . . . . . . . . . . . . . . . . . . . . . . . 29
Power losses across the sense resistor (I
LED
=350 mA) . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Maximum efficiency vs. voltage drop (I
LED
=700 mA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Power losses across the sense resistor (I
LED
=700 mA) . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Spread of the LED current vs. voltage drop (I
LED
=700 mA) . . . . . . . . . . . . . . . . . . . . . . . . 31
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Basic principles of LED driving
AN2823
1
Basic principles of LED driving
This paragraph deals with different ways to arrange a set of LEDs to generate the required
lumen output. Advantages and disadvantages for each topology are also provided.
The easiest and cheapest way to drive LEDs is to connect them in series, as shown in
Figure 2.
However, this implies that the LED driver must provide an output voltage that is the
sum of the forward voltage of the LEDs.
Figure 2.
Basic schematic of series-driven LEDs
Therefore, in other cases the preferred solution is to drive LEDs in parallel, matching the
different current sources (Figure
3).
Figure 3.
Basic schematic of parallel-driven LEDs
An easy and cost-effective way to design a constant current source to drive LEDs is to use a
DC-DC converter, as shown in
Figure 4.
Since the voltage control loop of the device
regulates the voltage at the FB pin, a constant current source can be obtained by simply
connecting a resistor between this pin and GND. Application note AN1891 from
STMicroelectronics provides helpful information for designing a similar application using
monolithic DC-DC converters.
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Doc ID 15015 Rev 1
AN2823
Figure 4.
Basic principles of LED driving
Basic schematic of a DC-DC converter driving LEDs
AM01354v1
The main drawback of the circuit shown in
Figure 4
is the amount of wasted power on the
sense resistor. This can affect the efficiency of the overall application, especially in cases
where the voltage reference of the embedded error amplifier is not negligible compared to
the voltage drop across the LEDs, or when high current devices are used.
The main objective of this document is to implement a more efficient current source by
converting the voltage loop of a monolithic DC-DC converter into a current loop.
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