SP4438EU

® SP4438 Ultra-Quiet Electroluminescent Lamp Driver For Cellular Phone Applications s Patent Pending Output Waveshaping s Waveshaped Output Optimized For Low Acoustic Noise And Maximum Efficiency s Reduced Supply Current And Standby Current s Up to15cd/m2 For Cell Phone Sized EL Lamps (2 to 3 sq. in.) s Waveshaped Output And Low Frequency Operation Minimize Stress On EL Lamp To Extend Its Lifetime s Uses 820µH, Sub-2mm Coils s Gradual Rising And Falling Edges Minimizes EMI s +2.7V To +3.3V Battery Operation s Space-Saving 8-pin MSOP package APPLICATIONS s Cellular Radios s Wireless Communication Products DESCRIPTION The SP4438 device is a low noise, high voltage output DC-AC inverter designed to drive electroluminescent lamps to backlight liquid crystal display and keypads used in cellular phones, cordless phones, 2-way radios, and other wireless communication products. The output waveform of the SP4438 device is ideal for cell phone applications requiring low acoustic noise performance. One external resistor is used to set the internal oscillator frequency and one inductor is required to generate the high voltage AC output to drive an EL lamp up to 3 square inches in size. The SP4438 operates from a +3.0V battery source and has a low power standby mode that draws less than 1µA, making it ideal for low-power cellular applications. All input pins are ESD protected with internal diodes to VDD and VSS. The SP4438 is offered in a space-saving 8-pin MSOP package. VBATT VDD 1 ELEN 2 ROSC 3 VSS 4 SP4438 8 EL1 ELEN 2 L1 820µH/14Ω 7 EL2 6 5 CINT COIL C1 0.1µF C2 1nF VDD ROSC 714kΩ 1 ROSC 3 4 SP4438 5 6 COIL D1 1N4148 CINT 8 VSS EL1 7 EL2 CINT 47nF EL Lamp Rev.11/13/00 SP4438 Ultra-Quiet Electroluminescent Lamp Driver © Copyright 2000 Sipex Corporation 1 ABSOLUTE MAXIMUM RATINGS These are stress ratings only and functional operation of the device at these ratings or any other above those indicated in the operation sections of the specifications below is not implied. Exposure to absolute maximum rating conditions for extended periods of time may affect reliability. Supply Voltage (VDD to VSS)........................-0.3V, +5V Operating Temperature.......................-40˚C to +85˚C Storage Temperature........................-65˚C to +150˚C Power Dissipation Per Package 8-pin MSOP (derate 4.85mW/OC above +70OC).........................400mW SPECIFICATIONS VDD = +2.7V to +3.3V, LCOIL = 820µH/14Ω, ROSC = 714kΩ, EL Lamp Load = (8nF + 2.5kΩ)/1MΩ, and TAMB = -40OC to +85OC. TAMB = 25OC for typical values unless otherwise noted. PARAMETER INPUT CHARACTERISTICS Supply Voltage, VDD Supply Current, ICOIL+IDD Coil Voltage, VCOIL ELEN Input Voltage, VELEN LOW: EL OFF HIGH: EL on ELEN Input Impedance Shutdown Current, ISD=ICOIL+IDD INDUCTOR DRIVE Coil Frequency, fCOIL Coil Duty Cycle Peak Coil Current, IPK-COIL EL LAMP OUTPUT EL Lamp Frequency, fLAMP Peak to Peak Output Voltage, VP-P MIN. TYP. MAX. UNITS CONDITIONS 2.7 3.0 3.3 30 V mA V VDD = +3.0V VDD -0.25 VDD-0.25 1 0 VDD 3 9 0.25V VDD+0.25 V MΩ 1 µA VELEN = LOW,VDD = +3.0V 26 23 32 43 45 kHz % ROSC = 714kΩ,TAMB = +25OC TAMB = -40OC to +85OC,VDD = +3.0V 90 100 mA 190 175 130 120 250 336 348 Hz TAMB = +25OC,VDD = +3.0V,with load TAMB = -40OC to +85OC TAMB = +25OC,VDD = +3.0V,with load TAMB = -40OC to +85OC TAMB = +25OC,VDD = +3.0V, NOTE 1 TAMB = +25OC,VDD = +3.0V,with load, measured from 10% to 90% 145 140 26 VP-P dBSPL 1.0 ms Audible Noise Rise Time .55 0.8 NOTE 1: Audible Noise is measured inside an acoustic sound chamber. The Sound Level Meter is a B&K Mediator 2238, A-weighted with Condenser Mic type 4188 positioned 1/4 inch above the lamp in an 8 cubic inch volume. See Figure 5 on page 6. Rev. 11/13/00 SP4438 Ultra-Quiet Electroluminescent Lamp Driver © Copyright 2000 Sipex Corporation 2 PINOUT VDD 1 ELEN 2 ROSC 3 VSS 4 SP4438 8 EL1 7 EL2 6 5 CINT COIL PIN ASSIGNMENTS Pin 1 — VDD — Positive Battery Power Supply. Connect such that +2.7V < VDD < +3.3V. Pin 2 — ELEN — Electroluminescent Lamp Enable. When driven HIGH, this input pin enables the EL driver outputs for EL1 and EL2. This pin has an internal pulldown resistor. Pin 3 — ROSC — Oscillator Resistor. Connecting a resistor to this input pin sets the frequency of the internal clock. Pin 4 — VSS — Power Supply Common. Connect to the lowest circuit potential, typically ground. Pin 5 — COIL — The inductor for the EL lamp is connected from VDD to this input pin. Pin 6 — CINT — Integrating Capacitor. An integrating capacitor (47nF typical) connected from this pin to ground filters out any coil switching spikes or ripple present in the output waveform to the EL lamp. Connecting a fast recovery diode from COIL to CINT increases the light output of the EL lamp. Pin 7 — EL2 — Electroluminescent Lamp Output 2. This is a high voltage lamp driver output pin to connect to the EL lamp. Pin 8 — EL1 — Electroluminescent Lamp Output 1. This is a high voltage lamp driver output pin to connect to the EL lamp. Rev.11/13/00 SP4438 Ultra-Quiet Electroluminescent Lamp Driver © Copyright 2000 Sipex Corporation 3 VBATT ELEN 2 L1 820µH/14Ω SP4438 5 VDD ROSC 714kΩ 1 6 ROSC 3 4 C1 0.1µF C2 1nF VSS 8 EL1 7 EL2 COIL D1 1N4148 CINT CINT 47nF EL Lamp Figure 1: Typical Operating Circuit for the SP4438. Note: C1 should be close to L1. Rev. 11/13/00 SP4438 Ultra-Quiet Electroluminescent Lamp Driver © Copyright 2000 Sipex Corporation 4 VBATT VDD 1 5 COIL L1 820µH/14Ω SP4438 ROSC 714kΩ ROSC 3 OSC D1 1N4148 6 CINT CINT 47nF C1 0.1µF C2 1nF PMOS PMOS Q FF7 Q ELEN 2 BRIDGE CONTROL DMOS DMOS 4 VSS 8 EL1 7 EL2 EL Lamp Figure 2: Internal Block Diagram of the SP4438 Figure 3: EL Differential Output Waveform of the EL1 and EL2 Outputs of the SP4438 Rev.11/13/00 SP4438 Ultra-Quiet Electroluminescent Lamp Driver © Copyright 2000 Sipex Corporation 5 EL Lamp Noise Measurement Set-Up 1” 2” Microphone Type 4188 Bruel & Kjaer 2238 Mediator A-Weighting dBSPL Measurement Photometer Tek-J17 Aux 1 Anechoic Chamber HP8903A Audio Analyzer - SPCL 3.1, 1.17 Preamplifier with 30kHz High Pass Filter 4” EL Lamp Differential Voltage Probe Current Probe Tek754C Digital Osciliscope Monitor Voltage Waveform Measurements EL Driver Figure 4: Dual Supply Application Circuit for the SP4438 EL Lamp Noise Measurement Set-Up 1” 2” Microphone Type 4188 Bruel & Kjaer 2238 Mediator A-Weighting dBSPL Measurement Photometer Tek-J17 Aux 1 Anechoic Chamber HP8903A Audio Analyzer - SPCL 3.1, 1.17 Preamplifier with 30kHz High Pass Filter 4” EL Lamp Differential Voltage Probe Current Probe Tek754C Digital Osciliscope Monitor Voltage Waveform Measurements EL Driver Figure 5. Electroluminescent Lamp Noise Measurement Setup for the SP4438 Rev. 11/13/00 SP4438 Ultra-Quiet Electroluminescent Lamp Driver © Copyright 2000 Sipex Corporation 6 DESCRIPTION The SP4438 Electroluminescent Lamp Driver is a low-cost low voltage device ideal for the replacement of LED backlighting designs in cell phones, PDAs and other portable designs desiring low acoustic noise. The SP4438 contains a DC-AC inverter that can produce an AC output of 145VP-P (typical) from a +2.7V to +3.3V input voltage. An internal block diagram of the SP4438 can be found in Figure 2. The SP4438 is built on Sipex's dielectrically isolated BiCMOS process that provides the isolation required to separate the high voltage AC signal used to drive the EL lamp from the low voltage logic and signal processing circuitry. This ensures latch-up free operation in the interface between the low voltage CMOS circuitry and the high voltage bipolar circuitry. The SP4438 is ideal for applications driving EL lamps to backlight LCD displays and keypads, used in cellular radios. A total of only six external components are required for the standard operation of the SP4438: an inductor, a fast recovery diode, three capacitors and a resistor. A diagram of the SP4438 in a typical application can be found in Figure 1. Electroluminescent Technology An EL lamp is a strip of plastic that is coated with a phosphorous material which emits light (fluoresces) when a (>40V) AC signal is applied across it. Long periods of DC voltages applied to the lamp tends to breakdown the material and reduce its lifetime. With these considerations in mind, the ideal signal to drive an EL lamp is a high voltage sine wave. Traditional approaches to achieving this type of waveform included discrete circuits incorporating a transformer, transistors, and several resistors and capacitors. This approach is large and bulky, and cannot be implemented in most hand held equipment. Sipex offers low power single chip driver circuits specifically designed to drive small to medium sized electroluminescent panels. Market Applications Electroluminescent backlighting is ideal when used with LCD displays, keypads, or other backlit readouts. Its main use is to illuminate displays Rev.11/13/00 in dim to dark conditions for momentary periods of time. EL lamps consume less power than LEDs or incandescent bulbs making them ideal for battery powered products. Also, EL lamps are able to evenly light an area without creating any undesirable "hot spots" in the display. THEORY OF OPERATION The SP4438 is a DC-AC inverter made up of: 1. The Oscillator/Frequency Divider, 2. The Coil/Boost Converter, 3. The Switched Hbridge Network, and 4. The Precision Bridge Control Logic. Further details of each element follow. The Oscillator/Frequency Divider The oscillator provides the SP4438 with an onchip clock used to control the coil switch (fCOIL) and the H-bridge network (fLAMP ). Although the oscillator frequency can be varied to optimize the lamp output, the ratio of fCOIL/fLAMP will always equal 128. Figure 2 shows the oscillator output driving the coil and through 7 flip flops, driving the lamp. The suggested oscillator frequency is 32kHz for fCOIL. The oscillator output is internally divided down by 7 flip flops to create a second internal control signal at 250Hz for fLAMP. The Coil/Boost Converter The supply VCOIL can range from +2.7V to +9V. See figure 4 on page 6. VCOILshould be chosen such that ICOIL does not exceed the maximum coil current specification. The majority of the current goes through the coil and is typically much greater than IDD. The inductor is an external component connected from VCOIL to the COIL pin of the SP4438. Energy is stored in the coil according to the equation EL = 1/2 x L x IP2 where IP, to the first approximation, is the product IP = (tON) x ((VBATT - VCE)/L) where tON is the time it takes for the coil to reach its peak current, VCE is the voltage drop across the internal NPN transistor and L is the © Copyright 2000 Sipex Corporation SP4438 Ultra-Quiet Electroluminescent Lamp Driver 7 inductance of the coil. When the NPN transistor switch is off, the energy is forced through an internal diode which drives the switched Hbridge network. This energy recovery is directly related to the brightness of the EL lamp output. There are many variations among coils; magnetic material differences, winding differences and parasitic capacitances. The fCOIL signal controls a switch that connects the coil at the COIL pin to ground or to open circuit. The fCOIL signal is a 90% duty cycle signal switching at the oscillator frequency, 32kHz. During the time when the fCOIL signal is HIGH, the coil is connected from VCOIL to ground and a charged magnetic field is created in the coil. When the fCOIL signal is LOW, the ground connection is switched open, the field collapses, and the energy in the inductor is forced to flow toward the high voltage H-bridge switches. The Switched H-Bridge Network Current sources and precision controlled timing of the SP4438 switched H-bridge network are designed to reduce EMI emissions, extend EL lamp life, and reduce the overall power dissipation of the device. Current sources were added to the high and low side of the H-bridge network to ensure control of the charge and discharge of the EL lamp. The precision MOSFET timing of the SP4438 allows for controlled charging and discharging of the EL lamp to minimize EMI and audible noise. Refer to Figure 7 for the single ended and differential output waveforms to the EL lamp. The Precision Bridge Control Circuitry This circuitry is driven by the internal oscillator to control the timing of the charge and discharge of the EL lamp to eliminate EMI and noise concerns. This control circuitry drives the Hbridge timing. Refer to Figure 2 for the internal block diagram of the SP4438. Fine Tuning Performance Circuit performance of the SP4438 can be improved with some of the following suggestions: Increase EL Lamp Light Output : By connecting a fast recovery diode from COIL (pin 5) to CINT (pin 6), the internal diode of the switched H-bridge network is bypassed resulting in an increase in light output at the EL lamp. We suggest a fast recovery diode, such as the industry standard 1N4148, be used for D1. This circuit connection can be found in Figure 2. Split Power Supplies: Instead of driving both the logic and power circuitry of the SP4438 from a single battery supply, two supplies can be implemented, one to drive the IC and the other to drive the inductor. The +2.7V to +3.3V power supply is connected to VCC. This power supply can be used to power the oscillator and control logic. The current draw on this power supply will be minimal, typically