LM2702 TFT Panel Module General Description Features The LM2702 is a compact bias solution for TFT displays. It has a current mode PWM step-up DC/DC converter with a 2A, 0.2Ω internal switch. Capable of generating 8V at 170mA from a Lithium Ion battery, the LM2702 is ideal for generating bias voltages for large screen LCD panels. The LM2702 operates at a switching frequency of 600kHz allowing for easy filtering and low noise. An external compensation pin gives the user flexibility in setting frequency compensation, which makes possible the use of small, low ESR ceramic capacitors at the output. The LM2702 uses a patented internal circuitry to limit startup inrush current of the boost switching regulator without the use of an external softstart capacitor. An external softstart pin enables the user to tailor the softstart to a specific application. The LM2702 has an internal controllable PMOS switch used for controlling the row driver voltages. The switch can be controlled externally with a control pin and delay time. The LM2702 contains a Vcom amplifier and a Gamma buffer capable of supplying 50mA source and sink. The TSSOP-16 package ensures a low profile overall solution. n n n n n n n n n n n 2A, 0.2Ω, internal power switch VIN operating range: 2.2V to 12V 600kHz switching frequency step-up DC/DC converter Inrush current limiting circuitry External softstart override Internal 7.3Ω PMOS switch PMOS switch control pin PMOS switch delay pin Vcom amplifier Gamma buffer 16 pin TSSOP package Applications n LCD Bias Supplies Typical Application Circuit 20051131 © 2002 National Semiconductor Corporation DS200511 www.national.com LM2702 TFT Panel Module November 2002 LM2702 Connection Diagram Top View 20051104 TJMAX TSSOP 16 package = 125˚C, θJA = 120˚C/W (Note 1) Pin Description Pin Name 1 Vcom+ Function 2 Vcom− Vcom Amplifier negative input. 3 Vcom Vcom Amplifier output. 4 Delay Switch delay. 5 Css Soft start pin. 6 VC Boost Compensation Network Connection. 7 FB Output Voltage Feedback input. 8 GND 9 SW NMOS power switch input. Main power input, step-up and switch circuitry. Vcom Amplifier positive input. Ground. 10 VIN 11 SWI PMOS switch input. 12 SWO PMOS switch output. 13 SWC PMOS switch control pin. 14 AVIN Analog power input (buffers). 15 GMA Gamma buffer output. 16 GMA+ Gamma buffer input. www.national.com 2 Vcom+(Pin 1): Positive input terminal of Vcom amplifier. Vcom−(Pin 2): Negative input terminal of Vcom amplifier. Vcom(Pin 3): Output terminal of Vcom amplifier. Delay(Pin 4): PMOS switch delay control pin. See Operation section for setting the delay time. The delay time begins when the output voltage of the DC/DC switching regulator reaches 85% of its true output voltage. This corresponds to a FB voltage of about 1.1V. The PMOS switch is controlled with both the delay time and the switch control pin, SWC. If no Cdelay capacitor is used, the PMOS switch is controlled solely with the SWC pin. SW(Pin 9): This is the drain of the internal NMOS power switch. Minimize the metal trace area connected to this pin to minimize EMI. VIN(Pin 10): Input Supply Pin. Bypass this pin with a capacitor as close to the device as possible. The capacitor should connect between VIN and GND. SWI(Pin 11): PMOS switch input. Source connection of PMOS device. SWO(Pin 12): PMOS switch output. Drain connection of PMOS device. SWC(Pin13): PMOS switch control pin. This pin creates an AND function with the delay time after the output of the switching regulator has reached 85% of its nominal value. To ensure the PMOS switch is in the correct state, apply a voltage above 1.5V to this pin to turn on the PMOS switch and apply a voltage below 0.7V to turn off the PMOS switch. AVIN(Pin 14): Supply pin for the Vcom opamp and the Gamma buffer. Bypass this pin with a capacitor as close to the device as possible, about 100nF. The capacitor should connect between AVIN and PGND. GMA(Pin 15): Gamma Buffer output pin. GMA+(Pin 16): Gamma Buffer input pin. Css(Pin 5): Softstart pin. Connect capacitor to Css pin and AGND plane to slowly ramp inductor current on startup. See Operation section for setting the softstart time. VC(Pin 6): Compensation Network for Boost switching regulator. Connect resistor/capacitor network between VC pin and AGND for boost switching regulator AC compensation. FB(Pin 7): Feedback pin. Set the output voltage by selecting values of R1 and R2 using: Connect the ground of the feedback network to the AGND plane, which should be tied directly to the GND pin. GND(Pin 8): Ground connect for LM2702. Connect all sensitive circuitry, ie. feedback resistors, softstart capacitor, de- Ordering Information Order Number Package Type NSC Package Drawing Supplied As LM2702MT-ADJ TSSOP-16 MTC16 73 Units, Rail LM2702MTX-ADJ TSSOP-16 MTC16 2500 Units, Tape and Reel 3 www.national.com LM2702 lay capacitor, and compensation network to a dedicated AGND plane which connects directly to this pin. Connect all power ground components to a PGND plane which should also connect directly to this pin. Please see Layout Considerations under the Operation section for more details on layout suggestions. Pin Functions LM2702 Block Diagrams 20051103 20051151 20051157 www.national.com 4 (Note 2) ESD Ratings (Notes 3, 4) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. VIN -0.3V to 12V SW Voltage -0.3V to 18V FB Voltage -0.3V to 7V VC Voltage 0.96V to 1.56V Css Voltage Human Body Model Amplifier/Buffer Input/Output Voltage Rail-to-Rail GND to 1.3V SWI -0.3V to 30V SWO -0.3V to 30V Operating Temperature −40˚C to +125˚C Storage Temperature −65˚C to +150˚C Supply Voltage, VIN -0.3V to 12V Delay 200V Operating Conditions -0.3V to 12V Supply Voltage, AVIN 2kV Machine Model -0.3V to 1.2V SWC Voltage LM2702 Absolute Maximum Ratings 2.2V to 12V SW Voltage 17.5V Supply AVIN 4V to 12V SWI 2.2V to 30V Electrical Characteristics Specifications in standard type face are for TJ = 25˚C and those with boldface type apply over the full Operating Temperature Range ( TJ = −40˚C to +125˚C). Unless otherwise specified, VIN =2.2V and AVIN = 8V, RCOM = RGAMMA = 50Ω, CCOM = CGAMMA = 1nF. Switching Regulator Symbol IQ Typ (Note 6) Max (Note 5) Not Switching, FB = 2V 1.6 2.3 Switching, switch open, FB = 0.1V 2.6 5.2 1.265 1.291 V 0.01 0.1 %/V 2 2.6 A Parameter Quiescent Current Conditions VFB Feedback Voltage %VFB/∆VIN Feedback Voltage Line Regulation ICL Switch Current Limit (Note 7) VIN = 2.7V VIN = 2.7V RDSON Switch RDSON (Note 8) IB FB Pin Bias Current (Note 9) Min (Note 5) 1.239 1.4 200 60 VIN Input Voltage Range ISS Soft Start Current TSS Internal Soft Start Ramp Time gm Error Amp Transconductance ∆I = 5µA 2.2 5 40 Units mA mΩ 500 nA 12 V 12 15 µA 7 10 mS 135 290 µmho AV Error Amp Voltage Gain DMAX Maximum Duty Cycle 78 85 fS Switching Frequency 480 600 135 V/V % 720 kHz IL Switch Leakage Current 0.1 20 µA UVP On Threshold 1.79 1.92 2.05 V Off Threshold 1.69 1.82 1.95 V VSW = 18V Hysteresis 100 5 mV www.national.com LM2702 Electrical Characteristics Specifications in standard type face are for TJ = 25˚C and those with boldface type apply over the full Operating Temperature Range ( TJ = −40˚C to +125˚C). Unless otherwise specified, VIN =2.2V and AVIN = 8V, RCOM = RGAMMA = 50Ω, CCOM = CGAMMA = 1nF. Vcom Amplifier Symbol VOS IB IOS CMVR Parameter Conditions Min (Note 5) 3.5 10 3 10 VCM = 7.5V Input Bias Current VCM = 1V 65 200 VCM = 7.5V 190 300 VCM = 1V 45 130 VCM = 7.5V 5 110 Input Common-mode Voltage Range VOUT Swing 0 RL=10k, Vo min. 7.94 RL=2k, Vo min. Large Signal Voltage Gain RL=2k, Vo max. 7.9 No Load, Vo = 2V to 7V 74.8 87.6 RL=10 kΩ, Vo = 2V to 7V 66.8 75.1 Supply Voltage CMRR Common Mode Rejection Ratio 4 VCM stepped from 3V to 8V 80 105 VCM stepped from 0V to 8V 57 80.7 70 77 Is+ Supply Current (Amplifier + Buffer) Vo = AVIN/2, No Load ISC Output Short Circuit Current Source Sink .02 nA V V dB 12 91.7 VCM = 0.5V, AVIN = 4 to 12V nA 55.8 72 Power Supply Rejection Ratio mV 7.95 VCM stepped from 0V to 1.1V PSRR Units .02 7.98 0.003 RL=2 kΩ, Vo = 2V to 7V AVIN 8 0.003 RL=10k, Vo max. AVOL Max (Note 5) Input Offset Voltage (Note 10) Input Offset Current VCM = 1V Typ (Note 6) V dB dB 2.2 4 40 50 70 40 50 60 mA mA Electrical Characteristics Specifications in standard type face are for TJ = 25˚C and those with boldface type apply over the full Operating Temperature Range ( TJ = −40˚C to +125˚C). Unless otherwise specified, VIN =2.2V and AVIN = 8V, RCOM = RGAMMA = 50Ω, CCOM = CGAMMA = 1nF. Gamma Buffer Symbol Parameter VOS Input Offset Voltage (Note 10) IB Input Bias Current VGR Gamma Input Voltage Range VOUT Swing Conditions Min (Note 5) RL=10k, Vo min. PSRR www.national.com Power Supply Rejection Ratio Units 1 10 mV 170 300 nA 8 V 0.05 7.9 RL=2k, Vo min. Voltage Gain Max (Note 5) 0 RL=10k, Vo max. AVCL Typ (Note 6) 7.94 0.05 RL=2k, Vo max. 7.865 7.9 No Load, Vo = 2V to 7V 0.995 0.999 RL=10 kΩ, Vo = 2V to 7V 0.995 0.999 RL=2 kΩ, Vo = 2V to 7V 0.993 0.998 70 77 AVIN = 4 to 12V 6 0.075 0.075 V V/V dB (Continued) Specifications in standard type face are for TJ = 25˚C and those with boldface type apply over the full Operating Temperature Range ( TJ = −40˚C to +125˚C). Unless otherwise specified, VIN =2.2V and AVIN = 8V, RCOM = RGAMMA = 50Ω, CCOM = CGAMMA = 1nF. Gamma Buffer Symbol Parameter Conditions Min (Note 5) Typ (Note 6) Max (Note 5) Units 12 V 2.2 4 mA AVIN Supply Voltage Is+ Supply Current (Amplifier + Buffer) Vo = AVIN/2, No Load ISC Output Short Circuit Current Source 50 66 75 Sink 40 56 65 4 mA Electrical Characteristics Specifications in standard type face are for TJ = 25˚C and those with boldface type apply over the full Operating Temperature Range ( TJ = −40˚C to +125˚C). Unless otherwise specified, VIN =2.2V and AVIN = 8V, RCOM = RGAMMA = 50Ω, CCOM = CGAMMA = 1nF. PMOS Switch Logic Control Symbol Parameter Conditions Min (Note 5) Typ (Note 6) Max (Note 5) Units 5.7 6.1 µA 7.3 20 IDELAY Delay Current RDSON PMOS Switch ON Resistance ISWO PMOS Switch Current Switch ON 20 ISWI PMOS Switch Input Current SWC = 0V, SWO Open, SWI = 30V 32 SWC = 1.7V, SWO Open, SWI = 30V 118 VSWC 5.1 Switch ON Ω mA µA 1.5 Switch OFF 1.1 1.1 0.7 V Note 1: The maximum allowable power dissipation is a function of the maximum junction temperature, TJ(MAX), the junction-to-ambient thermal resistance, θJA, and the ambient temperature, TA. See the Electrical Characteristics table for the thermal resistance of various layouts. The maximum allowable power dissipation at any ambient temperature is calculated using: PD (MAX) = (TJ(MAX) − TA)/θJA. Exceeding the maximum allowable power dissipation will cause excessive die temperature, and the regulator will go into thermal shutdown. Note 2: Absolute maximum ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions for which the device is intended to be functional, but device parameter specifications may not be guaranteed. For guaranteed specifications and test conditions, see the Electrical Characteristics. Note 3: The human body model is a 100 pF capacitor discharged through a 1.5kΩ resistor into each pin. The machine model is a 200pF capacitor discharged directly into each pin. Note 4: Vcom pin is rated for 1.5kV Human Body Model and 150V Machine Model. Note 5: All limits guaranteed at room temperature (standard typeface) and at temperature extremes (bold typeface). All room temperature limits are 100% production tested or guaranteed through statistical analysis. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL). Note 6: Typical numbers are at 25˚C and represent the most likely norm. Note 7: Duty cycle affects current limit due to ramp generator. Current limit is at 0% duty cycle and will decrease with higher duty cycles. See Typical Performance Characteristics for a graph of Power Switch Current Limit vs. VIN and Power Switch Current Limit vs. Temp. Note 8: See the graph titled "RDSON vs. VIN" for a more accurate value of the power switch RDSON. Note 9: Bias current flows into FB pin. Note 10: Refer to the graphs titled "Input Offset Voltage vs. Common Mode Voltage". 7 www.national.com LM2702 Electrical Characteristics LM2702 Typical Performance Characteristics Efficiency vs. Load Current (VOUT = 10V) Efficiency vs. Load Current (VOUT = 8V) 20051126 20051173 Power Switch Current Limit vs. Temperature (VOUT = 8V) Frequency vs. VIN 20051120 20051125 RDSON vs. VIN (ISW = 1A) Power Switch Current Limit vs. VIN 20051127 20051122 www.national.com 8 LM2702 Typical Performance Characteristics (Continued) IQ vs. VIN (not switching) IQ vs. VIN (switching) 20051129 20051121 Feedback Current vs. Temperature Soft Start Current vs. VIN 20051164 20051163 Delay Current vs. VIN PMOS RDSON vs. SWI Voltage 20051166 20051165 9 www.national.com LM2702 Typical Performance Characteristics (Continued) SWI Current vs. SWI Voltage (PMOS ON) SWI Current vs. SWI Voltage (PMOS OFF) 20051168 20051167 Load Transient Response PMOS Switching Waveform 20051116 20051158 VOUT = 8V, VIN = 2.5V VOUT = 8V, VIN = 2.5V, RLOAD = 40Ω, CSS = none 1) Load, 20mA to 155mA to 20mA, DC CD = 100nF, RSW = 10k\1.5k, SWI = 30V, 10% duty cycle 2) VOUT, 200mV/div, AC 1) SWC, 1V/div, DC 3) IL, 500mA/div, DC 2) SWO, 10V/div, DC T = 50µs/div T = 2.5µs/div PMOS Rising Edge PMOS Falling Edge 20051159 20051160 VOUT = 8V, VIN = 2.5V, RLOAD = 40Ω, CSS = none CD = 100nF, RSW = 10k\1.5k, SWI = 30V VOUT = 8V, VIN = 2.5V, RLOAD = 40Ω, CSS = none CD = 100nF, RSW = 10k\1.5k, SWI = 30V 1) SWC, 1V/div, DC 1) SWC, 1V/div, DC 2) SWO, 10V/div, DC 2) SWO, 10V/div, DC T = 50ns/div T = 50ns/div www.national.com 10 LM2702 Typical Performance Characteristics (Continued) Internal Soft Start and PMOS Delay External Soft Start and PMOS Delay 20051162 20051161 VOUT = 8V, VIN = 2.5V, RLOAD = 40Ω, CSS = 330nF CD = 100nF, RSW = 10k\1.5k, SWI = 30V, SWC = VIN VOUT = 8V, VIN = 2.5V, RLOAD = 40Ω, CSS = none CD = 100nF, RSW = 10k\1.5k, SWI = 30V, SWC = VIN 1) VIN, 2V/div, DC 1) VIN, 2V/div, DC 2) VOUT, 10V/div, DC 3) IL, 500mA/div, DC 2) VOUT, 10V/div, DC 3) IL, 500mA/div, DC 4) SWO, 20V/div, DC 4) SWO, 20V/div, DC T = 5ms/div T = 5ms/div Input Offset Voltage vs. Common Mode Voltage (Vcom, 3 units) Input Offset Voltage vs. Common Mode Voltage (Vcom Over Temperature) 20051175 20051174 Input Offset Voltage vs. Common Mode Voltage (Gamma, 3 units) Input Offset Voltage vs. Common Mode Voltage (Gamma Over Temperature) 20051176 20051177 11 www.national.com LM2702 Typical Performance Characteristics (Continued) Input Bias Current vs. Common Mode Voltage (Vcom) Input Bias Current vs. Common Mode Voltage (Gamma) 20051178 20051179 Output Voltage vs. Output Current (Vcom or Gamma, sinking) Output Voltage vs. Output Current (Vcom or Gamma, sourcing) 20051180 20051181 Supply Current vs. Common Mode Voltage (Both Amplifiers) Large Signal Step Response (50Ω, 1nF ext. compensation) 20051183 20051182 www.national.com 12 (Continued) Large Signal Step Response (no ext. compensation) Positive Slew Rate vs. Capacitive Load (Vcom or Gamma) 20051184 20051190 Negative Slew Rate vs. Capacitive Load (Vcom or Gamma) Phase Margin vs. Capacitive Load (Vcom) 20051185 20051186 Unity Gain Frequency vs. Capacitive Load (Vcom) CMRR vs. Frequency (Vcom) 20051188 20051187 13 www.national.com LM2702 Typical Performance Characteristics LM2702 Typical Performance Characteristics (Continued) PSRR vs. Frequency (Vcom) 20051189 www.national.com 14 LM2702 Operation 20051102 FIGURE 1. Simplified Boost Converter Diagram (a) First Cycle of Operation (b) Second Cycle Of Operation SETTING THE OUTPUT VOLTAGE The output voltage is set using the feedback pin and a resistor divider connected to the output as shown in the typical operating circuit. The feedback pin voltage is 1.265V, so the ratio of the feedback resistors sets the output voltage according to the following equation: CONTINUOUS CONDUCTION MODE The LM2702 is a TFT Panel Module containing a current-mode, PWM boost regulator. A boost regulator steps the input voltage up to a higher output voltage. In continuous conduction mode (when the inductor current never reaches zero at steady state), the boost regulator operates in two cycles. In the first cycle of operation, shown in Figure 1 (a), the transistor is closed and the diode is reverse biased. Energy is collected in the inductor and the load current is supplied by COUT. The second cycle is shown in Figure 1 (b). During this cycle, the transistor is open and the diode is forward biased. The energy stored in the inductor is transferred to the load and output capacitor. The ratio of these two cycles determines the output voltage. The output voltage is defined approximately as: SOFT-START CAPACITOR The LM2702 has patented internal circuitry that is used to limit the inductor inrush current on start-up of the boost DC/DC switching regulator. This inrush current limiting circuitry serves as a soft-start. However, many applications may require much more soft-start than what is available with the internal circuitry. The external SS pin is used to tailor the soft-start for a specific application. A 12µA current charges the external soft-start capacitor, CSS. The soft-start time can be estimated as: TSS = CSS*0.6V/12µA The minimum soft-start time is set by the internal soft-start circuitry, typically 7ms. Only longer soft-start times may be implemented using the SS pin and a capacitor CSS. If a shorter time is designed for using the above equation, the internal soft-start circuitry will override it. Due to the unique nature of the dual internal/external softstart, care was taken in the design to ensure temperature stable operation. As you can see with the Iss data in the Electrical Characterisitcs table and the graph "Soft-Start Current vs. VIN" in the Typical Performance Characterisitcs where D is the duty cycle of the switch, D and D' will be required for design calculations 15 www.national.com LM2702 Operation reaches 85% of the nominal output voltage. When this occurs, CD begins to charge. When the voltage on the Delay pin reaches 1.265V the PMOS switch will become active and can be controlled using the SWC pin. If no CD is used, the PMOS switch can be controlled immediately after VOUT reaches 85% of the nominal output voltage. The delay time can be calculated using the equation: TD = CD * (1.265V/5.7µA) (Continued) section, the soft start curent has a temperature coefficient and would lead one to believe there would be significant variation with temperature. Though the current has a temperature coefficient the actual programmed external soft start time does not show this extreme of a temperature variation. As you can see in the following transient plots: VOUT = 8V, VIN = 2.5V, RL = 51Ω, CSS = 330nF, T = 4ms/div. INTRODUCTION TO COMPENSATION Trace: 1) VIN, 5V/div, DC Coupled 2) VOUT, 5V/div, DC Coupled 3) IL, 0.5A/div, DC Coupled 4) VSW, 5V/div, DC Coupled 20051169 TA = −20˚C 20051105 FIGURE 2. (a) Inductor current. (b) Diode current. The LM2702 contains a current mode PWM boost converter. The signal flow of this control scheme has two feedback loops, one that senses switch current and one that senses output voltage. To keep a current programmed control converter stable above duty cycles of 50%, the inductor must meet certain criteria. The inductor, along with input and output voltage, will determine the slope of the current through the inductor (see Figure 2 (a)). If the slope of the inductor current is too great, the circuit will be unstable above duty cycles of 50%. A 4.7µH inductor is recommended for most applications. If the duty cycle is approaching the maximum of 85%, it may be necessary to increase the inductance by as much as 2X. See Inductor and Diode Selection for more detailed inductor sizing. The LM2702 provides a compensation pin (VC) to customize the voltage loop feedback. It is recommended that a series combination of RC and CC be used for the compensation network, as shown in the typical application circuit. For any given application, there exists a unique combination of RC and CC that will optimize the performance of the LM2702 circuit in terms of its transient response. The series combination of RC and CC introduces a pole-zero pair according to the following equations: 20051170 TA = 27˚C 20051171 TA = 85˚C When programming the softstart time externally, simply use the equation given in the Soft-Start Capacitor section above. This equation uses the typical room temperature value of the soft start current, 12µA, to set the soft start time. DELAY CAPACITOR The LM2702 has internal circuitry that can be used to set a delay time preventing control of the PMOS switch via SWC until a desired amount of time after the switcher starts up. The PMOS control circuitry remains inactive until VOUT www.national.com 16 The output diode for a boost regulator must be chosen correctly depending on the output voltage and the output current. The typical current waveform for the diode in continuous conduction mode is shown in Figure 2 (b). The diode must be rated for a reverse voltage equal to or greater than the output voltage used. The average current rating must be greater than the maximum load current expected, and the peak current rating must be greater than the peak inductor current. During short circuit testing, or if short circuit conditions are possible in the application, the diode current rating must exceed the switch current limit. Using Schottky diodes with lower forward voltage drop will decrease power dissipation and increase efficiency. (Continued) where RO is the output impedance of the error amplifier, approximately 1MΩ. For most applications, performance can be optimized by choosing values within the range 5kΩ ≤ RC ≤ 40kΩ (RC can be up to 200kΩ if CC2 is used, see High Output Capacitor ESR Compensation) and 680pF ≤ CC ≤ 4.7nF. Refer to the Typical Application Circuit and the Applications Information section for recommended values for specific circuits and conditions. Refer to the Compensation section for other design requirement. DC GAIN AND OPEN-LOOP GAIN Since the control stage of the converter forms a complete feedback loop with the power components, it forms a closedloop system that must be stabilized to avoid positive feedback and instability. A value for open-loop DC gain will be required, from which you can calculate, or place, poles and zeros to determine the crossover frequency and the phase margin. A high phase margin (greater than 45˚) is desired for the best stability and transient response. For the purpose of stabilizing the LM2702, choosing a crossover point well below where the right half plane zero is located will ensure sufficient phase margin. A discussion of the right half plane zero and checking the crossover using the DC gain will follow. COMPENSATION FOR BOOST DC/DC This section will present a general design procedure to help insure a stable and operational circuit. The designs in this datasheet are optimized for particular requirements. If different conversions are required, some of the components may need to be changed to ensure stability. Below is a set of general guidelines in designing a stable circuit for continuous conduction operation (Inductor current never reaches zero), in most all cases this will provide for stability during discontinuous operation as well. The power components and their effects will be determined first, then the compensation components will be chosen to produce stability. INDUCTOR AND DIODE SELECTION Although the inductor size mentioned earlier is fine for most applications, a more exact value can be calculated. To ensure stability at duty cycles above 50%, the inductor must have some minimum value determined by the minimum input voltage and the maximum output voltage. This equation is: INPUT AND OUTPUT CAPACITOR SELECTION The switching action of a boost regulator causes a triangular voltage waveform at the input. A capacitor is required to reduce the input ripple and noise for proper operation of the regulator. The size used depends on the application and board layout. If the regulator will be loaded uniformly, with very little load changes, and at lower current outputs, the input capacitor size can often be reduced. The size can also be reduced if the input of the regulator is very close to the source output. The size will generally need to be larger for applications where the regulator is supplying nearly the maximum rated output or if large load steps are expected. A minimum value of 10µF should be used for the less stressful conditions while a 22µF to 47µF capacitor may be required for higher power and dynamic loads. Larger values and/or lower ESR may be needed if the application requires very low ripple on the input source voltage. The choice of output capacitors is also somewhat arbitrary and depends on the design requirements for output voltage ripple. It is recommended that low ESR (Equivalent Series Resistance, denoted RESR) capacitors be used such as ceramic, polymer electrolytic, or low ESR tantalum. Higher ESR capacitors may be used but will require more compensation which will be explained later on in the section. The ESR is also important because it determines the peak to peak output voltage ripple according to the approximate equation: ∆VOUT ) 2∆iLRESR (in Volts) A minimum value of 10µF is recommended and may be increased to a larger value. After choosing the output capacitor you can determine a pole-zero pair introduced into the control loop by the following equations: where fs is the switching frequency, D is the duty cycle, and RDSON is the ON resistance of the internal switch taken from the graph "RDSON vs. VIN" in the Typical Performance Characteristics section. This equation is only good for duty cycles greater than 50% (D > 0.5), for duty cycles less than 50% the recommended values may be used. The corresponding inductor current ripple as shown in Figure 2 (a) is given by: The inductor ripple current is important for a few reasons. One reason is because the peak switch current will be the average inductor current (input current or ILOAD/D’) plus ∆iL. As a side note, discontinuous operation occurs when the inductor current falls to zero during a switching cycle, or ∆iL is greater than the average inductor current. Therefore, continuous conduction mode occurs when ∆iL is less than the average inductor current. Care must be taken to make sure that the switch will not reach its current limit during normal operation. The inductor must also be sized accordingly. It should have a saturation current rating higher than the peak inductor current expected. The output and input voltage ripples are also affected by the total ripple current. 17 www.national.com LM2702 Operation LM2702 Operation circuit. For improved transient response, higher values of RC should be chosen. This will improve the overall bandwidth which makes the regulator respond more quickly to transients. If more detail is required, or the most optimal performance is desired, refer to a more in depth discussion of compensating current mode DC/DC switching regulators. (Continued) Where RL is the minimum load resistance corresponding to the maximum load current. The zero created by the ESR of the output capacitor is generally very high frequency if the ESR is small. If low ESR capacitors are used it can be neglected. If higher ESR capacitors are used see the High Output Capacitor ESR Compensation section. HIGH OUTPUT CAPACITOR ESR COMPENSATION When using an output capacitor with a high ESR value, or just to improve the overall phase margin of the control loop, another pole may be introduced to cancel the zero created by the ESR. This is accomplished by adding another capacitor, CC2, directly from the compensation pin VC to ground, in parallel with the series combination of RC and CC. The pole should be placed at the same frequency as fZ1, the ESR zero. The equation for this pole follows: RIGHT HALF PLANE ZERO A current mode control boost regulator has an inherent right half plane zero (RHP zero). This zero has the effect of a zero in the gain plot, causing an imposed +20dB/decade on the rolloff, but has the effect of a pole in the phase, subtracting another 90˚ in the phase plot. This can cause undesirable effects if the control loop is influenced by this zero. To ensure the RHP zero does not cause instability issues, the control loop should be designed to have a bandwidth of less than 1⁄2 the frequency of the RHP zero. This zero occurs at a frequency of: To ensure this equation is valid, and that CC2 can be used without negatively impacting the effects of RC and CC, fPC2 must be greater than 10fZC. CHECKING THE DESIGN The final step is to check the design. This is to ensure a bandwidth of 1⁄2 or less of the frequency of the RHP zero. This is done by calculating the open-loop DC gain, ADC. After this value is known, you can calculate the crossover visually by placing a −20dB/decade slope at each pole, and a +20dB/decade slope for each zero. The point at which the gain plot crosses unity gain, or 0dB, is the crossover frequency. If the crossover frequency is less than 1⁄2 the RHP zero, the phase margin should be high enough for stability. The phase margin can also be improved by adding CC2 as discussed earlier in the section. The equation for ADC is given below with additional equations required for the calculation: where ILOAD is the maximum load current. SELECTING THE COMPENSATION COMPONENTS The first step in selecting the compensation components RC and CC is to set a dominant low frequency pole in the control loop. Simply choose values for RC and CC within the ranges given in the Introduction to Compensation section to set this pole in the area of 10Hz to 500Hz. The frequency of the pole created is determined by the equation: where RO is the output impedance of the error amplifier, approximately 1MΩ. Since RC is generally much less than RO, it does not have much effect on the above equation and can be neglected until a value is chosen to set the zero fZC. fZC is created to cancel out the pole created by the output capacitor, fP1. The output capacitor pole will shift with different load currents as shown by the equation, so setting the zero is not exact. Determine the range of fP1 over the expected loads and then set the zero fZC to a point approximately in the middle. The frequency of this zero is determined by: mc ) 0.181fs (in V/s) Now RC can be chosen with the selected value for CC. Check to make sure that the pole fPC is still in the 10Hz to 500Hz range, change each value slightly if needed to ensure both component values are in the recommended range. After checking the design at the end of this section, these values can be changed a little more to optimize performance if desired. This is best done in the lab on a bench, checking the load step response with different values until the ringing and overshoot on the output voltage at the edge of the load steps is minimal. This should produce a stable, high performance www.national.com where RL is the minimum load resistance, VIN is the minimum input voltage, gm is the error amplifier transconductance found in the Electrical Characteristics table, and RDSON is the value chosen from the graph "RDSON vs. VIN " in the Typical Performance Characteristics section. 18 LAYOUT CONSIDERATIONS The LM2702 uses a single ground connection, GND. The feedback, softstart, delay, and compensation networks should be connected directly to a dedicated analog ground plane and this ground plane must connect to the GND pin, as shown in Figure 3. If no analog ground plane is available then the ground connections of the feedback, softstart, delay, and compensation networks must tie directly to the GND pin, as show in Figure 4. Connecting these networks to the PGND plane can inject noise into the system and effect performance. The input bypass capacitor CIN must be placed close to the IC. This will reduce copper trace resistance which effects input voltage ripple of the IC. For additional input voltage filtering, a 100nF bypass capacitor can be placed in parallel with CIN, close to the VIN pin, to shunt any high frequency noise to ground. The output capacitor, COUT, should also be placed close to the IC. Any copper trace connections for the COUT capacitor can increase the series resistance, which directly effects output voltage ripple and efficiency. The feedback network, resistors R1 and R2, should be kept close to the FB pin, and away from the inductor, to minimize copper trace connections that can inject noise into the system. Trace connections made to the inductor and schottky diode should be minimized to reduce power dissipation and increase overall efficiency. (Continued) Vcom AND Gamma COMPENSATION The architecture used for the amplifiers in the LM2702 requires external compensation on the output. Depending on the equivalent capacitive load of the TFT-LCD panel, external components at the amplifier outputs may or may not be necessary. If the capacitance presented by the load is equal to or greater than 5nF no external components are needed as the TFT-LCD panel will act as compensation itself. Distributed resistive and capacitive loads enhance stability and increase performance of the amplifiers. If the capacitance presented by the load is less than 5nF external components will be required as the load itself will not ensure stability. No external compensation in this case will lead to oscillation of the amplifier and an increase in power consumption. A single 5nF or greater capacitor on the output will ensure a stable amplifier with no oscillations. For applications requiring a higher slew rate, a good choice for compensation is to add a 50Ω (RCOM or RGAMMA) in series with a 1nF (CCOM or CGAMMA) capacitor from the output of the amplifier to ground. This allows for driving zero to infinite capacitance loads with no oscillations, minimal overshoot, and a higher slew rate than using a large capacitor. The high phase margin created by the external compensation will guarantee stability and good performance for all conditions. For noise sensitive applications greater output capacitance may be desired. When the power supply for the amplifiers (AVIN) is connected to the output of the switching regulator, the output ripple of the regulator will produce ripple at the output of the amplifiers. 20051152 FIGURE 3. Multi-Layer Layout 19 www.national.com LM2702 Operation LM2702 Operation (Continued) 20051153 FIGURE 4. Single Layer Layout Application Information 20051172 FIGURE 5. 5V to 10V TFT Application www.national.com 20 LM2702 TFT Panel Module Physical Dimensions inches (millimeters) unless otherwise noted TSSOP-16 Pin Package (MTC) For Ordering, Refer to Ordering Information Table NS Package Number MTC16 LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. 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