DESIGN FEATURES Triple Output LCD Power Supply Delivers 95% Efficiency from a Tiny 3mm x 3mm Package by John Bazinet Introduction Today’s handheld products pack more functionality in less space while demanding improved battery life over products of the previous generations. The only way to achieve both is to improve power efficiency in the device wherever possible. The color LCD display system is a good place to start, since it is an increasingly popular, but power hungry feature. The LTC3450 improves battery life and saves space by delivering a 95% efficient color LCD bias solution in a low profile (0.8mm tall), 3mm × 3mm package. Figure 1 shows a block diagram of the LTC3450—a complete triple output LCD power converter—in a low noise 5.1V, 10mA output synchronous step up DC/DC converter. The charge pump based voltage tripler develops a 15V output and a voltage inverter develops –10V. The 15V and –10V outputs are used in the LCD display for VGL and VGH supplies, while the 5.1V output is used to provide the main panel power. The 5.1V converter switches at a constant 550kHz, which enables very low AVDD ripple voltage even when using tiny ceramic capacitors and one small inductor. The output voltages of the LTC3450 are sequenced to be compatible with color LCD displays with AVDD powering up first followed by VGL and then VGH. The LTC3450 also provides inrush current limiting during start-up (Figure 2), as well as output disconnect and active discharge in shutdown 8 VIN SW Some types of color LCD displays switch to an ultra low power state while the display is static, which allows for increased battery life. The LTC3450 supports this mode of operation by SYNCHRONOUS PWM BOOST CONVERTER 6 7 VOUT C2 2.2µF SHUTDOWN CHARGE PUMP DOUBLER IN OUT OSCILLATOR OFF ON Power Saving Mode L1 47µH VIN 1.5V TO 4.6V C1 2.2µF BLANK SCAN mode. The LTC3450 is stable with ceramic capacitors and its internal compensation eliminates the need for an external R-C compensation network. The LTC3450 also features a wide input voltage range of 1.5V to 4.6V, making it compatible with a wide variety of battery or fixed DC voltage inputs. Very low quiescent currents allow the LTC3450 to deliver excellent efficiency over the entire input voltage range (Figure 3). MODE SHDN 10 12 C1+ C1– CF1 0.1µF V2X 10V C7 1µF SHUTDOWN 550kHz 69kHz CHARGE PUMP TRIPLER IN 4 5 11 OUT GLOBAL SHUTDOWN 14 13 15 C2 + C2 – CF2 0.1µF V3X VGH (3 × AVDD) 15V/500µA C8 0.47µF SHUTDOWN 16 CHARGE PUMP INVERTER IN OUT SHUTDOWN AVDD 5.1V/10mA 1 2 3 VINV C3 + C3 – VNEG CF3 0.1µF VGL –10V/500µA C11 0.47µF LTC3450 9 GND Figure 1. LTC3450 block diagram Linear Technology Magazine • May 2004 19 DESIGN FEATURES 100 95 0 AVDD 2V/DIV 10mA 90 EFFICIENCY (%) INDUCTOR CURRENT 100mA/DIV L = 47µH 5mA 85 2mA 80 0 75 70 20µs/DIV VIN = 3.6V Figure 2. AVDD turn on showing inrush current limiting 1.5 2.0 3.0 3.5 VIN (V) 2.5 4.0 4.5 5.0 Figure 3. LTC3450 AVDD efficiency vs VIN and load current 47µH 8 2.2µF 6 BLANK SCAN 4 7 SW VOUT 11 C1 + 10 C1 – VIN MODE V2X LTC3450 OFF ON 5 SHDN 9 14 C2 – 13 GND VINV VNEG C3 – C3 + 3 2 AVDD 5.1V/10mA 100 0.1µF 100µH 0.47µF 0.1µF 15 VGH (3 × AVDD) 15V/500µA 16 5mA LOAD 95 12 C2 + V3X 2.2µF 90 EFFICIENCY (%) VIN 1.5V TO 4.6V 47µH 85 80 75 0.1µF 1 70 1.5 2.0 0.1µF 0.1µF 2.5 3.0 3.5 VIN (V) 4.0 4.5 5.0 VGL –10V/500µA Figure 4. 5.1V, 15V, –10V application circuit and efficiency reducing its own quiescent current to a mere 30µA from the battery while maintaining all three regulated voltage outputs. This “Blank” mode operation is programmed via the Mode pin of the LTC3450. Driving the SHDN pin low reduces the LTC3450’s quiescent current to 10nA (typical) and all three voltage outputs are actively discharged to ground. LCD Bias Power Supply Circuits Figure 4 shows a 1.5V to 4.6V input to a triple output (5.1V/10mA, 15V/ 500µA and –10V/500µA) application circuit. Greater than 90% efficiency is maintained over the Li-Ion battery’s voltage range. This is far superior to an all charge pump approach that can only deliver efficiency approaching the LTC3450 when VIN is approximately 1/2 of AVDD. Figure 5 shows a 1.5V to 4.6V input to 5.1V/10mA, 15V/500µA and 20 –15V/500µA converter circuit. A tiny external dual diode is added to the circuit to get the converter to deliver the –15V and 15V outputs together. Figure 6 shows a 1.5V to 4.6V input to 5.1V/10mA, 15V and –5V circuit. VIN 1.5V TO 4.6V Peak efficiency is greater than 90%. The magnitude of the negative output voltage (VGL) is equal to the positive voltage applied to VINV. VINV is connected to either AVDD (for –5V), V2X (–10V), or with the dual diode (Figure 4) continued on page 24 L1 47µH C1 2.2µF BLANK SCAN 8 6 4 7 SW VIN VOUT 11 C1 + 10 C1 – MODE V2X LTC3450 OFF ON 5 V3X 9 GND VINV VNEG C3 – C3 + 3 2 AVDD 5.1V/10mA CF1 0.1µF 12 14 C2 + C2 – SHDN C2 2.2µF 13 CF2 0.1µF 15 16 D2 C4 0.47µF D1 VGH 15V/500µA C6 0.1µF 1 0.1µF CF3 0.1µF C5 0.1µF VGL –15V/500µA D1, D2: DUAL SCHOTTKY DIODE, PANASONIC MA704WKCT L1: SUMIDA CMD4D08-470 Figure 5. 5.1V, 15V, –15V application circuit Linear Technology Magazine • May 2004 DESIGN FEATURES Dual 5V Example Figure 2 shows an example of a redundant 5V supply. In the event that one supply goes down, the back up supply would take over. In this application, back-to-back MOSFETs are used to prevent the body diode of the MOSFET from conducting in the event that a 5V supply looses regulation and goes into an overvoltage condition. Resistive dividers from IN to UV and OV set the fault detection thresholds. In this example the UV fault occurs at 4.5V with 0.25V of hysteresis and the OV fault occurs at 5.5V. L1 and D1 are the boost regulator components. The LT4351 creates a VDD supply of 10.5V above IN. If an external supply that can provide sufficient gate drive is available, that supply can be used instead of the boost regulator. The MOSFETs are sized based on desired voltage drop with considerations for power dissipation. In this case the Si4838DY has a worst case 4.5mΩ RDS(ON) (at temperature) so the back-to-back pair is 9mΩ. These MOSFETs come in SO-8 packages so if power is limited to 1W in each then they can handle 14.9A. The LTC3450, continued from page 20 for –15V. If desired, an independent positive voltage source between 5V and 15V can be connected to VINV to produce any desired negative voltage between –5 and –15V. voltage drop across both MOSFETs at this current is 2 • 4.5mΩ • 14.9A = 0.134V. If more current is required, use MOSFETs with lower RDS(ON) and/ or better thermal resistance, or add parallel MOSFETs. The LT4351 is useful in any ORing situation benefiting from low power dissipation—not just redundant supplies. Different types of power sources can also be ORed together, and because the LT4351 diode function is gated, power sequencing of different supplies is relatively easy. For example, Figure 3 shows a system with two redundant supplies and a battery backup. The two redundant supplies are ORed via the ideal diodes, so power is delivered from the higher of the in-range supplies. Their undervoltage and overvoltage thresholds are set based on the input supply range. The LT4351 circuit for the battery disconnects the battery when power is supplied from either system supply. Its OV pin is above threshold if the FAULT is off on either system supply (UV is set above threshold). If both system supplies are disabled (FAULT of both systems are VIN 1.5V TO 4.6V The LTC3450 delivers a highly compact and efficient power supply solution for small LCD displays. Its wide input voltage range makes it easy to drop into a variety of applications. Built-in inrush current limiting, output disconnect and power saving controls simplify the task of implementing power friendly LCD displays. Conclusion The trend in today’s power supplies is toward higher currents, lower voltages, higher efficiency and increased reliability. These needs are forcing designers away from traditional Schottky ORing diodes. The LT4351 provides an improved ORing solution by controlling low RDS(ON) MOSFETs to create a near ideal diode. In addition the LT4351 adds increased functionality with supply monitoring that can disable power path conduction. An LT4351 solution has significantly lower power dissipation than a Schottky diode and offers protection features that a Schottky cannot. L1 47µH 8 C1 2.2µF 6 BLANK SCAN 4 7 SW VIN VOUT 11 C1 + 10 C1 – MODE V2X LTC3450 Conclusion low) then the battery’s LT4351 OV pin is pulled below threshold to allow the battery to provide power . Figure 4 shows an example of combining the LT4351 ideal diode function with a Hot Swap controller. This can be used to create ORed redundant supplies on a plug-in board. The Hot Swap controller provides current limiting, circuit breaker functions and reset timing while the LT4351 provides the ideal diode behavior. OFF ON 5 V3X 9 GND VINV VNEG C3 – C3 + 3 L1: SUMIDA CMD4D08-470 2 AVDD 5.1V/10mA CF1 0.1µF 12 14 C2 + C2 – SHDN C2 2.2µF 13 CF2 0.1µF 15 C4 0.47µF VGH (3 × AVDD) 15V/500µA 16 C6 0.1µF 1 C5 0.1µF CF3 0.1µF VGL –5V/500µA Figure 6. 5.1V, 15V, –5V application circuit To view this and past issues of LT Magazine online, see http://www.linear.com/go/ltmag 24 Linear Technology Magazine • May 2004