Discrete LED driver

AN10739
Discrete LED driver
Rev. 2 — 21 June 2010
Application note
Document information
Info
Content
Keywords
LED, constant current source, buck converter
Abstract
This application note describes a 300 mA discrete LED driver, based on a
buck-converter principle, with a cycle-by-cycle current control. It includes a
proposal for a BOM and layout of a low cost, low component count
solution.
AN10739
NXP Semiconductors
Discrete LED driver
Revision history
Rev
Date
Description
02
20100621
Corrected version, Figure 4 figure notes corrected
01
20090211
Initial version
Contact information
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For sales office addresses, please send an email to: salesaddresses@nxp.com
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1. Introduction
This application note describes a 300 mA discrete LED driver, based on a buck-converter
principle with an efficiency of 80 to 90 %. It includes a proposal for a BOM and layout
leading to a low cost, low component count solution to drive a single LED or a string of
LEDs connected in series.
The choice of the discrete parts is discussed with respect to NXP’s bipolar low
VCEsat (BISS) and ultra low VF MEGA Schottky technologies, i.e. the PBSSxxx series
and the PMEGxxx series.
Key applications for the driver are lighting applications, where constant LED brightness,
high efficiency and low cost are important features. For example automotive lighting
applications require that general illumination and signage should not consume too much
power when the motor is not running. The input voltage of +6 V to +18 V supports
automotive requirements, too.
Besides, battery driven handhelds like flash lights or head lamps will benefit from the
topology and efficiency the driver delivers.
2. Operating principle
Vin
C1
R2
R1
TR2
TR1
L1
Vout
TR3
D1
R3
C2
C3
D2
006aab405
Fig 1.
LED driver schematic
2.1 Basic operating principle
The 300 mA discrete LED driver is based on the buck-converter principle with a
cycle-by-cycle current control. The input peak current is set by resistor R1 and by
modifying R1, the current can easily be set to lower or higher values, i. e. designing a
driver from 20 mA to 1 A.
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When applying supply voltage Vin, TR3 is switched on, providing the base current for the
PNP transistor TR1 and switching it on. With diode D1 reversed biased, current starts to
flow through inductor L1 and LED D2.
The coil equation described by Equation 1 shows that a desired rise or fall of the inductor
current requires a certain voltage step applied to the inductor, with the factor of
proportionality L, called the self-inductance of the coil:
Δi L ( t )
v L ( t ) = L × --------------Δt
(1)
With an LED as load and a constant Vin the result is a linearly increasing input current, as
depicted in Figure 2:
Imax
Vin
t
ton
tperiod
006aab406
Fig 2.
Input voltage and current
As the collector current of TR1 increases, the voltage drop at the current sense
resistor R1 increases, too. When the voltage drop reaches TR2s base-emitter turn-on
voltage VBE(on) of about 0.65 V, TR2 switches on and pulls the base of TR1 to the supply
voltage, i. e. turns TR1 off.
The value of R1, therefore, sets the maximum input current in the application, which flows
through R1, TR1 and the inductor L1.
When switching TR1 off, its collector current almost immediately drops back to zero.
The inductor, however, cannot change its current suddenly, according to ΔI/Δt = V/L.
The current will decrease but continues to flow in the same direction, with diode D1
now conducting.
As D1 is forward biased, the voltage over L1 reverses when TR1 is switched off.
The voltage level at the cathode is −VF of the Schottky diode, as long as there is energy
stored in the inductor.
Solving the inductor equation for this case and taking iL(0) = Imax as boundary condition
leads to:
– V out × t
v L(t) × t
i L(t) = ------------------- + I max = --------------------- + Imax
L
L
(2)
The current is decreasing until it reaches zero, depicted in Figure 3.
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Discrete LED driver
Imax
A
Iout
B
tperiod
t
006aab407
Fig 3.
Inductor current and optimal (DC) output current
When all the energy that was stored in the inductor is delivered to the output, D1 becomes
reversed biased again and the procedure is restarted.
2.2 Current, current ripple and switching frequency
The slope of the current is set by the voltage step across the inductor, and for a fixed
input voltage this voltage step is constant because the voltage drop across the LED
is nearly independent from its current.
The constant voltage step at the inductor leads to a linearly increasing current
(remember: ΔI/Δt = V/L) flowing through the inductor and the LED - neglecting losses
and other parasitic effects.
When no output capacitors are used, the output current is exactly the coil current and the
ripple height would be ±50 % (see Figure 3).
To get smaller output ripples, the capacitor C2 is added, acting as a charge storage device
and smoothing the sawtooth ripple. The value of the capacitor must be chosen according
to LED current and flicker requirements of the specific application, the larger the capacitor,
the less the ripple.
The most important design value for the LED driver is the average output current,
which is half the peak current of the coil set by R1 (Imax = VBE(TR2)/R1).
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Discrete LED driver
Looking at Equation 1 the energy absorption at the circuit input for one period can be
determined as:
1
W in = --- × I max × V in × t on
2
(3)
The energy provided to the LED is:
W out = Iout × V F × ( t on + t off )
(4)
Iout is the desired DC LED current.
ton and toff are the turn-on time and the turn-off time of TR1, and the rise time tr and the fall
time tf of the coil current, respectively. Their values can be calculated using the two
solutions of the coil equation derived above. During ton the coil current needs to rise
from 0 to Imax. Thus, using Equation 2, the turn-on time can be calculated to:
L
t on = I max × -----------------------V in – V out
(5)
The time the current needs to drop back to 0 A is:
L
t off = I max × ---------V out
(6)
Pasting Equation 5 and Equation 6 into Equation 3 and Equation 4 and applying the
power conservation law yields (assuming no losses in the circuit):
1
W out = W in ⇒ I out = --- × I max
2
(7)
ton and toff determine the switching frequency of the circuit:
V in – V out V out
1
f = -------------------- = ------------------------ × ---------t on + t off
V in
L × I max
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Discrete LED driver
3. Dimensioning and choice of discrete parts
The choice of the discrete parts on one hand is dependent on the input requirements like
input voltage range, LED current and switching frequencies. On the other hand, the
performance of the devices like their on-state losses, switching losses or the power
dissipation capabilities of a specific package influence the efficiency and the costs of
the circuit.
3.1 Inductor L1, Transistor TR1 and Schottky diode D1
3.1.1 Inductor L1
The switching frequency of the circuit is determined by the input voltage Vin, the
LED forward voltage VF, the peak current Imax, and the inductor value L (see Equation 8).
With given input conditions one can calculate the resulting switching frequency for
different values of L to get a guideline for the choice of the inductor. In general, L shall be
as small as possible to reduce costs and package size of the device.
Smaller inductors usually have a smaller DC resistance, too, leading to higher efficiency
of the whole circuit. The minimum coil saturation current rating should be 1.2 times the
peak current.
Alternatively, one can specify a maximum switching frequency of the application to derive
the required inductor, using Equation 8. For the example below, fmax was set to 100 kHz,
which is an appropriate value for a bipolar switch and also noise immunity.
Example:
For fmax = 100 kHz, Imax = 0.6 A, Vin(max) = 18 V, VF = 3.2 V
18V – ( 3, 2V ) 3, 2V
L = -------------------------------------- × ------------- = 43, 85μH
100kHz × 0, 6A 18V
Taking 47 μH will result in a maximum switching frequency of < 100 kHz for Vin = 18 V.
3.1.2 Transistor TR1
A bipolar transistor in a small SMD package shall be used for the switch as it offers an
excellent performance-cost ratio for this application. The final choice of the device is
dependent on the required performance. IC and VCEO are given by the input conditions
but also the losses of the device, i. e. Ptot, during operation are important. The main
parameters contributing to the losses are the saturation voltage VCEsat and the power loss
during the fall time tf during turn-off.
The best choice to keep the on-state losses low, is using a low VCEsat (BISS) transistor
whereas BVCEO shall be at least 1.2 × Vin(max), and IC(max,DC) shall be at least 1.2 × Imax.
Besides the on-state losses, switching times are an important factor for the efficiency
whereas the main contributor is the fall time. Losses during the rise time are nearly zero
as with an inductive load the collector current rises slowly.
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Discrete LED driver
PBSS5220T is a good choice for a 300 mA driver with an input voltage from +6 V
to +18 V.
The device is a 2 A, 20 V bipolar low VCEsat (BISS) transistor, with a typical VCEsat
of 70 mV at IC = 600 mA and reasonable switching times. It comes in the very
cost-efficient SOT23 package, with a Ptot of 250 mW on standard footprint.
To assure saturation for TR1 - in order to benefit from the low VCEsat technology - R3 must
be chosen in a way that with IC,TR3 (which equals IB,TR1) an IC/IB ratio of about 30 is
adjusted.
For a maximum TR1 collector current of 600 mA, IB shall be tuned to 20 mA, with
R3 = 510 Ω.
For the resulting IC/IB = 30, there is no VCEsat curve in the set of curves shown below for
PBSS5220T. To get an idea of the power dissipation during ton, the value for an IC/IB = 50
is taken, which will be at least equal or worse.
006aab408
1
VCEsat
(V)
10−1
IC/IB = 100
10−2
IC/IB = 50
IC/IB = 10
10−3
10−1
1
10
102
103
104
IC (mA)
VCEsat,typ (at IC = 600 mA and IB = 20 mA) = 70 mV
Fig 4.
AN10739
Application note
PBSS5220T: Collector-emitter saturation voltage as a function of
collector current; typical values
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3.1.3 Schottky diode D1
A Schottky diode is chosen for the ’catch diode’, to provide a current path for the
LED current during toff.
NXP’s MEGA Schottky PMEG series offers an ultra low forward voltage VF, resulting in
reduced heat generation during operation and an increased efficiency.
PMEG2010EJ is proposed for a 300 mA LED driver, which is a 20 V, 1 A MEGA Schottky
diode in the SOD323F (SC-90) package. It offers a Ptot of 360 mW on standard footprint
with a VF of typically 340 mV at 0.6 A DC current, whereas the SOD323F (SC-90)
package is a cost-efficient solution, which can not only serve for the 300 mA LED driver
but also for modifications up to higher output currents.
4. Demo-Board and measurements
To demonstrate the performance of the application discussed above, a demonstrator was
realized on a 16.5 mm × 49.5 mm PCB, with the BOM proposed (see Section 4.1).
Input requirements were an input voltage range from +6 V to +18 V, low LED current ripple
and a maximum switching frequency < 100 kHz.
006aab409
Fig 5.
AN10739
Application note
NXP Discrete LED Driver and OSRAM Golden Dragon
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4.1 BOM proposal: 300 mA driver
Table 1.
BOM proposal 350 mA LED current
BOM part
Proposal
R1
1.2 Ω (2010), 1 W resistor
R2
10 kΩ (0603)
R3
510 Ω (0603)
C1
1 μF
C2
220 μF
C3
not connected
L1
47 μH, LQH55D series from Murata
D1
PMEG2010EJ; 20 V, 1 A Schottky diode (SOD323F/SC-90), NXP
D2
1 A LED; OSRAM Golden Dragon LW W5SM
TR1
PBSS5220T; 20 V, 2 A PNP low VCEsat (BISS) transistor (SOT23), NXP
TR2, TR3
BC847BPN; NPN/PNP general-purpose double transistor (SOT363), NXP
4.1.1 Measurements
Measurements have been performed on the final layout regarding efficiency and switching
frequencies.
006aab410
100
η
(%)
(1)
80
(2)
60
40
20
0
6
8
10
12
14
16
18
Vin (V)
(1) L1 = 33 μH
(2) L1 = 47 μH
Fig 6.
Efficiency = f(Vin)
The efficiency Pin/Pout of the board as shown with L1 = 47 μH is about 80 % for a supply
voltage range from 9 V to 12 V.
Choosing lower inductor values would result in a higher efficiency, as usually a smaller
inductor comes with a lower DC resistance as well as lower inductor core losses.
But, the smaller the inductor, the higher the maximum switching frequency of the
application.
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Using 33 μH instead of 47 μH would increase the efficiency to > 85 %. With 18 V input
voltage, the switching frequency would be about 120 kHz (see Equation 8), and the
resulting effects on increased switching losses and noise immunity may become an issue
for certain application areas. However, the layout should be able to handle a minimum
value of L1 of 22 μH.
Using an LED string instead of a single LED would result in an increased efficiency, too,
as the ratio between input and output voltage in that case would be beneficial for a buck
converter.
As with increasing input voltage also the switching frequency increases, the efficiency
drops because of higher switching losses in the discrete devices.
The increase of frequency is shown below as a comparison between the theoretical
values using Equation 8 and a real measurement.
006aab411
100
f
(kHz)
(1)
80
(2)
60
40
20
0
6
8
10
12
14
16
18
Vin (V)
(1) measured data
(2) calculated data
Fig 7.
AN10739
Application note
Frequency = f(Vin), L1 = 47 μH
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5. Conclusion
• Highly efficient constant current LED driver using a switching power conversion
solution based on a buck-converter principle, supported by NXP’s low VCEsat BISS
and MEGA Schottky technologies
• Applicable for a wide input voltage range from +6 V to +18 V
• Applicable for a wide range of ambient temperatures due to low power dissipation /
low heat generation of the driver
• Low cost, low component count solution
• Modifiable for a wide range of output currents from 300 mA up to 1 A
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6. Legal information
6.1
Definitions
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information included herein and shall have no liability for the consequences of
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6.2
Disclaimers
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6.3
Trademarks
Notice: All referenced brands, product names, service names and trademarks
are the property of their respective owners.
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7. Contents
1
2
2.1
2.2
3
3.1
3.1.1
3.1.2
3.1.3
4
4.1
4.1.1
5
6
6.1
6.2
6.3
7
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Operating principle . . . . . . . . . . . . . . . . . . . . . . 3
Basic operating principle . . . . . . . . . . . . . . . . . 3
Current, current ripple and switching frequency 5
Dimensioning and choice of discrete parts. . . 7
Inductor L1, Transistor TR1 and Schottky diode
D1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Inductor L1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Transistor TR1 . . . . . . . . . . . . . . . . . . . . . . . . . 7
Schottky diode D1 . . . . . . . . . . . . . . . . . . . . . . 9
Demo-Board and measurements . . . . . . . . . . . 9
BOM proposal: 300 mA driver . . . . . . . . . . . . 10
Measurements . . . . . . . . . . . . . . . . . . . . . . . . 10
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Legal information. . . . . . . . . . . . . . . . . . . . . . . 13
Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Disclaimers . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Trademarks. . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Please be aware that important notices concerning this document and the product(s)
described herein, have been included in section ‘Legal information’.
© NXP B.V. 2010.
All rights reserved.
For more information, please visit: http://www.nxp.com
For sales office addresses, please send an email to: salesaddresses@nxp.com
Date of release: 21 June 2010
Document identifier: AN10739