ZXSC400EV5 User Guide Issue 2

ZXSC400EV5 USER GUIDE
DESCRIPTION
The ZXSC400 is a voltage mode boost
converter in a SOT23-6 package. The low
feedback voltage VFB input allows the
current in a chain of LEDs to be set
accurately. The ZXSC400EV5 is designed to
drive one or two 3W LEDs at a constant
current of 700mA from an input voltage
between 1.8V and 3.0V. The board is
compatible with Future LED luminary boards
for use with a variety of LED types. The
board is open-circuit-LED protected and
intended to be used with two alkaline, NiCad
or NiMH batteries.
FEATURES
• Drives one or two 3W LEDs.
• Typical efficiency of 80%.
• Open-circuit-LED protected.
• High-sided current sensing.
• Compatible with Future LED boards.
APPLICATIONS
• LED torches and flashlights.
• Portable illumination.
• Emergency lights.
TYPICAL APPLICATION CIRCUIT
ORDERING
INFORMATION
ORDER NUMBER
ZXSC400EV5
Please note evaluation
boards are subject to
availability and qualified
leads.
Figure 1. ZXSC400EV5 Evaluation Board.
Issue 2 – February 2009
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ZXSC400EV5
REFERENCE DESIGN
The ZXSC400EV5 uses the circuit below. The target application is a 3W white LED
being driven from two NiCd/NiMH batteries for torches, but the device is also
excellent for general high powered LED driving.
Q1 and Q2 form a pseudo-Darlington pair, which provides enough current gain for
switching currents of above 1.5A. The turnoff speed is enhanced by D2, which
provides a discharge path for Q1.
The current monitor ZXCT1009 is used for monitoring the LED current flowing
through the low-ohmic resistor R2. The resistor R2 transforms the LED current into
an accurate and low-loss sensing voltage for the VFB pin of the ZXSC400. Open
circuit protection is provided by Zener diode D3, which causes VFB to be held above
300mV when the output is over voltage.
Since the Isense threshold of ZXSC400 is quite low (typically 28mV), noise due to
heavy collector switching currents may cause the system to work in unstable
manner. In order to prevent this from happening, the waveform on the Isense pin is
enhanced by adding the edges from the collector waveform via the high pass filter
formed by C4 and R6.
The ZXSC400EV5 operates from a supply voltage between approximately 1.8V and
3.0V.
For other reference designs or further applications information, please refer to the
ZXSC400 datasheet.
Figure 2. ZXSC400EV5 Schematic Diagram.
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ZXSC400EV5
COMPONENTS LIST
Ref
Value
Package
Part Number
Manufacturer
Notes
U1
U2
Q1
Q2
D1
D2
D3
L1
R1
R2
R3
R4
R5
R6
R7
C1
C2
N/A
N/A
N/A
N/A
40V/2A
40V/400mA
6.8V/0.25W
15uH/3A
20mΩ
50mΩ
820Ω
82Ω
4.7Ω
10Ω
100Ω
22µF/16V
4.7µF/10V
SOT23-6
SOT23
SOT23
SOT23
SOT23-6
SOD323
SOT23
N/A
0805
0805
0805
0805
0805
0805
0805
1210
1206
ZXSC400E6
ZXCT1009
ZXTN25012EFH
ZXTN25012EFL
ZHCS2000
ZHCS400
BZX84-C6V8
NPI31W150MTRF
Boost converter
Current monitor
Low sat. NPN transistor
Low sat. NPN transistor
40V/1A Schottky diode
40V/400mA Schottky diode
6.8V/ 0.25W Zener diode
15uH/3A SMT Inductor
0805 1% tolerance
0805 1% tolerance
0805 1% tolerance
0805 5% tolerance
0805 5% tolerance
0805 5% tolerance
0805 5% tolerance
C3
C4
0.22µF/6V3
330pF/6V3
0805
0805
Diodes Zetex
Diodes Zetex
Diodes Zetex
Diodes Zetex
Diodes Zetex
Diodes Zetex
Diodes Zetex
NIC Comps.
Generic
Generic
Generic
Generic
Generic
Generic
Generic
Murata
Murata
NIC Comps
Generic
Generic
GRM32ER61C226KE20L
GRM31CR71A475KA01L
NMC1206X7R475K10
1206 X7R
Figure 3. Performance graphs.
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ZXSC400EV5
ZXSC400EV5 VERIFICATION
Figure 4. Evaluation PCB
ZXSC400EV5 Set-up and Test
1. Connect VIN and GND to positive and zero volts of PSU supply respectively.
2. Set the PSU to 3V.
3. Turn on PSU.
4. The LED should illuminate and be regulated at 700mA +/-5%.
5. The input current should measure between 1A ~ 1.2A
THIS IS A FUNCTIONAL EVAL BOARD.
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ZXSC400EV5
LAYOUT CONSIDERATIONS
The PCB tracks have been kept as short as possible to minimise ground bounce,
and the ground pin of the device is soldered directly to the ground plane. The
inductor, and the input and output capacitors, have been mounted close to the
device. This is particularly important in order to minimise the parasitic resistance and
inductance, which would degrade efficiency. The Isense pin is a high impedance
input, so PCB track lengths to pin this have been kept as short as possible to reduce
noise pickup.
Top Silk
Top Copper
Bottom Copper
Drill File
Figure 5. Layout of the ZXSC400EV5.
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ZXSC400EV5
INTENTIONALLY BLANK
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ZXSC400EV5
INTENTIONALLY BLANK
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ZXSC400EV5
Definitions
Product change
Diodes Incorporated reserves the right to alter, without notice, specifications, design, price or conditions of supply of any product or service.
Customers are solely responsible for obtaining the latest relevant information before placing orders.
Applications disclaimer
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 assumed by
Diodes Inc. with respect to the accuracy or use of such information, or infringement of patents or other intellectual property rights arising from
such use or otherwise. Diodes Inc. does not assume any legal responsibility or will not be held legally liable (whether in contract, 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.
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1. are intended to implant into the body
or
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To ensure quality of service and products we strongly advise the purchase of parts directly from Zetex Semiconductors or one of our regionally
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Semiconductor devices are susceptible to damage by ESD. Suitable precautions should be taken when handling and transporting devices. The
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Product status key:
“Preview”
“Active”
“Last time buy (LTB)”
“Not recommended for new designs”
“Obsolete”
Datasheet status key:
“Draft version”
“Provisional version”
“Issue”
Future device intended for production at some point. Samples may be available
Product status recommended for new designs
Device will be discontinued and last time buy period and delivery is in effect
Device is still in production to support existing designs and production
Production has been discontinued
This term denotes a very early datasheet version and contains highly provisional
information, which may change in any manner without notice.
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.
This term denotes an issued datasheet containing finalized specifications. However, changes to
specifications may occur, at any time and without notice.
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