DN63

DN63
ZXSC310 Solution to drive 8 LEDs connected in series
Khagendra Thapa, Principal Systems Engineer, Zetex Semiconductors
Description
Low cost, small simple and low power multi-LED drive solutions are important in applications
including LCD backlight, key illuminations and effects for handheld devices (e.g. cell phones),
signage and indicators. The LED current is generally between 10mA to 30mA and is powered from
a single cell Li-Ion or three cell alkaline/NiMH/NiCad batteries. For battery powered applications
low shutdown quiescent current is important to conserve battery life.
Figure 1 shows a simple low cost boost convertor, ZXSC310, driving eight series connected LEDs.
ZXSC310 is in a small SOT23-5 package. The design solution is for an application with an input
voltage range of 4.5V to 2.5V (e.g. a single cell Li-Ion can have a voltage range of 4.3V to 2.6V)
with LED current optimized at 20mA typical, at 4.0V supply. The LED current at 4V is chosen to
match the 20mA typical forward current of the LED used.
D1
ZHCS1000
VIN
4.5V to 2.5V
L1
68␮H
LED1
U1
Q1
ZXTN25040DFH
VCC
LED2
Drv
Stdn
Stdn
L1
68␮H
ISENSE
GND
C1
100nF
LED7
ZXSC310
R1
100k⍀
RSENSE
200m⍀
LED8
GND
Figure 1
Schematic diagram
With a single cell Li-Ion battery, the circuit in Figure 1 can drive 3 or more series connected LEDs,
the maximum number of LEDs limited by the breakdown voltage of the bipolar transistor Q1.
Depending on the number of LEDs connected in series, the sense resistor, RSENSE, will have to be
adjusted to obtain the required LED current at a certain supply voltage.
The ZXSC310 can be shutdown by pulling the Stdn pin low. The quiescent current in the
shutdown mode is typically 5␮A. If shutdown feature is not required tie the Stdn pin to the VCC
pin.
Figure 2 shows the efficiency and the LED current against supply voltage. The LED current
decreases with the supply voltage. This helps to draw less current from a discharged battery.
The bill of materials for the circuit in Figure 1 is shown in Table 1.
Issue 5 - August 2007
© Zetex Semiconductors plc 2007
1
www.zetex.com
DN63
Efficiency vs input voltage
LED current vs supply voltage
100
20
90
LED Current (mA)
Efficiency (%)
80
70
60
50
40
30
20
3 LEDs in Series
10
0
4.5
4
3.5
3
15
10
5
3 LEDs in Series
0
4.5
2.5
4
Input voltage (V)
3.5
3
2.5
Input voltage (V)
Figure 2
Performance graphs
Ref.
Part no.
Value
Manufacturer Contact details
U1
Q1
ZXSC310E5
NA
ZXTN25040DFH NPN, VCEO = 40V
D1
ZHCS1000
Zetex
Zetex
www.zetex.com
www.zetex.com
1A, low forward voltage VF Zetex
www.zetex.com
RSENSE Generic
200m⍀
Generic
NA
R1
C1
C2
L1
LED1-8
100k⍀
100nF, 6.3V, X7R
2.2␮F, 35V
68␮H
White LED
Generic
Generic
Generic
Coilcraft
Nichia
NA
NA
NA
www.coilcraft.com
www.nichia.com
Generic
Generic
Generic
DO1608P-683
NSPMW500BS
Table 1
www.zetex.com
Bill of materials
2
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DN63
Definitions
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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
the user’s application and meets with the user’s requirements. No representation or warranty is given and no liability whatsoever is
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tort (including negligence), breach of statutory duty, restriction or otherwise) for any damages, loss of profit, business, contract,
opportunity or consequential loss in the use of these circuit applications, under any circumstances.
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A. Life support devices or systems are devices or systems which:
1. are intended to implant into the body
or
2. support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the
labelling can be reasonably expected to result in significant injury to the user.
B. A critical component is any component in a life support device or system whose failure to perform can be reasonably expected to
cause the failure of the life support device or to affect its safety or effectiveness.
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The possible damage to devices depends on the circumstances of the handling and transporting, and the nature of the device. The extent
of damage can vary from immediate functional or parametric malfunction to degradation of function or performance in use over time.
Devices suspected of being affected should be replaced.
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Future device intended for production at some point. Samples may be available
“Active”
Product status recommended for new designs
“Last time buy (LTB)”
Device will be discontinued and last time buy period and delivery is in effect
“Not recommended for new designs” Device is still in production to support existing designs and production
“Obsolete”
Production has been discontinued
Datasheet status key:
“Draft version”
This term denotes a very early datasheet version and contains highly provisional information, which
may change in any manner without notice.
“Provisional version”
This term denotes a pre-release datasheet. It provides a clear indication of anticipated performance.
However, changes to the test conditions and specifications may occur, at any time and without notice.
“Issue”
This term denotes an issued datasheet containing finalized specifications. However, changes to
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© 2007 Published by Zetex Semiconductors plc
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