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A Product Line of
Diodes Incorporated
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Using the ZXSC310 to drive high power LEDs from 2
alkaline cell with hysteretic UVLO
Dr. Kit Latham, Senior appplications engineer, Diodes Incorporated
Introduction
This note describes how the ZXSC310 boost convertor can be used to drive high power LED’s
from batteries of less than 1V to greater than 6V .
The option of under voltage lockout (UVLO) to protect the batteries from deep discharge is also
discussed. Two options are described one with hysteresis and a simpler one without.
The device is available in a SOT23-5 packag and requires only a few small external components
in this application.
Rather than a classical supply philosophy, which would supply the full current to the LED until the
battery was exhausted and then switch off suddenly, the ZXSC310 has been designed to allow a
graceful fading of the intensity so that the user knows that it is time to replace the battery but can
continue to use the torch (flashlight); albeit at gradually reducing intensity. (If a steady current is
preferred the ZXSC400 can be used in circuits very similar to those to be described in this note.)
The following paragraphs describe some of the design considerations and trade-off’s relevant to
such an application.
Heatsinking
The High Power LED’s can be driven at up to 5W and under such drives typically 4W will need to
be extracted from the device without the junction temperature exceeding it’s rated value. If
maximum life is required then there should be a margin between the rated temperature and the
actual temperature. If the device is expected to be used in an ambient temperature of say 30degC
at 4W dissipation then the necessary thermal resistance form the junction to ambient needs to be
no more than
(Tjmax-Tamb)/Pdiss thus:-
(150-30)/4 K/W = 30K/W.
The LED data sheet for the Luxeon K2 defines the thermal resistance (junction to thermal bonding
pad) as 9K/W, leaving a maximum of 21K/W from the mounting point to ambient to be guaranteed
by the thermal design of the product. One square cm of FR4 pcb 1.6mm thick has a thermal
resistance of about 50K/W from the top layer to the bottom layer, so it is clear that attention needs
to be paid to the pcb mounting as well as the thermal path to the outside. Frequently ISM
(insulated metal substrate) pcbs are used in all but the most cost constrained applications. If this
is not possible then thermal vias can be used to conduct the heat to the bottom of the pcb from
where it would need to be conducted to ambient and this is a requirement of the mechanical
design of the product.
Sometimes, as in the designs presented below, 5W represents too much battery drain and a lower
current is used which eases the thermal design. One K2 LED running at 500mA has a dissipation
Issue 1 - November 2008
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1
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AN65
of about 1.5W so the maximum thermal resistance then becomes 70K/W - rather easier to
implement.
Vbatt
D1
L1
1
3
C1
2
U1
Vcc
Drive
Q1
5
En
GND
C2
Sense
D2
4
R1
ZXSC310:
GND
Figure 1 - Basic boost circuit
Choice of inductor
The inductor is usually the physically largest component in this type of design and there are tradeoffs to be made in it’s selection.
For the best efficiency (read battery life) the inductor needs to be operating within it’s rating for
saturation current and to have an acceptably low series resistance. It should also be physically
small and inexpensive. These two pairs of attributes are usually mutually exclusive so a suitable
compromise has to be reached and this is a function of the particular application.
Two examples are shown below.
•
Driving a K2 at 350mA from 2 alkaline cells.
•
Driving a K2 at 500mA from 2 alkaline cells.
The first graph shows how the LED current varies with battery voltage for a 350mA initial current.
500
mA
400
300
batt m A
led m A
200
100
0
1000
1500
2000
2500
3000
b a tt m V
Figure 2 - Performance at a nominal 350mA drive
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This characteristic applies with the following BoM
Table 1. Bill of materials for figure 1
Ref
U1
Q1
D1
R1
D2
C1
C2
L1
Part No
ZXSC310
ZXTN25012
SBR2A40P1
Value
Manufacturer
Zetex
Zetex
Diodes
generic
Lumileds
generic
generic
Web
1µF 6.3V
1uF 25V
Package
sot23-5
sot23
di123
0805
proprietary
0805
0805
10uH
proprietary
CoilCraft
www.coilcraft.com
50mΩ
LXK2-xxx
LPO2506OB103
www.zetex.com
www.zetex.com
www.diodes.com
www.lumileds.com
A photograph of this inductor is shown in Figure 3, as can be seen it is low profile and compact
.
Figure 3 - The low profile inductor
Higher power circuits
For more LED current an inductor with lower resistance and higher saturation is preferred, the
data shown in Figure 4 was obtained using a slightly larger inductor, pictured below the circuit
diagram in Figure 5.
25mR & 40mR 6.8uH
600
Iled 25m R
Iled 33m R
500
Iled 40m R
mA
400
300
200
100
0
1
1.5
2
2.5
3
V b a tt V
Figure 4 - Performance for a nominal 500mA drive
Issue 1 - November 2008
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Figure 5 - The slightly larger inductor
In this case the BoM is:
Table 2 - BoM for a 500mA drive
Ref
U1
Q1
D1
R1
D2
C1
C2
L1
Part No
ZXSC310
ZXTN25012
SBR2A40P1
Value
25/33/40mΩ
LXK2-xxx
DO3316P-223
1µF 6.3V
1uF 25V
22uH
Package
sot23-5
sot23
di123
0805
proprietary
0805
0805
proprietary
Manufacturer
Zetex
Zetex
Diodes
generic
Lumileds
generic
generic
CoilCraft
Web
www.zetex.com
www.zetex.com
www.diodes.com
www.lumileds.com
www.coilcraft.com
Shutdown
This circuit has no switch off functionality and there is therefore the risk that the cells could go
into deep discharge which could lead to leakage. The addition of a few low cost components can
get round this problem as shown in the circuit diagram below.
Here the shutdown capability of the ZXSC310 is used to switch the unit off before the cell voltage
falls below the specified value. There is then only a very small current drawn from the battery.
This circuit will exhibit some temperature dependence as the Vf’s of the diodes are temperature
dependent, so it may not be suitable for applications requiring constant performance over a wide
temperature range .
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Vbatt
D1
L1
D2
U1
D3
1
Vcc
3
C1
5
Q1
En
2
D5
Drive
GND Sense
R1
C2
4
ZXSC310:
D4
R2
GND
Figure 6 - A circuit for simple UVLO
For this design the BoM is:
Table 3 - BoM for the Simple UVLO circuit
Ref
U1
Q1
D1
D2,D3,D5
R1
R2
D4
C1
C2
L1
Part No
ZXSC310
ZXTN25012
SBR2A40P1
1N4148WT
Value
Package
sot23-5
sot23
di123
sod523
0805
0805
proprietary
0805
0805
proprietary
see text
50mΩ
LXK2-xxx
1µF 6.3V
1µF 25V
10µH
LPO2506OB-103
Manufacturer
Zetex
Zetex
Diodes
Diodes
generic
generic
Lumileds
generic
generic
CoilCraft
Web
www.zetex.com
www.zetex.com
www.diodes.com
www.diodes.com
www.lumileds.com
www.coilcraft.com
The performance of this circuit is shown in Figure 7 for different combinations of diodes and
resistors.
E n a b le C h a ra c te ris tic w ith D io d e /R c c t
1.80
5.1
1.70
1.60
10
15
20
V b att
1.50
33
1.40
5.1
1.30
1.20
1.10
3@ 1N4148 +R
51
5.1 10
15
10
15 20
20
2@ 1N4148 +R
75
1.00
51
51
0.90
75
75
0.80
0
20
2@ 1N4148+ Sc hottk y
33
33
40
60
80
kR
Figure 7 - Selection of components for the required UVLO value
Issue 1 - November 2008
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As an example 3 1N4148 diodes and a 62k resistor would give a shutdown at about 1.3V.
Resistance values above 75k are too high to give reliable shutdown .
Switch off with hysteresis
The circuit above can be improved by the addition of a few more components to give some
hysteresis. This avoids the possible scenario where the device switches off reducing the drain on
the battery whose voltage then rises and switches the LED’s back on again. The circuit shown in
Figure 8 uses 2 transistors in a type of Schmitt circuit to avoid this possibility. The actual
switching levels do depend on the transistors chosen. A dual transistor would save a little space
but the switching levels may have to be adjusted. The performance of the example circuit is
shown in Figure 9.
This circuit will have less temperature dependence than the previous one.
The ZXTD09N50DE6 is a dual in SOT23-6 which gives very similar performance to the 2
ZXTN2040F’s and offers space saving at a slightly higher price.
Vbatt
D1
L1
R3
R2
R1
U1
1
C1
Q2
Q3
3
2
R4
Vcc
Drive
5
Q1
GND Sense
D2
4
ZXSC310:
R5
C2
En
R6
GND
Figure 8. UVLO with hysteresis.
H y s te re s is P e rfo rm a n c e
450
400
350
Iled m A
300
250
V batt ris ing
200
V batt falling
150
100
50
0
1
1.5
2
2.5
3
V b a tt V
Figure 9 - Hysteretic UVLO Performance.
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Table 4 - BOM for hysteretic UVLO
Ref
U1
Q1
Q2,Q3
D1
R1
R2, R3, R4
R5
R6
D4
C1
C2
L1
Part No
ZXSC310
ZXTN25012
ZXTN2040F
SBR2A40P1
Value
16kΩ
10kΩ
1k2Ω
50mΩ
LXK2-xxx
LPO2506OB-103
1µF 6.3V
1µF 25V
10µH
Package
sot23-5
sot23
sot23
di123
Manufacturer
Zetex
Zetex
Zetex
Diodes
0805
0805
0805
0805
proprietary
0805
0805
proprietary
generic
generic
generic
generic
Lumileds
generic
generic
CoilCraft
Web
www.zetex.com
www.zetex.com
www.zetex.com
www.diodes.com
www.lumileds.com
www.coilcraft.com
There is a sample schematic which will run with the free downloadable Zetex Simulator available
at www.zetex.com and this is a good way of testing the effects of different component values.
Choice of switching transistor
The switching transistor Q1 needs high current gain and low Vcesat for the circuit to work
properly. The Zetex ZXTN25012 has been developed with this application in mind and whilst
other types could be used they would not give the same efficiency and would run hotter.
Conclusion
We have shown how, with the addition of a few small components, the ZXSC310 can drive high
power LED’s from 2 alkaline cells. Additionally 2 methods of adding UVLO to the designs have
been described.
The same principles can be applied to the ZXSC400 which has feedback capability to maintain the
LED current more constant as the battery becomes discharged.
Please see the full data sheets (and other Application and Design Notes) at www.zetex.com.
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The circuits in this design/application note are offered as design ideas. It is the responsibility of the user to ensure that the circuit is fit for
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or
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