Design Alternatives To The TC682 For Performing Inverting Voltage Doubler Functions. DC/DC Converter +5V 6 V IN V OUT TC682 NC GND 5

Transcription

1 M AN80 Design Alternatives To The TC8 For Performing Inverting Voltage Doubler Functions Author: INTRODUCTION Pat Maresca Microchip Technology Inc. Creating a negative DC bias voltage from a positive DC supply often is required in battery-powered, portable, hand-held instruments that use a Liquid Crystal Display (LCD). Many LCDs require a relatively large negative bias (on the order of -0V). Since many portable systems (such as cellular subscriber units) often have a regulated V DC bias available, the critical design task involves converting this supply voltage to a negative DC bias for the LCD. This application note discusses the advantages and disadvantages of several alternatives for implementing the -0V bias required by the LCD. TRADITIONAL TC8 IMPLEMENTATION Figure shows a circuit implementation using Microchip Technology's TC8 to generate the -0V LCD bias from a regulated V input. Assuming that a single-cell battery (with a nominal terminal voltage of.v) is powering the system, a regulating DC/DC boost converter is needed to generate the V regulated input supply to the TC8. Although the TC8 requires only three external tantalum capacitors to generate the -0V bias, its package is a large 8-pin SOIC, which occupies approximately square millimeters of circuit board space. The TC8 active supply current is typically 8 µa, with the internal charge pump switching frequency being khz. The TC8 has no shutdown features, so it always consumes power when the V input bias is active..v Battery - DC/DC Converter C C. Requires one 8-pin SOIC package, sq. millimeters.. Requires three tantalum capacitors.. TC8 supply current approximately 8 µa.. Charge pump switching frequency: khz.. No shutdown mode available. 7 V V TC8 NC 8-0V to LCD Display C FIGURE : TC8 Circuit Implementation. 00 Microchip Technology Inc. DS0080B-page

2 AN80 DUAL TCM88 IMPLEMENTATION Figure shows a two-chip inverting doubler solution using Microchip s TCM88. Like the TC8 example, a regulating DC/DC boost converter is needed to provide the V regulated input voltage to the first TCM88, assuming the system is powered from a single-cell Li- Ion battery. The second TCM88 is level-shifted (i.e., powered from ground and -V) to generate the -0V LCD bias. Although this implementation requires four external tantalum capacitors, the total package size of the two TCM88s occupies only 8 square millimeters of board space, which is considerably less than the TC8 solution. The dual-tcm88 approach consumes only 00 µa (again, considerably less than the TC8) and has the same internal charge pump switching frequency as the TC8 (i.e., khz). As in the TC8 example, the dual TCM88 solution has no shutdown feature. FIGURE :.V Battery - C DC/DC Converter V C TCM88 TCM88 C Dual TCM88 implementation. -V. Requires two SOT-A- packages, 8 sq. millimeters total.. Requires four tantalum capacitors.. Combined TCM88 supply current approximately 00 µa.. Charge pump switching frequency: khz.. No shutdown mode available. -0V to LCD Display C DS0080B-page 00 Microchip Technology Inc.

3 AN80 DUAL TCM89 IMPLEMENTATION Figure shows another two-chip inverting doubler solution using Microchip s TCM89. This implementation is almost identical to the dual TCM88 approach. The primary difference being that the two TC89s switch at a higher frequency ( khz) than the dual TCM88 or TC8. This allows the designer to use smaller external capacitors with the dual TCM89 solution at the expense of a higher supply current (0 µa) than the dual TCM88 approach. All other features of the dual TCM88 solution apply to the dual TCM89 solution. FIGURE :.V Battery - C. µf DC/DC Converter Dual TCM89 implementation. V TCM89 C. µf. Requires two SOT-A- packages, 8 sq. millimeters total.. Requires four. µf tantalum capacitors.. Combined TCM89 supply current approximately 0 µa.. Charge pump switching frequency: khz.. No shutdown mode available. TCM89 -V -0V to LCD Display C C. µf. µf 00 Microchip Technology Inc. DS0080B-page

4 AN80 DUAL TC9 IMPLEMENTATION A two-chip solution using Microchip s TC9 is shown in Figure. This implementation is similar to the dual TCM88 approach with one key difference: the TC9 has a shutdown feature that can power down the negative bias generator during low-power operating modes of the battery-powered instrumentation. An example of this is the sleep mode of a Code Division Multiple Access (CDMA) cellular subscriber unit. By applying a low-level logic signal to the first TC9, the second TC9 will automatically be powered down because the -V bias to its pin is eliminated. All other features of the dual TCM88 solution apply to the dual TC9 approach, with the exception that the active supply current of the dual TC9 solution (0 µa) is higher than the dual TCM88 solution (which can be negated by the shutdown feature of the TC9). Note: When the first TC9 is shut down, V this TC9 is automatically shut down ON because the V bias is removed. DC/DC OFF Converter.V Shutdown Control Logic from the CPU Battery V - V IN SHDN SHDN C C TC9 TC9-0V -V to LCD Display C C. Requires two SOT-A- packages, 8 sq. millimeters total.. Requires four tantalum capacitors.. Combined TC9 supply current approximately 0 µa.. Charge pump switching frequency: khz.. Total supply current in shutdown mode approximately 0. µa. FIGURE : Dual TC9 implementation. * See the TC data sheet (DS0) for details. DS0080B-page 00 Microchip Technology Inc.

5 AN80 DUAL TC0 IMPLEMENTATION Figure illustrates still another two-chip solution using Microchip s TC0. This implementation is almost identical to the dual TC9 approach, with the main difference being that the two TC0s switch at a higher frequency ( khz) than the dual TC9. This allows the designer to use smaller external capacitors with the dual TC0 solution at the expense of a higher active supply current (0 µa) than the dual TC9 approach. All other features of the dual TC9 solution apply to the dual TC0 approach. Note: When the first TC0 is shut down, V this TC0 is automatically shut ON down because the V bias is removed. DC/DC OFF Converter.V Shutdown Control Logic from the CPU Battery - V V IN SHDN SHDN C C TC0. µf TC0. µf -0V -V to LCD Display C. µf C. µf. Requires two SOT-A- packages, 8 sq. millimeters total.. Requires four. µf tantalum capacitors.. Combined TC0 supply current approximately 0 µa.. Charge pump switching frequency: khz.. Total supply current in shutdown mode approximately 0. µa. FIGURE : Dual TC0 Implementation. 00 Microchip Technology Inc. DS0080B-page

6 AN80 TC/TCM88 IMPLEMENTATION Figure shows a two-chip inverter solution using Microchip s TC-.0 and TCM88. The most important difference of this implementation, when compared to the previous solutions, is that the regulating DC/DC boost converter is no longer required. The TC-.0 is a regulating -x boost/buck inverter that generates a -V output from any positive DC bias ranging from.v to.v. This feature allows the TC-.0 to connect directly to the single-cell battery without the need for a regulating V DC/DC converter. The TC-.0 also can be placed in a low power shutdown mode (with a logic-low on the CCLK input*), which also will power down the TCM88 because the -V bias to its pin is eliminated. The TC-.0 runs off an internal oscillator of 00 khz, but this can be overridden by connecting an external oscillator to the CCLK input*. The solution requires less circuit board space than any of the previous solutions due to the DC/DC boost converter not being required. The total active supply current for this solution is approximately 0 µa. V ON OFF.V Shutdown Control Logic 7 from the CPU Battery C 8 CCLK 0.7 µf C TC-.0 TCM88 -V C V 0.7 µf C.7 µf. Requires one SOT-A- package and one 8-pin MSOP,. sq. millimeters total.. Requires two, one.7 µf, and two 0.7 µf tantalum capacitors.. Total active supply current approximately 0 µa.. Charge pump switching frequency: 00 khz for TC, khz for TCM88.. Shutdown mode available; total supply current in shutdown mode approximately 0. µa.. TC provides regulated voltage output. 7. Unregulated input voltage (.V to.v) allowed; can be powered directly off battery. Note: When the TC-.0 is shut down, the TCM88 is automatically shut down because the V bias is removed. -0V to LCD Display C FIGURE : TC/TCM88 implementation. DS0080B-page 00 Microchip Technology Inc.

7 AN80 TC/TCM89 IMPLEMENTATION Figure 7 shows a two-chip inverter solution using Microchip s TC-.0 and TCM89. This implementation is almost identical to the TC/ TCM88 approach, with the only difference being that the TCM89 switches at a higher frequency ( khz) than the TCM88. This allows the designer to use smaller external capacitors on the TCM89. The total active supply current for this solution is approximately µa..v Battery V ON OFF Shutdown Control Logic 7 from the CPU V V IN IN 8 C CCLK 0.7 µf C TC-.0. µf TCM89 -V V C 0.7 µf C.7 µf Note: When the TC-.0 is shut down, the TCM89 is automatically shut down because the V bias is removed. -0V to LCD Display C. µf. Requires one SOT-A- package and one 8-pin MSOP,. sq. millimeters total.. Requires two. µf, one.7 µf, and two 0.7 µf tantalum capacitors.. Total active supply current approximately µa.. Charge pump switching frequency: 00 khz for TC, khz for TCM89.. Shutdown mode available; total supply current in shutdown mode approximately 0. µa.. TC provides regulated voltage output. 7. Unregulated input voltage (.V to.v) allowed; can be powered directly off battery. FIGURE 7: TC/TCM89 implementation. 00 Microchip Technology Inc. DS0080B-page 7

8 AN80 SUMMARY There are many methods of implementing an inverting voltage doubler function using Microchip s power management ICs. Table summarizes each approach and identifies the advantages and disadvantages of each. Newer state-of-the-art components, such as the TCM88, TCM89, TC9, TC0 and TC, are providing designers of battery-powered instrumentation with greater circuit flexibility than the older traditional TC8 method of implementing a negative DC bias. Designers now can select the most optimal solution that best meets the technical criteria of their systems. TABLE : INVERTING DOUBLER TECHNICAL SUMMARY Approach IC Area (sq. mm) # Caps Active Supply Current (µa) Switching Frequency (KHz) Shutdown Regulated Input Required Primary Advantage(s) () TC8 8 No Yes Component Count () TCM No Yes Supply Current Primary Disadvantage(s) No Shutdown/ Physical Area/ Supply Current/ Regulated Supply Required No Shutdown/ Regulated Supply Required () TCM No Yes Physical Area No Shutdown/ Regulated Supply Required () TC9s 8 0 Yes Yes Supply Current/ Shutdown () TC0s 8 0 Yes Yes Physical Area/ Shutdown () TC/ () TCM88 () TC/ () TCM / Yes No Unregulated Input Allowed/ Shutdown. 00/ Yes No Unregulated Input Allowed/ Shutdown Regulated Supply Required Regulated Supply Required Active Supply Current Active Supply Current DS0080B-page 8 00 Microchip Technology Inc.

9 Note the following details of the code protection feature on Microchip devices: Microchip products meet the specification contained in their particular Microchip Data Sheet. Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the intended manner and under normal conditions. There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip's Data Sheets. Most likely, the person doing so is engaged in theft of intellectual property. Microchip is willing to work with the customer who is concerned about the integrity of their code. Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not mean that we are guaranteeing the product as unbreakable. Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our products. Attempts to break microchip s code protection feature may be a violation of the Digital Millennium Copyright Act. If such acts allow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act. Information contained in this publication regarding device applications and the like is intended through suggestion only and may be superseded by updates. It is your responsibility to ensure that your application meets with your specifications. No representation or warranty is given and no liability is assumed by Microchip Technology Incorporated with respect to the accuracy or use of such information, or infringement of patents or other intellectual property rights arising from such use or otherwise. Use of Microchip s products as critical components in life support systems is not authorized except with express written approval by Microchip. No licenses are conveyed, implicitly or otherwise, under any intellectual property rights. Trademarks The Microchip name and logo, the Microchip logo, Accuron, dspic, KEELOQ, MPLAB, PIC, PICmicro, PICSTART, PRO MATE and PowerSmart are registered trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. AmpLab, FilterLab, microid, MXDEV, MXLAB, PICMASTER, SEEVAL and The Embedded Control Solutions Company are registered trademarks of Microchip Technology Incorporated in the U.S.A. Application Maestro, dspicdem, dspicdem.net, ECAN, ECONOMONITOR, FanSense, FlexROM, fuzzylab, In-Circuit Serial Programming, ICSP, ICEPIC, microport, Migratable Memory, MPASM, MPLIB, MPLINK, MPSIM, PICkit, PICDEM, PICDEM.net, PowerCal, PowerInfo, PowerMate, PowerTool, rflab, rfpic, Select Mode, SmartSensor, SmartShunt, SmartTel and Total Endurance are trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. Serialized Quick Turn Programming (SQTP) is a service mark of Microchip Technology Incorporated in the U.S.A. All other trademarks mentioned herein are property of their respective companies. 00, Microchip Technology Incorporated, Printed in the U.S.A., All Rights Reserved. Printed on recycled paper. Microchip received QS-9000 quality system certification for its worldwide headquarters, design and wafer fabrication facilities in Chandler and Tempe, Arizona in July 999 and Mountain View, California in March 00. The Company s quality system processes and procedures are QS-9000 compliant for its PICmicro 8-bit MCUs, KEELOQ code hopping devices, Serial EEPROMs, microperipherals, non-volatile memory and analog products. In addition, Microchip s quality system for the design and manufacture of development systems is ISO 900 certified. 00 Microchip Technology Inc. DS0090B-page 9

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