ZXSC100 SINGLE CELL DC-DC CONVERTER SOLUTION DESCRIPTION The ZXSC100 series is designed for DC-DC applications where step-up voltage conversion from very low input voltages is required. These applications mainly operate from single nickel cadmium or nickel metal hydride battery cells. The IC and discrete combination offers the ultimate cost vs performance solution for single cell DC-DC conversion. The circuit can start up under full load with regulation maintained down to an input voltage of only 0.926 volts. The solution configuration ensures optimum efficiency over a wider range of load currents, several circuit configurations are possible with power dissipation up to 2W. The step up output voltage is easily programmed with external resistors, the non-synchronous architecture and SuperSOT4™ device enabling an output voltage down to the input voltage level. For best performance the ZXSC100 quiescent current is a small 150µA ensuring minimum battery drain in no load conditions. FEATURES APPLICATIONS(continued) • Efficiency maintained over a wide range of input • Hand held instruments • Portable medical equipment • Solar powered equipment voltages and load currents 82% efficiency @ VBATT=1V • • • • Startup under full load Minimum operating input voltage VBATT=0.926V TYPICAL APPLICATION CIRCUIT Adjustable output voltage down to VBATT Quiescent current typically 150µA referred to input voltage • MSOP8 package L1 VBATT D1 • SO8 package R1 Q1 Cordless telephones MP3 players PDA Pagers Battery backup supplies Electronic toothbrush GPS receivers Digital camera Palmtop computers R3 FMMT617 U1 APPLICATIONS • • • • • • • • • 3.3V/0.1A ZHCS1000 EM VDRIVE BAS ISENSE C3 C2 RE FB VCC GND ZXSC100 R2 C1 R4 ORDERING INFORMATION DEVICE Package Partmarking Reel size Tape width Quantity per reel ZXSC100X8 MSOP8 ZXSC100 7” 12mm 1,000 ZXSC100N8 SO8 ZXSC100 7” 12mm 500 ISSUE 3 - JANUARY 2004 1 SEMICONDUCTORS ZXSC100 ABSOLUTE MAXIMUM RATING Supply voltage Maximum voltage other pins 0.3 to 3.5V 0.3 to VCC+0.3V Operating temperature Storage temperature Junction temperature Power dissipation (25°C) MSOP8 SO8 500mW 780mW 0 to 70°C -55 to 150°C 150°C ELECTRICAL CHARACTERISTICS (Unless otherwise stated) VCC=1.2V, TA = 25°C Symbol Parameter Conditions I CC Quiescent current Not switching I DRIVE Base drive current V RE = V CC 5 V DRIVE V DRIVE o/p voltage V RE = V CC , I DRIVE = 5mA V CC - 0.17 V FB Feedback voltage 708 730 752 mV V ISENSE Output current reference voltage 12 17.5 24 mV T CVISENSE I SENSE voltage temp co. V DREF Drive current reference voltage T CVDREF V DREF temp co. V CC(SRT) Startup voltage V CC(min) Minimum operating input voltage V CC(hys) Supply start up to shutdown hysteresis 80 I FB Feedback input current 100 200 nA I ISENSE I SENSE input current 4 5.5 µA V O(min) Minimum output voltage V O(max) Maximum output voltage T OFF Discharge pulse width (1) Min. Typ. Max. Units 150 200 µA 10 mA V 0.4 Measured with respect to VCC 20 30 %/°C 40 1 Any output load V ISENSE = 0V mV %/°C 1.01 1.06 1.1 V 0.926 0.98 1 V 3 mV V CC V FMMT617as pass element (1) 1.7 3 20 V 4 µs Depends on breakdown voltage of pass device. See FMMT617 datasheet ISSUE 3 - JANUARY 2004 SEMICONDUCTORS 2 ZXSC100 OPERATING CONDITIONS Symbol F OSC 2 3 Parameter Conditions Recommended operating frequency Min Typ 3 Max Units 200 kHz These parameters guaranteed by design and characterization Operating frequency is application circuit dependant. See applications section FMMT617 For the circuits described in the applications section, Zetex FMMT617 is the recommended pass transistor. The following indicates outline data for the transistor, more detailed information can be found at www.zetex.com ELECTRICAL CHARACTERISTICS (at TA = 25°C unless otherwise stated) PARAMETER SYMBOL MIN. Collector-emitter breakdown voltage V (BR)CEO 15 Collector-emitter saturation voltage V CE(sat) TYP. MAX. 18 UNIT CONDITIONS V I C =10mA* 8 14 mV I C =0.1A, I B =10mA* 70 100 mV I C =1A, I B =10mA* 150 200 mV I C =3A, I B =50mA* *Measured under pulsed conditions. Pulse width=300µs. Duty cycle ≤ 2% ZHCS1000 For the circuits described in the applications section Zetex ZHCS1000 is the recommended Schottky diode. The following indicates outline data for the ZHCS, more detailed information is available at www.zetex.com ELECTRICAL CHARACTERISTICS (at Tamb = 25°C unless otherwise stated) PARAMETER SYMBOL Forward voltage MIN. TYP. MAX. UNIT CONDITIONS VF 500 mV I F =1A Reverse current IR 100 µA V R =30V Reverse recovery time t rr ns Switched from IF = 500mA to IR = 500mA. Measured at IR=50mA 12 *Measured under pulsed conditions. Pulse width=300µs. Duty cycle ≤ 2% ISSUE 3 - JANUARY 2004 3 SEMICONDUCTORS ZXSC100 TYPICAL CHARACTERISTICS 2.0 1.5 Output Voltage (%) Quiescent Current (µA) 300 250 200 150 100 1.0 1.5 2.0 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 -10 2.5 0 10 20 30 40 50 60 70 80 Temperature (°C) Input Voltage (V) Quiescent Current v Input Voltage Output Voltage v Temperature 5.0 2.0 Line Regulation (%) Load Regulation (%) 1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 0 50m 2.5 0.0 -2.5 -5.0 1.0 100m Output Current (A) 1.5 2.0 2.5 Input Voltage (V) Line Regulation Load Regulation ISSUE 3 - JANUARY 2004 SEMICONDUCTORS 4 ZXSC100 DEVICE DESCRIPTION The ZXSC100 is non-synchronous PFM, DC-DC controller IC which, when combined with a high performance external transistor, enables the production of a high efficiency boost converter for use in single cell applications. A block diagram is shown for the ZXSC100 in Figure 1. The driver circuit supplies the external switching transistor with a defined current, which is programmed by an external resistor connected between the RE pin and VCC. The internal reference voltage for the circuit is 25mV below VCC. To maximise efficiency the external transistor is switched quickly, typically being forced off within 20ns. VCC In higher power applications more current can be supplied to the switching transistor by using a further external component. The driver transistor in the IC can be bypassed with the addition of a discrete PNP. More information on this circuit configuration can be found in the applications section. Drive Shutdown RE Comp1 I EM R1 BAS VREF R2 VDRIVE Comp2 ISENSE FB GND Figure 1 ZXSC100 Block Diagram A shutdown circuit turns the device on or off at VCC=1V with a hysteresis of typically 80mV. At start up, comparator Comp1 turns the driver circuit and therefore the external switching transistor on. This circuit will remain active until the feedback voltage at the pin FB rises above VREF, which is set to 730mV. An external resistive divider on the FB pin sets the output voltage level. Comparator Comp2 forces the driver circuit and the external switching transistor off, if the voltage at ISENSE exceeds 25mV. 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 Comp2 extends the turn-off time of the output stage by a minimum of 2us. This ensures that there is sufficient time to discharge the inductor coil before the next on period. The AND gate between the monostable and Comp1 output ensures that the switching transistor always remains on until the ISENSE threshold is reached and that the minimum discharge period is always asserted. The pulse width is constant, the pulse frequency varies with the output load. ISSUE 3 - JANUARY 2004 5 SEMICONDUCTORS ZXSC100 PIN DESCRIPTIONS Pin No. Name Description 1 EM Emitter of internal drive transistor. Connect to RE in lower power applications. Must be unconnected in higher power applications 2 BAS Not connected in lower power applications. Connect to base of external drive transistor in higher power applications 3 RE Drive current sense input. Internal threshold voltage set 25mV below V CC . Connected external sense resistor. Connect emitter of external drive transistor in higher power applications 4 V CC Supply voltage, generally NiMH, NiCd single cell 5 I SENSE Inductor current sense input. Internal threshold voltage set to 25mV. Connect external sense resistor 6 FB Feedback sense. Internal threshold set to 730mV. Connect external resistive divider to output voltage 7 G ND Ground VDRIVE GND EM 1 8 BAS 2 7 RE 3 6 FB VCC 4 5 ISENSE REFERENCE DESIGNS Three typical DC-DC step-up converter applications for the ZXSC300 are shown. Firstly with a maximum output power of 0.33W, secondly with a maximum output power of 1.0W and finally driving white LED’s in a flashlight application. Low power solution (330mW) efficiency ISSUE 3 - JANUARY 2004 SEMICONDUCTORS 6 ZXSC100 Low power solution, VOUT=3.3V, PL=0.33W L1 VBATT D1 3.3V/0.1A ZHCS1000 R1 Q1 R3 FMMT617 U1 EM VDRIVE BAS ISENSE RE FB VCC GND ZXSC100 C1 C3 C2 R2 R4 MATERIALS LIST Ref Value Part Number Manufacturer Comments U1 N/A ZXSC100X8 Zetex Plc Single cell converter, MSOP8 Q1 20V, 13mΩ, 7A FMMT617 Zetex Plc Low VCE(sat) NPN, SOT23 D1 0.5V, 2A ZHCS1000 Zetex Plc 1A Shottky diode R1 0Ω* Generic Various 0805 Size R2 33mΩ Generic Various 0805 Size R3 110kΩ Generic Various 0805 Size R4 30kΩ Generic Various 0805 Size C1 220µF TPSD227M010R0100 AVX Low ESR tantalum capacitor C2 220µF TPSD227M010R0100 AVX Low ESR tantalum capacitor C3 1nF Generic Various 0805 Size L1 22µH D01608C-223 D03316P-223 Coilcraft Low profile SMT * Note: Refer to External Transistor base drive selection in the Applications Section. ISSUE 3 - JANUARY 2004 7 SEMICONDUCTORS ZXSC100 Higher power solution, VOUT=3.3V, PL=1W L1 VBATT D1 3.3V/0.33A ZHCS1000 R1 Q2 Q1 R3 C3 FMMT617 U1 EM VDRIVE BAS ISENSE RE FB VCC GND C2 ZXSC100 R2 C1 R4 MATERIALS LIST Ref Value Part Number Manufacturer Comments U1 N/A ZXSC100X8 Zetex Plc Single cell converter, MSOP8 Q1 20V, 13mΩ, 7A FMMT617 Zetex Plc Low VCE(SAT) NPN, SOT23 Q2 N/A 2N2907 Various Small signal transistor D1 0.5V, 2A ZHCS1000 Zetex Plc 1A Shottky diode R1 3.3Ω* Generic Various 0805 Size R2 33mΩ Generic Various 0805 Size R3 110kΩ Generic Various 0805 Size R4 30kΩ Generic Various 0805 Size C1 220µF TPSD227M010R0100 AVX Low ESR tantalum capacitor C2 220µF TPSD227M010R0100 AVX Low ESR tantalum capacitor C3 1nF Generic Various 0805 Size L1 22µH D01608C-223 D03316P-223 Coilcraft Low profile SMT * Note: Refer to External Transistor base drive selection in the Applications Section. ISSUE 3 - JANUARY 2004 SEMICONDUCTORS 8 ZXSC100 OTHER APPLICATIONS The circuit itself is very simple, a minimum number of components are used and they are all small size. The ZXSC uses the very small MSOP8 package, the pass transistor is SOT23. No capacitors are required as the circuit is stable under all conditions. The inductor recommended is a low cost miniature component. L1 VBATT 100µH Q1 No compromise is made on efficiency however. In a standard configuration efficiency well over 80% can be achieved. With careful inductor selection efficiency over 90% is possible. FMMT617 U1 EM VDRIVE BAS ISENSE RE FB VCC GND D1 WHITE LED The inherent flexibility of the ZXSC circuit means that parallel or series LEDs can be driven depending on application needs. A simple modification to the application circuit means that the maximum pulse current can be programmed to match the characteristics of the chosen LED load, pulse current in the range 10mA to 3A and beyond can be easily achieved. R2 0.22R ZXSC100 Driving white LED’s in a flashlight application The ZXSC100 solution is ideal for LED lamp driving applications operating from a single cell. In principal conversion from 1.2V to the 3.6V, typically required by white LEDs, is necessary. Load currents in the region of 20mA to 50mA being required for a single LED element. An application note (AN33) is available describing various circuits for driving white LEDs. This application note includes details of circuits that optimise battery life, maximise brightness and can be constructed for minimal cost. Contact your local Zetex office for further details. To minimise size, weight and cost, single cell operation is an advantage. The ZXSC is well matched to single cell NiCd and NiMH characteristics. The circuit will turn on at 1.06V, to maximise the life the battery can offer, the converter does not turn off until the battery voltage falls to 0.93V. ISSUE 3 - JANUARY 2004 9 SEMICONDUCTORS ZXSC100 APPLICATIONS INFORMATION Schottky diode selection As with the switching transistor the Schottky rectifier diode has a major impact on the DC-DC converter efficiency. A Schottky diode with a low forward voltage and fast recovery time should be used for this application. The majority of losses in the diode are, ‘on-state’ and can be calculated by using the formula below: The following section is a design guide for optimum converter performance. Switching transistor selection The choice of switching transistor has a major impact on the DC-DC converter efficiency. For optimum performance, a bipolar transistor with low VCE(SAT) and high gain is required. The majority of losses in the transistor are, ‘on-state’ and can be calculated by using the formula below: PQ1 = PD1 = ((IAVxVCE(SAT) ) + ( IBx VBE(SAT) ))xTON (TON + TOFF) ) where IAV IAV x VF(MAX) x TDIS (TOn + TOFF ) where IAV = I PK = 2 I PK 2 From the calculations above the impact on converter efficiency can be seen. 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. External drive transistor selection For higher power applications an external transistor is required to provide the additional base drive current to the main switching transistor. For this, any small signal PNP transistor is sufficient. Please see reference designs for recommended part numbers. Inductor selection The Zetex ZHCS1000 meets these needs. A data sheet for the ZHCS1000 is available on the Zetex web site or through your local Zetex sales office. Outline information is included in the characteristics section of this data sheet. ISSUE 3 - JANUARY 2004 SEMICONDUCTORS 10 ZXSC100 The inductor value must be chosen to satisfy performance, cost and size requirements of the overall solution. For the reference designs we recommend an inductor value of 22µH with a core saturation current rating greater than the converter peak current value. Figure 3 shows the discontinuous inductor current and the relationship between output power, TON, TDIS and TOFF. Inductor selection has a significant impact on the converter efficiency. For applications where efficiency is critical, a 5% improvement can be achieved with a high performance inductor. This should be selected with a core saturation current rating much higher than the peak current of the converter, say 3 times greater. The resultant reduction in core losses brings about the efficiency improvement. IPK Peak current definition The peak current rating is a design parameter whose value is dependent upon the overall application. For the reference designs, a peak current of 1.2A was chosen to ensure that the converter could provide the required output power. 0A TDIS TOFF 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. Figure 3 Discontinuous inductor current Once IPK is decided the value of RSENSE can be determined by: RSENSE = Output capacitors Output capacitors are a critical choice in the overall performance of the solution. They are required to filter the output and supply load transient currents. There are three parameters which are paramount in the selection of the output capacitors; their capacitance value, IRIPPLE and ESR. The capacitance value is selected to meet the load transient requirements. The capacitors IRIPPLE rating must meet or exceed the current ripple of the solution. VISENSE IPK Output power definition By making the above assumptions for the inductor and IPK the output power can be determined by: Output Power = (VOUT − VIN) x IPK x TDIS 2 x (TOn + TOFF ) The ESR of the output capacitor can also affect loop stability and transient performance. The capacitors selected for the solution, and indicated in the reference designs, are optimised to provide the best overall performance. where TON = and TDIS = TON IPK xL VIN IPK xL (VOUT − VIN) Note: VOUT = output voltage + rectifier diode VF ISSUE 3 - JANUARY 2004 11 SEMICONDUCTORS ZXSC100 Input capacitors The input capacitor is chosen for its voltage and RMS current rating. The use of low ESR electrolytic or tantalum capacitors is recommended. Capacitor values for optimum performance are suggested in the reference design section. VOUT RA Also note that the ESR of the input capacitor is effectively in series with the input and hence contributes to efficiency losses in the order of IRMS2 x ESR. VFB Output voltage adjustment The ZXSC100 is an adjustable converter allowing the end user the maximum flexibility in output voltage selection. For adjustable operation a potential divider network is connected as indicated in the diagram. RB 0V The output voltage is determined by the equation: VOUT= VFB (1 + RA / RB), where VFB=730mV External transistor base drive selection Optimisation of the external switching transistor base drive may be necessary for improved efficiency in low power applications. This can be achieved by introducing an external resistor between the supply and the RE pin of the ZXSC300. The resistor value can be determined by: The resistor values, RA and RB, should be maximised to improve efficiency and decrease battery drain. Optimisation can be achieved by providing a minimum current of IFB(MAX)=200nA to the VBATT pin. The output is adjustable from VFB to the (BR)VCEO of the switching transistor, Q1. R1 = Note: For the reference designs, RA is assigned the label R3 and RB the label R4. VDREF IB ISSUE 3 - JANUARY 2004 SEMICONDUCTORS 12 ZXSC100 Layout issues Layout is critical for the circuit to function optimally in terms of electrical efficiency, thermal considerations and noise. minimising parasitic inductance, capacitance and resistance. Also the sense resistor R2 should be connected, with minimum trace length, between emitter lead of Q1 and ground, again minimising stray parasitics. 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. The layout for the 0.33W solution is shown below. Actual Size To optimise for best performance each of these loops should be kept separate from each other and interconnections made with short, thick traces thus Top silk Drill holes Top Copper Bottom Copper 0.33W solution demo board layout ISSUE 3 - JANUARY 2004 13 SEMICONDUCTORS ZXSC100 Designing with the ZXSC100 Introduction Main switching waveforms This section refers to the ZXSC100, 3.3V/100mA output reference design and demonstrates the dynamic performance of the solution. Steady state operation under constant load gives an excellent indication of ZXSC100 performance. Represented in Figure 3. is the main switching waveform, measured at the collector of Q1, indicating the transistor on-state and the diode energy transfer to the output. L1 VBATT D1 3.3V/0.1A 22µH ZHCS1000 R1 0R U1 EM VDRIVE BAS ISENSE Q1 R3 C3 FMMT617 110K 1NF C2 220µF FB RE VCC GND ZXSC100 C1 R2 R4 0.033R 30K 220µF Figure 1. ZXSC100 low power solution, 3.3V/100mA output. Figure 3. Switching waveform Efficiency The peak switching current is derived from the threshold of the ISENSE pin and the sense resistor value (see Applications section for calculations). Figure 4. shows the switching waveform associated with the ISENSE pin Efficiency is often quoted as one of the key parameters of a DC-DC converter. Not only does it give an instantaneous idea of heat dissipation, but also an idea as to the extent battery life can be extended. Figure 2. Shows the efficiency of the ZXSC100 low power solution. Efficiency v Output current is shown for a 3.3V output at various input voltages. L1 VBATT D1 3.3V/0.1A 22µH ZHCS1000 R1 0R U1 EM VDRIVE BAS ISENSE Q1 R3 C3 FMMT617 110K 1NF C2 220µF RE VCC FB GND ZXSC100 C1 R2 R4 0.033R 30K 220µF Figure 2. ZXSC100 efficiency v output current Figure 4. ISENSE threshold ISSUE 3 - JANUARY 2004 SEMICONDUCTORS 14 ZXSC100 Shown in Figure 5. is the discontinuous inductor current. The ramp-up current stores energy in the inductor. The switching transistor,Q1, is on during this time and has an equivalent current ramp-up, shown in Figure 6. The ramp-down current is associated with the energy being delivered to the output via the Schottky diode, D1. The diode current is equivalent to this ramp-down current and is shown in figure 7. Figure 7. Diode current (200mA/div) Figure 5. Inductor current (200mA/div) Figure 6. Transistor current (200mA/div) ISSUE 3 - JANUARY 2004 15 SEMICONDUCTORS ZXSC100 Output voltage ripple Output voltage ripple is shown in Figure 8. The circuit is operated with a 1.2V input voltage, 3.3V output voltage and 100mA load current. Output voltage ripple will be dependent, to a large extent, on the output capacitor ESR. (see Applications section for recommended capacitors). Figure 8. Output voltage ripple for 3.3V/100mA output. Transient response Transient response to step changes in load is a critical feature in many converter circuits. The ZXSC100 operates a pulse by pulse regulation scheme and therefore corrects for changes in the output every pulse cycle, giving excellent response characteristic. Measurement with a power supply When measuring with a power supply it is important to realise that the impedance is much greater than that of a secondary battery (NiCd or NiMH). To simulate the lower impedance of the battery x10 low ESR 1000uF capacitors where placed in parallel at the input of the c o n v e r t er . A l l t he dyna m i c per f o r m a n ce measurements were taken using this technique. ISSUE 3 - JANUARY 2004 SEMICONDUCTORS 16 ZXSC100 CONNECTION DIAGRAMS VDRIVE GND EM 1 8 BAS 2 7 RE 3 6 FB VCC 4 5 ISENSE MSOP8 Millimeters D Inches MAX MIN MAX 0.91 1.11 0.036 0.044 A1 0.10 0.20 0.004 0.008 B 0.25 0.36 0.010 0.014 C 0.13 0.18 0.005 0.007 D 2.95 3.05 0.116 0.120 8 7 6 5 2 3 4 E MIN A H DIM 0.65NOM e1 eX6 θ° 0.0256NOM 0.33NOM 0.0128NOM 2.95 3.05 0.116 0.120 H 4.78 5.03 0.188 0.198 A1 E A e 1 B C L SO8 DIM A Millimeters Inches MIN MAX MIN MAX 4.80 4.98 0.189 0.196 B 1.27 BSC 0.05 BSC C 0.53 REF 0.02 REF D 0.36 0.46 0.014 0.018 E 3.81 3.99 0.15 0.157 F 1.35 1.75 0.05 0.07 G 0.10 0.25 0.004 0.010 J 5.80 6.20 0.23 0.24 © Zetex plc 2004 Europe Americas Asia Pacific Corporate Headquaters Zetex GmbH Streitfeldstraße 19 D-81673 München Germany Zetex Inc 700 Veterans Memorial Hwy Hauppauge, NY 11788 USA Zetex (Asia) Ltd 3701-04 Metroplaza Tower 1 Hing Fong Road, Kwai Fong Hong Kong Zetex plc Fields New Road, Chadderton Oldham, OL9 8NP United Kingdom Telefon: (49) 89 45 49 49 0 Fax: (49) 89 45 49 49 49 [email protected] Telephone: (1) 631 360 2222 Fax: (1) 631 360 8222 [email protected] Telephone: (852) 26100 611 Fax: (852) 24250 494 [email protected] Telephone (44) 161 622 4444 Fax: (44) 161 622 4446 [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 3 - JANUARY 2004 17 SEMICONDUCTORS