ZXSC400 LED DRIVER BOOST CONVERTER DESCRIPTION The ZXSC400 is a voltage mode boost converter in the SOT23-6 package. Its low feedback voltage allows the current in a chain of LEDs to be set and accurately monitored with a single resistor giving minimal losses. Its excellent load and line regulation means that for the full supply range from lithium-ion cells, the LED current will typically change by less than 1%. Using high efficiency Zetex switching transistors with ratings of 20V and higher allow many LEDs to be chained in series for the best LED current matching possible. FEATURES SOT23-6 • 1.8V to 8V supply range • Typical output regulation of ⫾1% • Over 80% typical efficiency • 4.5A typical shutdown current • Series connection for ultimate LED current matching APPLICATIONS • White LED backlighting for colour LCD panels • General LED backlighting • High performance white LED flashlights PINOUT • General LED driving from batteries ORDERING INFORMATION DEVICE REEL SIZE TAPE WIDTH QUANTITY PER REEL 7“ 8mm 3000 units ZXSC400E6TA VCC GND DRIVE VFB STDN SENSE Top View DEVICE MARKING • C400 ISSUE 1 - JANUARY 2003 1 ZXSC400 ABSOLUTE MAXIMUM RATINGS VCC DRIVE EOR STDN SENSE Operating Temp. Storage Temp. Power Dissipation -0.3V to -0.3V to -0.3V to -0.3V to -0.3V to -40°C to -55°C to 450mW +10V VCC + 0.3V VCC + 0.3V The lower of (+5.0V) or (VCC + 0.3V) The lower of (+5.0V) or (VCC + 0.3V) +85°C +125°C ELECTRICAL CHARACTERISTICS Test Conditions VCC= 3V, T= -40°C to 85°C unless otherwise stated. Symbol Parameter Conditions Limits Min Typ Units Max Supply parameters V IN V CC Range Iq 1 Quiescent Current I STDN Shutdown Current Eff 1 Efficiency 30mA > I LED > 10mA Acc REF Reference tolerance 1.8V < V CC < 8V TCO REF Reference Temp Co T DRIVE Discharge pulse width F OSC Operating Frequency 1.8 8 V CC = 8V 220 A 4.5 80 -3.0 % 3.0 0.005 1.8V < V CC < 8V V A % %/⬚C s 1.7 200 kHz Input parameters V SENSE Sense voltage I SENSE Sense input current V FB Feedback voltage 2 V FB = 0V, V SENSE = 0V 22 28 34 mV -1 -7 -15 A 291 300 309 mV -1.2 -4.5 A V IH Shutdown high voltage 1.5 VCC V V IL Shutdown low voltage 0 0.55 V dV LN Line voltage regulation I FB Feedback input current V FB = 0V, V SENSE = 0V 0.5 %/V Output parameters I OUT 3 Output current Vin > 2V, Vout = 10.5V I DRIVE Transistor drive current Vdrive = 0.7V 2 V DRIVE Transistor voltage drive 1.8V < V CC < 8V 0 C DRIVE Mosfet gate drive cpbty dILD 4 Load current regulation 50 mA 3.5 5 V C C -0.4 300 mA V pF 0.1 mA/V Note 1 2 3 4 Excluding gate/base drive current. IFB is typically half these values at 300mV. System not device spec, including recommended transistors. 10.5V represents 3 white LEDs. Change in LED current with changing supply voltage (LED VF x number of series LEDs). ISSUE 1 - JANUARY 2003 2 ZXSC400 TYPICAL CHARACTERISTICS ISSUE 1 - JANUARY 2003 3 ZXSC400 DESIGN INFORMATION IC operation description Block diagram Bandgap Reference All threshold voltages and internal currents are derived from a temperature compensated bandgap reference circuit with a reference voltage of 1.22V nominal. Dynamic Drive Output Depending on the input signal, the output is either “LOW” or “HIGH”. In the high state a 2.5mA current source (max drive voltage = VCC-0.4V) drives the base or gate of the external transistor. In order to operate the external switching transistor at optimum efficiency, both output states are initiated with a short transient current in order to quickly discharge the base or the gate of the switching transistor. 2µs Switching Circuit The switching circuit consists of two comparators, Comp1 and Comp2, a gate U1, a monostable and the drive output. Normally the DRIVE output is “HIGH”; the external switching transistor is turned on. Current ramps up in the inductor, the switching transistor and external current sensing resistor. This voltage is sensed by comparator, Comp2, at input ISENSE. Once the current sense voltage across the sensing resistor exceeds 30mV, comparator Comp2 through gate U1 triggers a re-triggerable monostable and turns off the output drive stage for 2s. The inductor discharges to the load of the application. After 2s a new charge cycle begins, thus ramping the output voltage. When the output voltage reaches the nominal value and FB gets an input voltage of more than 300mV, the monostable is forced “on” from Comp1 through gate U1, until the feedback voltage falls below 300mV. The above action continues to maintain regulation. Pin functions Pin No. Name Description 1 V CC Supply voltage, 1.8V to 8V. 2 GND Ground 3 STDN Shutdown 4 SENSE Inductor current sense input. Internal threshold voltage set to 30mV. Connect external sense resistor 5 V FB Reference voltage. Internal threshold set to 300mV. Connect external resistor to program LED current 6 DRIVE Drive output for external switching transistor. Connect to base or gate of external switching transistor. ISSUE 1 - JANUARY 2003 4 ZXSC400 APPLICATIONS INFORMATION Switching transistor selection Capacitor 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 VCEO of the switching transistor is also an important parameter as this sees the full output voltage when the transistor is switched off. Zetex SuperSOT™ transistors are an ideal choice for this application. A small value, low ESR ceramic capacitor is required to filter the output, typically 1F to 4.7F. Schottky diode selection The inductor value must be chosen to satisfy performance, cost and size requirements of the overall solution. Generally an input capacitor is not required, but a small ceramic capacitor may be added to aid EMC, typically 1F to 4.7F. Inductor Selection 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. Inductor selection has a significant impact on the converter performance. For applications where efficiency is critical, an inductor with a series resistance of 250m⍀ or less should be used. The diode should be selected so that the maximum forward current rating 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 ZHCS Series meet these needs. A list of recommended inductors is listed in the table below: Combination devices To minimise the external component count Zetex recommends the ZX3CDBS1M832 combination of NPN transistor and Schottky diode in a 3mm x 2mm MLP package. This device is recommended for use in applications using 1 to 4 white LEDs. The IC is also capable of driving MOSFETs. Zetex recommends the ZXMNS3BM832 combination of low threshold voltage N-Channel MOSFET and Schottky diode in a 3mm x 2mm MLP package. This device is recommended for use in applications using 1 to 8 white LEDs. Part No. Manufacture L I PK (A) R DC ( ) CMD4D11-100MC Sumida 10H 0.5 0.457 CMD4D11-220MC Sumida 22H 0.4 0.676 LPO2506OB-103 Coilcraft 10H 1.0 0.24 ST2006103 Standex Electronics Inc 10H 0.6 0.1 ISSUE 1 - JANUARY 2003 5 ZXSC400 Peak current definition Programming LED current 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 the required output power is determined, the LED current can be programmed by adding a single resistor in the LED chain. The resistor value is determined by the following: Once I PK is decided the value of R SENSE can be determined by: RSENSE = VSENSE IPK Sense Resistor A low value sense resistor is required to set the peak current. Power in this resistor is negligible due to the low sense voltage threshold, VISENSE. At the bottom of the page there is a table of recommended sense resistors. Output power calculation ILED = By making the above assumptions for inductance and peak current the output power can be determined by: Where VFB = 300mV POUT = IAV x VIN x (Watts) RLED selection table VFB RLED where IAV = IPK (TON + TDIS) × 2 (TON + TOFF) and TON IPK × L = VIN and TDIS = IPK × L (VLED − VIN) RLED 40mA 7.5⍀ 30mA 10⍀ 20mA 15⍀ 15mA 20⍀ 10mA 30⍀ Shutdown Control The ZXSC400 offers a shutdown mode that consumes a standby current of less than 5A. When the voltage at the STDN pin between 1V and 8V (and also open circuit), the ZXSC400 is enabled and the driver is in normal operation. When the voltage at the STDN pin is 0.7V or lower, the ZXSC400 is disabled and the driver is in shutdown mode. The SHDN input is a high impedance current source of 1A typ. The driving device can be an open collector or an open drain or a logic output with a “High“ voltage of 5V max. The device shutdown current depends of the supply voltage, see typical characteristics graph and TOFF ≅ 1.7s (internally set by ZXSC400) and = efficiency e.g. 100% = 1 Operating frequency can be derived by: F = ILED 1 TON + TOFF Manufacture Series RDC( ) Range Size Tolerance URL Cyntec RL1220 0.022 - 10 0805 ±5% www.cyntec.com IRC LR1206 0.010 - 1.0 1206 ±5% www.irctt.com ISSUE 1 - JANUARY 2003 6 ZXSC400 Open-circuit protection Dimming Control using the shutdown pin For applications where the LED chain might go open-circuit a Zener diode can be connected across the LED chain preventing over-voltage and possible damage to the main switching transistor. The Zener diodes should be selected by ensuring its voltage rating is higher than the combined forward voltage of the LED chain. Under open circuit conditions the curren in the Zener diode defines the output current as: The first method uses the shutdown pin. By injecting a PWM waveform on this pin and varying the duty cycle, LED current and hence LED brightness can be adjusted. IZ = To implement this method of brightness control on the ZXSC400, apply PWM signal with amplitude of between 0.7V and Vcc at a frequency of 120Hz or above (to eliminate LED flicker). The LED current and hence LED brightness is linearly proportional to the duty cycle ratio, so for brightness control adjust duty cycle ratio as necessary. For example, a 10% duty cycle equates to 10% of full LED brightness. VFB RZ The circuit example below give an open circuit output current of 300A. Dimming Control There are four types of dimming control that can be implemented for the ZXSC400. ISSUE 1 - JANUARY 2003 7 ZXSC400 Dimming Control using a DC voltage Dimming Control using a logic signal For applications where the shutdown pin is not available a DC voltage can be used to control dimming. By adding resistors R2 and R3 and applying a DC voltage, the LED current can be adjusted from 100% to 0%. As the DC voltage increases, the voltage drop across R2 increases and the voltage drop across R1 decreases, thus reducing the current through the LEDs. Selection of R2 and R3 should ensure that the current from the DC voltage is much less than the LED current and much larger than the feedback current. The component values in the diagram below represent 0% to 100% dimming control from a 0 to 2V DC voltage. For applications where the LED current needs to be adjusted in discrete steps a logic signal can be applied as shown in the diagram below. When Q1 os ‘off’, R1 sets the minimum LED current. When Q1 is ‘on’, R2 sets the LED current that will be added to the minimum LED current. The formula for selecting values for R1 and R2 are given below: ZXSC400 VFB R2 ZXSC400 VFB VDC R3 R2 67k 10k LOGIC SIGNAL Q1 R1 R1 MOSFET ‘off’ VFB ILED(MIN) = RLEB MOSFET ‘on’ VFLB ILED(MAX) = + ILED(MIN) RLEB Dimming Control using a filtered PWM signal The filtered PWM signal can be considered as an adjustable DC voltage by applying a RC filter. The values shown in the diagram below are configured to give 0% to 100% dimming for a 1kHz to 100kHz PWM signal with a 2V amplitude. e.g. a 50% duty cycle will give 50% dimming. Where VFB = 300mV ZXSC400 VFB PWM R4 R3 R2 10k 67k 10k R1 C1 0.1µF ISSUE 1 - JANUARY 2003 8 ZXSC400 Layout considerations To optimise for best performance each of these loops kept separate from each other and interconnected with short, thick traces thus minimising parasitic inductance, capacitance and resistance. Also the RSENSE resistor should be connected, with minimum trace length, between emitter lead of Q1 and ground, again minimising stray parasitics. Layout is critical for the circuit to function in the most efficient manner in terms of electrical efficiency, thermal considerations and noise. For ‘step-up converters’ there are four main current loops, the input loop, power-switch loop, rectifier loop and output loop. The supply charging the input capacitor forms the input loop. The power-switch loop is defined when Q1 is ‘on’, current flows from the input through the inductor, Q1, RSENSE and to ground. When Q1 is ‘off’, the energy stored in the inductor is transferred to the output capacitor and load via D1, forming the rectifier loop. The output loop is formed by the output capacitor supplying the load when Q1 is switched back off. ISSUE 1 - JANUARY 2003 9 ZXSC400 REFERENCE DESIGNS Li - Ion to 2 LED converter 30 100 Efficiency (%) 90 Load Current (mA) 2 White LEDs ILED=20mA 80 70 60 2.5 3.0 3.5 2 White LEDs 25 20 15 10 5 0 4.0 2.5 Input voltage (V) 3.0 3.5 4.0 Input Voltage (V) Input Voltage vs Efficiency Input Voltage vs LED Current Bill of Materials Ref Value U1 U2 Part Number Manufacture Comments ZXSC400 Zetex LED driver IC ZX3CDBS1M832 Zetex NPN + Schottky dual L1 10H CMD4D11-100MC Sumida 1mm height profile R1 100m⍀ LR1206 / RL1220 IRC / Cyntec 1206 / 0805 size R2 15⍀ Generic Generic 0603 size C1 2.2F/6V3 GRM Series Murata 0805 size C2 2.2F/16V GRM Series Murata 1206 size NSCW215 Nichia White SMT LED LED ISSUE 1 - JANUARY 2003 10 ZXSC400 Li - Ion to 3 LED converter 90 30 3 White LEDs 3 White LEDs ILED=20mA LED Current (mA) Efficiency (%) 100 80 70 60 2.5 3.0 3.5 20 10 0 2.5 4.0 3.0 3.5 4.0 Input Voltage (V) Input Voltage (V) Input Voltage v Efficiency Input Voltage v LED Current Bill of Materials Ref Value U1 U2 Part Number Manufacture Comments ZXSC400 Zetex LED driver IC ZX3CDBS1M832 Zetex NPN + Schottky dual L1 10H CMD4D11-100MC Sumida 1mm height profile R1 68m⍀ LR1206 / RL1220 IRC / Cyntec 1206 / 0805 size R2 15⍀ Generic Generic 0603 size C1 2.2F/6V3 GRM Series Murata 0805 size C2 2.2F/16V GRM Series Murata 1206 size NSCW215 Nichia White SMT LED LED ISSUE 1 - JANUARY 2003 11 ZXSC400 Li - Ion to 4 LED converter 90 30 80 L=22µH RSENSE=80mΩ Load Current (mA) Efficiency (%) 4 White LEDs ILED=20mA L=10µH RSENSE=47mΩ 70 60 2.5 3.0 3.5 20 15 10 5 0 4.0 4 White LEDs 25 2.5 Input voltage (V) 3.0 3.5 4.0 Input Voltage (V) Input Voltage vs Efficiency Input Voltage vs LED Current Bill of Materials Ref Value U1 U2 Part Number Manufacture Comments ZXSC400 Zetex LED driver IC ZX3CDBS1M832 Zetex NPN + Schottky dual L1 10H CMD4D11-100MC Sumida 1mm height profile R1 47m⍀ LR1206 / RL1220 IRC / Cyntec 1206 / 0805 size R2 15⍀ Generic Generic 0603 size C1 2.2F/6V3 GRM Series Murata 0805 size C2 2.2F/16V GRM Series Murata 1206 size NSCW215 Nichia White SMT LED LED ISSUE 1 - JANUARY 2003 12 ZXSC400 Li - Ion to 6 LED converter 90 30 Load Current (mA) Efficiency (%) 6 White LEDs ILED=20mA 80 70 60 2.5 3.0 3.5 20 15 10 5 0 4.0 6 White LEDs 25 2.5 Input voltage (V) 3.0 3.5 4.0 Input Voltage (V) Input Voltage vs Efficiency Input Voltage vs LED Current Bill of Materials Ref Part Number Manufacture Comments U1 Value ZXSC400 Zetex LED driver IC Q1 FMMT619 Zetex 40V NPN D1 ZHCS1000 Zetex 1A Schottky diode L1 22H CMD4D11-220MC Sumida 1mm height profile R1 47m⍀ LR1206 / RL1220 IRC / Cyntec 1206 / 0805 size R2 15⍀ Generic Generic 0603 size C1 2.2F/6V3 GRM Series Murata 0805 size C2 2.2F/25V GRM Series Murata 1206 size NSCW215 Nichia White SMT LED LED ISSUE 1 - JANUARY 2003 13 ZXSC400 Li - Ion to 8 LED converter 90 30 Load Current (mA) Efficiency (%) 8 White LEDs ILED=20mA 80 70 60 2.5 3.0 3.5 8 White LEDs 25 20 15 10 5 0 2.5 4.0 Input voltage (V) 3.0 3.5 4.0 Input Voltage (V) Input Voltage vs Efficiency Input Voltage vs LED Current Bill of Materials Ref Part Number Manufacture Comments U1 ZXSC400 Zetex LED driver IC Q1 FMMT619 Zetex 40V NPN D1 ZHCS1000 Zetex 1A Schottky diode 22H CDRH4D28-220 D01608C-223 Sumida Coilcraft 0.7A, 0.235⍀ 0.7A, 0.37⍀ L1 or Value R1 33m⍀ LR1206 / RL1220 IRC / Cyntec 1206 / 0805 size R2 15⍀ Generic Generic 0603 size C1 2.2F/6V3 GRM Series Murata 0805 size C2 2.2F/35V GRM Series Murata 1206 size NSCW215 Nichia White SMT LED LED ISSUE 1 - JANUARY 2003 14 ZXSC400 Notes ISSUE 1 - JANUARY 2003 15 ZXSC400 PACKAGE OUTLINE PAD LAYOUT DETAILS e b L 2 E1 E DATUM A a e1 D C A A2 A1 CONTROLLING DIMENSIONS IN MILLIMETRES APPROX CONVERSIONS INCHES. PACKAGE DIMENSIONS Millimetres Inches DIM Millimetres Inches DIM Min Max Min Max Min Max Min Max A 0.90 1.45 0.35 0.057 E 2.60 3.00 0.102 0.118 A1 0.00 0.15 0 0.006 E1 1.50 1.75 0.059 0.069 A2 0.90 1.30 0.035 0.051 L 0.10 0.60 0.004 0.002 b 0.35 0.50 0.014 0.019 e 0.95 REF 0.037 REF C 0.09 0.20 0.0035 0.008 e1 1.90 REF 0.074 REF D 2.80 3.00 0.110 0.118 L 0° 10° 0° 10° © Zetex plc 2003 Americas Asia Pacific Zetex GmbH Streitfeldstraße 19 D-81673 München Zetex Inc 700 Veterans Memorial Hwy Hauppauge, NY11788 Germany Telefon: (49) 89 45 49 49 0 Fax: (49) 89 45 49 49 49 [email protected] USA Telephone: (631) 360 2222 Fax: (631) 360 8222 [email protected] Zetex (Asia) Ltd 3701-04 Metroplaza, Tower 1 Hing Fong Road Kwai Fong Hong Kong Telephone: (852) 26100 611 Fax: (852) 24250 494 [email protected] Europe Zetex plc Fields New Road Chadderton Oldham, OL9 8NP United Kingdom Telephone (44) 161 622 4422 Fax: (44) 161 622 4420 [email protected] 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 ISSUE 1 - JANUARY 2003 16