ST AN2754 Application note
AN2754
Application note
Buck high-brightness LED driver based on the ST1S10
step-down DC-DC converter voltage regulator
Introduction
High-brightness LEDs are becoming a prominent source of light because of their long life,
ruggedness, design flexibility, small size and energy efficiency. LEDs are now available in
higher and higher wattages per package (1 W, 3 W and 5 W) with currents up to 1.5 A. At
these current levels, the traditional means of limiting current with a resistor is not sufficiently
accurate nor efficient. Today, single-dice, white HBLEDs capable of delivering up to
90 lm/W of light are available. A typical 1 W white LED delivers an optical efficiency of
30 lm/W, whereas a typical 60 W light bulb delivers 15 lm/W.
It is known that the brightness of an LED is proportional to the forward current, so the best
way to supply LEDs is to control the forward current to get good matching of the output light.
LED manufacturers specify the characteristics (such as lumens, beam pattern) of their
devices at a specified forward current (I
This application note describes how to implement a constant current control to drive highbrightness LEDs by a step-down DC-DC converter voltage regulator. A switching regulator
is the best choice for driving HBLEDs when high efficiency and low power dissipation are
required.
), not at a specific forward voltage (VF).
F
The circuit uses the ST1S10 high-efficiency buck converter configured to drive a single
HBLED in constant current mode.
The ST1S10 is a step-down monolithic power switching regulator which needs few external
passive components and it is capable of delivering 3 A. An internal oscillator fixes the
switching oscillation at 900 kHz, and it is possible to synchronize the switching frequency
with an external clock from 400 kHz to 1.2 MHz.
This application note includes a schematic diagram, bill of material (BOM), and test data.
August 2008 Rev 1 1/20
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Contents AN2754
Contents
1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2 Buck topology switching power supply . . . . . . . . . . . . . . . . . . . . . . . . . 6
3 Design example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1 Design parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.2 Power stage selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.3 Current sense . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.4 Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.5 Output capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.6 Input capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4 Description of the board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.1 Input/output connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5 Schematic and bill of material (1 A LED current) . . . . . . . . . . . . . . . . . 14
6 Board layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
7 Typical application waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.1 Startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.2 Switching waveform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.3 PWM dimming using the enable function . . . . . . . . . . . . . . . . . . . . . . . . . 18
8 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2/20
AN2754 List of figures
List of figures
Figure 1. Constant voltage control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Figure 2. Constant current control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Figure 3. Constant current control with V
Figure 4. Buck topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 5. Buck converter circuit while the switch is in position 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 6. Inductor current waveform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Figure 7. Buck converter circuit while the switch is in position 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Figure 8. Output impedence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Figure 9. Schematic - LED current 1 A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Figure 10. Assembly layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Figure 11. Top layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Figure 12. Bottom layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Figure 13. Startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Figure 14. Steady-state operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Figure 15. PWM dimming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
amplification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
SENSE
3/20
Background AN2754
1 Background
When designing a power supply for a white high-brightness LED, the main requirements are
efficiency, size and cost of the complete solution.
A standard buck converter is the best choice for providing a constant current because only
the buck converter among the switching topologies has an average inductor current that is
equal to the average load current. For this reason, the conversion of a constant voltage into
constant current is much easier.
LEDs are current-driven devices whose brightness is proportional to their forward current.
Forward current can be controlled in two ways: voltage mode and current mode. The first
method uses the LED V-I curve to determine what voltage has to be applied to the LED in
order to generate the desired forward current. This is typically accomplished by applying a
voltage source and using a ballast resistor as shown in Figure 1. This method has two
serious drawbacks. The first is that every change in LED forward voltage creates a change
in LED current. The second problem is the power lost across the ballast resistor which
reduces the efficiency.
Figure 1. Constant voltage control
Equation 1
⎛
⎜
1VV ×+×=
+=
FBOUT
⎜
⎝
LEDs are PN junction devices with a steep I - V curve. For this reason, driving an LED with
a voltage source can lead to large swings of forward current in response to even a small
change in forward voltage. In general, to meet the needs of a driver for an HBLED, the
current output must be in the ±5% to ±20% range.
The best way to drive the LEDs is to control the forward current so that it eliminates changes
in current due to variations in forward voltage, which translates into a constant LED
brightness. Figure 2 illustrates the configuration of a typical buck converter driver circuit.
The value of current-sense resistor (R
SENSE
feedback voltage that the buck converter requires. Multiple LEDs should be connected in a
series configuration to keep an identical current flowing in each LED.
4/20
⎞
R
H
⎟
⎟
R
L
⎠
RIVn
FBFMAX_F
) depends on the desired LED current and the
AN2754 Background
Figure 2. Constant current control
VOUT
Voltage Regulator
FB
R
SENSE
Equation 2
V
FB
I =
F
R
FB
Accuracy and efficiency are the two main goals of the current sensing even if they are in
direct conflict. The higher the sense voltage is, the higher the signal-noise ratio, but the
higher the power dissipated on R
To reduce the power dissipated in the series resistance,
SENSE
.
Figure 3 shows a simple method of
amplifying the current sense signal by using a single supply op-amp. This method allows the
user to select the current sense resistor R
while setting the average value of I
with the gain of the op-amp.
F
according to the desired power dissipation
SENSE
Figure 3. Constant current control with V
Voltage Regulator
VOUT
FB
Equation 3
I
=
F
SENSE
5/20
amplification
SENSE
+
OUT
-
RFB RSENSERIN
V
FB
⎛
R
FB
⎜
1R
+⋅
⎜
R
IN
⎝
⎞
⎟
⎟
⎠
Buck topology switching power supply AN2754
2 Buck topology switching power supply
The buck topology switching power supply is an efficient voltage regulator which produces
an output voltage always less than or equal to the source voltage in the same polarity. The
first step of conversion is to generate a chopped version of input source. A single-pole
double-throw (SPDT) switch is connected as shown in
Figure 4. Buck topology
The switch output voltage is equal to the converter input voltage when the switch is in
position 1 and equal to zero when the switch is position 2. The position is varied periodically
at a frequency of 1/T, where T represents the switching cycle period. The ratio of the on-time
to the period is referred to as the duty cycle D. So the switch output is a rectangular
waveform having amplitude equal to the source voltage, frequency equal to 1/T and duty
cycle equal to D. By inserting a low-pass filter between the (SPDT) switch and the load, a
basic buck topology is formed. The DC value of switch output voltage is simply the source
voltage multiplied by the duty cycle. The L-C filter cutoff is selected to pass the desired lowfrequency components of the switch output but also to attenuate the high-frequency
switching harmonics.
Figure 4.
A power stage can operate in continuous or discontinuous inductor current mode.
Continuous inductor current mode is characterized by current flowing continuously in the
inductor during the entire switching cycle in steady-state operation. In discontinuous mode
the inductor current drops to zero for a portion of the switching cycle. In this section we will
derive the voltage conversion relationship for the continuous conduction mode buck power
stage. In continuous conduction mode, the power stage assumes two states per switching
cycle.
The ON state is when the high-side switch is ON and the low-side switch is OFF.
Figure 5. Buck converter circuit while the switch is in position 1
During the ON state the voltage applied on the inductor is given by:
Equation 4
VVsv −=
outL
6/20
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