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ZXSC310
LED DRIVER SOLUTION FOR LCD BACKLIGHTING
DEVICE DESCRIPTION The ZXSC310 is a single or multi cell LED driver designed for LCD backlighting applications. The input voltage range of the device is between 0.8V and 8V. This means the ZXSC310 is compatible with single NiMH, NiCd or Alkaline cells, as well as multi-cell or LiIon batteries. The device features a shutdown control, resulting in a standby current less than 5A, and an output capable of driving serial or parallel LED's. The circuit generates constant power output, which are ideal for driving single or multiple LED's over a wide range of operating voltages. These features make the device ideal for driving LED's particularly in LCD backlight applications for Digital Still cameras and PDA's. The ZXSC310 is a PFM DC-DC controller IC that drives an external Zetex switching transistor with a very low saturation resistance. These transistors are the best switching devices available for this type of conversion enabling high efficiency conversion with low input voltages. The drive output of the ZXSC310 LED driver generates a dynamic drive signal for the switching transistor. The circuit can start up under full load and operates down to an input voltage of 0.8 volts. The solution configuration ensures optimum efficiency over a wider range of load currents; several circuit configurations are possible depending on battery life versus brightness considerations. The ZXSC310 is offered in the SOT23-5 package which, when combined with a SOT23 switching transistor, generates a high efficiency small size circuit solution. The IC and discrete combination offers the ultimate cost Vs performance solution for LED backlight applications.
FEATURES
* 94% efficiency * Minimum operating input voltage 0.8V * Maximum operating input voltage 8V * Standby current less than 5A * Programmable output current * Series or parallel LED configuration * Low saturation voltage switching transistor * SOT23-5 package
TYPICAL APPLICATIONS CIRCUIT
APPLICATIONS
* LCD backlights: Digital still camera PDA Mobile phone * LED flashlights and torches * White LED driving * Multiple LED driving
V CC VDRIVE STDN I SENSE
Gnd
VIN = 3.3V / 5V
ORDERING INFORMATION
DEVICE REEL SIZE ZXSC310E5 180mm TAPE WIDTH 8mm QUANTITY PER REEL 3000
DEVICE MARKINGS
* C310
PROVISIONAL ISSUE A - SEPTEMBER 2001 1
ZXSC310
ABSOLUTE MAXIMUM RATINGS:
Supply Voltage Maximum Voltage other pins Power Dissipation Operating Temperature Storage Temperature ELECTRICAL CHARACTERISTICS: -0.3 to 10V -0.3 to VCC+0.3V 450mW -40 to 85 C -55 to 125C
Test conditions unless otherwise stated: VCC=1.5V, TAMB=25 C
Symbol Parameter Efficiency 1 V CC V CC(min) Recommended supply voltage range Minimum startup and operating voltage Quiescent current 2 Base drive current Supply current 3 I DRIVE =-600A , V DRIVE =0.7V I DRIVE =-600A, V DRIVE =0.7V, T AMB =-10C3 V EN = V CC (enabled) V EN = 0V (standby) I VDRIVE I CC V VDRIVE(high) V VDRIVE(low) V EN(high) V EN(low) I EN V ISENSE
(threshold)
Conditions
Min
Typ
Max 94
Units % V V
0.8 0.8 0.9 0.2 4 1.5 2
8 0.92
IQ
mA 5 3.5 4 A mA mA
V DRIVE = 0.7V, V ISENSE = 0V V DRIVE = 0.7V, V ISENSE = 0V V ISENSE = 0V, I VDRIVE =-0.5mA V ISENSE = 50mV, I VDRIVE = 5mA
High level drive voltage Low level drive voltage Device enabled Device in standby mode Enable input current Output current reference voltage I SENSE voltage temp co. 2 I SENSE input current
V CC -0.3 0 0.7
V CC 0.2
V V V
0.15 -1 14 19 0.4 V ISENSE = 0V 0 -30 -65 1 24
V A mV %/C A
T CVISENSE I ISENSE
ELECTRICAL CHARACTERISTICS: AC PARAMETERS2 Test conditions unless otherwise stated: VCC=1.5V, TAMB=0 to 70 C
Symbol T DRV F OSC Parameter Discharge Pulse Width Recommended operating frequency 4 Conditions Min 1.2 Typ 1.7 Max 3.2 200 Units s kHz
1 Application dependent, see reference designs 2 These parameters guaranteed by Design 3 Total supply current =IQ + IVDRIVE, see typical characteristics 4 Operating frequency is application circuit dependent. See applications section.
PROVISIONAL ISSUE A - SEPTEMBER 2001 2
ZXSC310
FMMT618
For the circuits described in the applications section Zetex FMMT618 is the recommended pass transistor. The following indicates outline data for the device, more detailed information can be found in the Zetex surface mount products data book or on Zetex Web page: www.zetex.com
ELECTRICAL CHARACTERISTICS: Test conditions unless otherwise stated: TAMB=25 C
Symbol V CE(sat) Parameter Collector-Emitter Saturation Voltage 5 Conditions I C =0.1A, I B =10mA I C =1A, I B =10mA I C =2.5A, I B =50mA V (BR)CEO
5
Min
Typ 8 70 130
Max 15 150 200
Units mV
Collector-Emitter Breakdown Voltage 5
I C =10mA
20
27
V
Measured under pulse conditions. Pulse width=300s. Duty cycle
2%
ZHCS1000
For the maximum brightness circuit described in the a p p l i c a ti ons s ec t i on Zet e x ZH C S 1000 i s t h e recommended Schottky diode. The following indicates outline data for the ZHCS, more detailed information can be found on Zetex Web page: www.zetex.com
ELECTRICAL CHARACTERISTICS: Test conditions unless otherwise stated: TAMB=25 C
Symbol VF t rr Parameter Forward voltage Reverse Recovery Time Conditions I F = 500mA I F = 1A Switched from I F =500mA to I R =500mA. Measured at I R =50mA IR Reverse Current V R = 30V 50 100 A 12 Min Typ Max 400 500 ns Units mV
Part Number
VR Max. V
IF Max. mA 200 2000 1000 750 500 400
I FSM Max. A 0.6 20 12 12 6.75 6.75
V F at Max. mV 500 500 500 540 550 500 IF mA 30 2000 1000 750 500 400
I R at Max. A 250 1000 100 100 40 40 VR V 25 30 30 30 30 30
Capacitance at V R = 25V, f = 1MHz Typ. pF 10 60 25 25 20 20
Package SOT23 SOT23-6 SOT23 SOT23 SOT23 SOT23 SOT323
BAT54 ZHCS2000 ZHCS1000 ZHCS750 ZHCS500 ZHCS400
30 40 40 40 40 40
PROVISIONAL ISSUE A - SEPTEMBER 2001 3
ZXSC310
TYPICAL CHARACTERISTICS
PROVISIONAL ISSUE A - SEPTEMBER 2001 4
ZXSC310
DEVICE DESCRIPTION
The ZXSC310 is PFM, controller IC which, when combined with a high performance external transistor, enables the production of a high efficiency boost converter for LED driving applications. A block diagram is shown for the ZXSC310 below. The on chip comparator forces the driver circuit and therefore the external switching transistor off if the voltage at ISENSE exceeds 19mV. An internal reference circuit and divider set this threshold. The voltage at ISENSE is taken from a current sense resistor connected in series with the emitter of the switching transistor. A monostable following the output of the comparator forces the turn-off time of the output stage to be typically 1.7us. This ensures that there is sufficient time to discharge the inductor coil before the next on period. With every on pulse the switching transistor is kept on until the voltage across the current-sense resistor exceeds the threshold of the ISENSE input. The on-pulse length, and therefore the switching frequency, is determined by the programmed peak current, the input voltage and the input to output voltage differential. See applications section for details. The driver circuit supplies the external switching transistor with a fixed drive current. To maximise efficiency the external transistor switched quickly, typically being forced off within 30ns.
VCC
I 2
STDN
Drive
R1 VRE I R2
VDRIVE
ISENSE GND
ZXSC310 Block Diagram
PROVISIONAL ISSUE A - SEPTEMBER 2001 5
ZXSC310
REFERENCE DESIGNS
Three typical LED driving applications are shown. Firstly a typical LCD backlight circuit, then maximum brightness LED driving solution and lastly an optimised battery life LED driving solution.
LCD backlight circuit
VIN = 3.3V / 5V
V CC VDRIVE STDN I SENSE
Gnd
(Notes)
This application shows the ZXSC310 in a typical LCD backlight application for Digital Still Cameras and PDA's. The input voltage for these backlight circuits are usually fixed from the main system power, typically 3.3V or 5V. The LED's are connected serially so that the light is distributed uniformly in each LED. The current provided to the LED's can either be pulsed or DC. The DC current is programmable via a sense resistor, RSENSE, and is set to an optimum LED current of 20mA for the reference designs. DC current is achieved by adding a Schottky rectifying diode and an output capacitor, as shown in the reference design below.
Materials list
Ref U1 Q1 D1 C1 R1
6
Value N/A N/A N/A 2.2 F 150m 250m 68H
Part Number ZXSC310E5 FMMT618 ZHCS1000 Generic Generic Generic
Manufacture Zetex Plc Zetex Plc Zetex Plc Various Various Various
Comments Single cell converter, SOT23-5 Low V CE(sat ) NPN, SOT23 1A Schottky diode, SOT23 0805 Size 1206 Size 1206 Size Surface mount inductor
R1 7 L1 8
6 7
Used for 3.3V input, ILED set to 20mA 10%. Used for 5V input, ILED to 20mA 10%. 8 See Application section. PROVISIONAL ISSUE A - SEPTEMBER 2001 6
ZXSC310
Maximum brightness solution (Notes)
VBATT
L1 U1
VCC V DRIVE STDN I SENSE G ND
D1 Q1
ZHCS1000
FMMT617
D2
C1 R1
ZXSC310
This circuit provides a continuous current output to the LED by rectifying and buffering the DC-DC output. This ensures maximum LED brightness.
Materials list
Ref U1 Q1 D1 R1 C1 L1
8
Value N/A N/A 1A 100m 2.2 F 100H 8
Part Number ZXSC310E5 FMMT617 ZHCS1000 Generic
Generic
Manufacture Zetex Plc Zetex Plc Zetex Plc Various
Various
Comments Single cell converter, SOT23-5 Low
VCE(sat)
NPN, SOT23
1A Shottky diode, SOT23 0805 Size Low ESR ceramic capacitor Surface mount inductor
See Application section.
PROVISIONAL ISSUE A - SEPTEMBER 2001 7
ZXSC310
(Notes) Maximum battery life solution
To ensure optimum efficiency, and therefore maximum battery life, the LED is supplied with a pulsed current. Maximum efficiency is ensured with the removal of rectifier losses experienced in the maximum brightness solution.
Materials list
Ref U1 Q1 R1 L1
8
Value N/A N/A 330m 100 H 8
Part Number ZXSC310E5 FMMT617 Generic
Manufacture Zetex Plc Zetex Plc Various
Comments Single cell converter, SOT23-5 Low V CE(SAT) NPN, SOT23 0805 Size Surface mount inductor
See Application section. PROVISIONAL ISSUE A - SEPTEMBER 2001 8
ZXSC310
APPLICATIONS INFORMATION
The following section is a design guide for optimum converter performance.
Schottky diode selection
For the maximum battery life solution a Schottky rectifier diode is required. As with the switching transistor the Schottky rectifier diode has a major impact on the converter efficiency. A Schottky diode with a low forward voltage and fast recovery time should be used for this application. The diode should be selected so that the maximum forward current is greater or equal to the maximum peak current in the inductor, and the maximum reverse voltage is greater or equal to the output voltage. The Zetex ZHCS1000 meets these needs. Datasheets for the ZHCS Series are available on Zetex web site or through your local Zetex sales office. Outline information is included in the characteristics section of this data sheet. For the maximum brightness solution a pulsed current is supplied to the LED and thus a Schottky rectifier diode is not required.
Switching transistor selection
The choice of switching transistor has a major impact on the converter efficiency. For optimum performance, a bipolar transistor with low VCE(SAT) and high gain is required. The Zetex FMMT618 is an ideal choice of transistor, having a low saturation voltage. A data sheet for the FMMT618 is available on Zetex web site or through your local Zetex sales office. Outline information is included in the characteristics section of this data sheet.
C / = 50
100 C
Inductor selection
The inductor value must be chosen to satisfy performance, cost and size requirements of the overall solution. For the LCD backlight reference design we recommend an inductor value of 68uH with a core saturation current rating greater than the converter peak current value and low series resistance. Inductor selection has a significant impact on the converter performance. For applications where efficiency is critical, an inductor with a series resistance of 500m or less should be used. A list of recommended inductors is shown in the table below:
50 C 10 C
10A
Part No. DO1608C-683 CR54-680 P1174.683 SFOP5845-R61680 SIS43-680
Manufacture Coilcraft Sumida Pulse Samwha Delta
L 68 H 68 H 68 H 68 H 68 H
I PK 0.4A 0.61A 0.4A 0.61A 0.4
R DC 0.86 0.46 0.37 0.46 1.125
PROVISIONAL ISSUE A - SEPTEMBER 2001 9
ZXSC310
Peak current definition
The peak current rating is a design parameter whose value is dependent upon the overall application. For the high brightness reference designs, a peak current of was chosen to ensure that the converter could provide the required output power to the LED. In general, the IPK value must be chosen to ensure that the switching transistor, Q1, is in full saturation with maximum output power conditions, assuming worse-case input voltage and transistor gain under all operating temperature extremes. Once I PK is decided the value of R SENSE can be determined by: VISENSE IPK
RSENSE =
A selection guide of sense resistor and inductor values for given input voltages, output currents and number of LED connected in series is provided in the table below.
Input Voltage (V) 3.3V 3.3V 3.3V 3.3V 3.3V 3.3V 3.3V 3.3V 3.3V 5V 5V 5V 5V 5V 5V 5V 5V 5V
LED current (mA) 10 10 10 20 20 20 30 30 30 10 10 10 20 20 20 30 30 30
No. of LED's 3 4 6 3 4 6 3 4 6 3 4 6 3 4 6 3 4 6
R SENSE (m ) 510 330 150 220 150 77 170 100 47 750 510 330 440 250 150 330 170 100
Inductor ( H) 68 68 68 68 68 68 68 68 68 68 68 68 68 68 68 68 68 68
Efficiency (%) 80 81 79 84 93 79 84 84 77 83 84 79 85 85 82 86 85 83
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ZXSC310
Output Power Calculation
By making the above assumptions for inductance and peak current the output power can be determined by:
Capacitor selection
For pulsed operation, as in the maximum battery life solution, no capacitors are required at the output to the LED. For rectified operation, as in the maximum brightness solution, a small value ceramic capacitor is required, typically 2.2uF. Generally an input capacitor is not required, but a small ceramic capacitor may be added to aid EMC, typically 470nF to 1uF. (notes)
POUT = (VOUT - VIN) x IAV x
TOFF TON + TOFF
Note:VOUT=output voltage + Schottky rectifier voltage drop. Where TOFF 1.7us (internally set by ZXSC310) and TON = TOFF (VOUT - VIN) VIN
and
IAV =
IPK + IMIN 2
Where
(VOUT - VIN) x TOFF
IMIN = IPK -
L
Operating frequency can be derived by:
1 F= ( TON + TOFF )
PROVISIONAL ISSUE A - SEPTEMBER 2001 11
ZXSC310
Shutdown Control
The ZXSC310 offers a shutdown mode that produces a standby current of less than 5uA when in operation. When the voltage at the STDN pin is 0.7V or higher the ZXSC310 is enabled, hence the driver is in normal operation. When the voltage at the STDN pin is 0.1V or lower the ZXSC310 is disabled, hence the driver is in shutdown mode. If the STDN pin is open circuit the ZXSC310 is also enabled.
Demonstration board
A demonstration board for the LCD backlighting solution, is available upon request. These can be obtained through your local Zetex office or through Zetex web pages. For all reference designs Gerber files and bill of materials can be supplied.
Layout of LCD backlighting solution
Top Silk
Drill File
Top Copper
Bottom Copper
PROVISIONAL ISSUE A - SEPTEMBER 2001 12
ZXSC310
PINOUT DIAGRAM
VCC
1
5 VDRIVE 4 ISENSE
Gnd 2 STDN 3 Top View
PIN DESCRIPTIONS
Pin No. 1 2 3 4 5 Name V CC G nd S TDN I SENSE V DRIVE Description Supply voltage, generally Alkaline, NiMH or NiCd single cell Ground Shutdown Inductor current sense input. Internal threshold voltage set to 19mV. Connect external sense resistor Drive output for external switching transistor. Connect to base of external switching transistor.
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ZXSC310
PACKAGE DIMENSIONS
SOT23-5
DIM MIN A A1 A2 b C D E E1 e 0.90 0.00
0.90 Millimetres
MAX 1.45 0.15 1.3 0.50 0.20
3.00
0.35 0.09 2.80 2.60 1.50 0.95 REF
3.00 1.75
(c) Zetex plc 2001
Zetex plc Fields New Road Chadderton Oldham, OL9 8NP United Kingdom Telephone (44) 161 622 4422 Fax: (44) 161 622 4420 Zetex GmbH Streitfeldstrae 19 D-81673 Munchen Germany Telefon: (49) 89 45 49 49 0 Fax: (49) 89 45 49 49 49 Zetex Inc Suite 315 700 Veterans Memorial Highway Hauppauge NY11788 USA Telephone: (631) 360 2222 Fax: (631) 360 8222 Zetex (Asia) Ltd 3701-04 Metroplaza, Tower 1 Hing Fong Road Kwai Fong, Hong Kong China Telephone: (852) 26100 611 Fax: (852) 24250 494
These offices are supported by agents and distributors in major countries world-wide. This publication is issued to provide outline information only which (unless agreed by the Company in writing) may not be used, applied or reproduced for any purpose or form part of any order or contract or be regarded as a representation relating to the products or services concerned. The Company reserves the right to alter without notice the specification, design, price or conditions of supply of any product or service. For the latest product information, log on to www.zetex.com
PROVISIONAL ISSUE A - SEPTEMBER 2001 14

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