19-1406; Rev 1; 1/99 KIT ATION EVALU SHEET A T A WS D FOLLO ECB and LCD Display Bias Supply with Accurate Output Voltage and Temperature Compensation The MAX1729 micropower step-up/step-down DC-DC converter is ideally suited for electrically controlled birefringence (ECB) and liquid-crystal-display (LCD) biassupply generation. It provides step-up/step-down voltage conversion and reduces output ripple by using a step-up DC-DC converter followed by a linear regulator. This architecture permits a physically smaller inductor than those used in competing SEPIC and flyback topologies. This device features low quiescent current (67µA typical). A logic-controlled shutdown mode further reduces quiescent current to 0.4µA typical. The MAX1729 features an input that dynamically adjusts the output voltage to control display color or contrast. It offers two feedback modes: internal and external. Internal feedback mode allows output voltages between 2.5V and 16V, and is specifically designed to hold temperature drift to ±11ppm/°C. External feedback mode allows the MAX1729 output voltage range to be tailored for various displays. An on-chip temperature sensor with a positive temperature coefficient provides compensation for LCD/ECB display temperature characteristics. In internal feedback mode, the buffered temperature sensor output is read and used to adjust the output voltage via a digital control signal. External feedback mode features an additional compensation method in which the temperature output is summed directly into the feedback network to provide first-order negative temperature compensation of the output voltage. The MAX1729 is available in the space-saving 10-pin µMAX package. Features ♦ High-Accuracy Reference Voltage (±1%) ♦ ±11ppm/°C Output Voltage Drift ♦ On-Chip Temperature Sensor Output ♦ Accurate Voltage and Temperature Provide: Consistent ECB Colors Consistent LCD Gray-Scale Contrast ♦ +2.7V to +5.5V Input Voltage Range ♦ Output Voltage Range +2.5V to +16V in Internal Feedback Mode Programmable in External Feedback Mode ♦ Dynamic Control of the Output Voltage ♦ 67µA Supply Current ♦ 0.4µA Shutdown Current ♦ 10-Pin µMAX Package (1.09mm max height) ♦ Evaluation Kit Available (MAX1729EVKIT) Ordering Information PART MAX1729EUB TEMP. RANGE PIN-PACKAGE -40°C to +85°C 10 µMAX Typical Operating Circuit Applications ECB Display Bias & Color Adjustment LCD Display Bias & Contrast Adjustment VIN 2.7V to 5.5V Cellular Phones Personal Digital Assistants IN GND Pin Configuration REF TOP VIEW IN 1 10 GND TC 2 9 LX REF 3 8 PS COMP 4 7 OUT FB 5 6 CTLIN MAX1729 LX PS DIGITAL PWM CONTROLLER ADC MAX1729 CTLIN TC OUT VOUT +2.5V to +16V FB COMP µMAX ________________________________________________________________ Maxim Integrated Products 1 For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800. For small orders, phone 1-800-835-8769. MAX1729 General Description MAX1729 ECB and LCD Display Bias Supply with Accurate Output Voltage and Temperature Compensation ABSOLUTE MAXIMUM RATINGS IN to GND .................................................................-0.3V to +6V LX, PS, OUT to GND...............................................-0.3V to +20V CTLIN, FB, REF, COMP, TC to GND ...........-0.3V to (VIN + 0.3V) LX to PS ..................................................................-20V to +1.0V LX, PS, OUT Current ...........................................................60mA Continuous Power Dissipation (TA = +70°C) 10-pin µMAX (derate 5.6mW/°C above +70°C) ..........444mW Operating Temperature Range ...........................-40°C to +85°C Junction Temperature ......................................................+150°C Storage Temperature Range .............................-65°C to +160°C Lead Temperature (soldering, 10sec) .............................+300°C Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. ELECTRICAL CHARACTERISTICS (VIN = +3V, CTLIN = IN, FB = GND, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note 1) MAX UNITS Operating Voltage Range PARAMETER VIN 2.7 5.5 V Undervoltage Lockout Threshold (Note 2) VLO 2.0 2.6 V 37 50 µA IN Supply Current PS Supply Current Shutdown Supply Current SYMBOL IPS ISHDN VREF Minimum Output Voltage VOUT (MIN) VOUT (MAX) Output Voltage Temperature Coefficient TCOUT Maximum Output Current IOUT TC Output Voltage VTC TC Output Current Feedback Set Voltage (FB) FB Mode Threshold FB Bias Current CTLIN High Voltage 2 CTLIN = GND, ISHDN = IIN + IPS IREF = 0 FB = GND, CTLIN = 0.1% duty cycle, IOUT = 0 to 0.5mA TCTC VPS = +18V (Note 3) VIH TA = -40°C to +85°C 1.200 TA = 0°C to +85°C 2.35 TA = -40°C to +85°C 2.35 30 40 µA 0.4 2 µA 1.228 1.241 1.256 2.45 V 2.5 V 2.52 16 16.40 TA = 0°C to +85°C 13.90 13.95 TA = -40°C to +85°C 13.60 V 14.00 V/100% 14.20 TA = 0°C to +85°C ±11 ±30 TA = -40°C to +85°C ±18 ±65 0.5 2.5 1.178 1.228 1.278 TA = 0°C to +85°C 15.5 16.5 17.5 TA = -40°C to +85°C 14.5 16.5 18.5 TA = +25°C 1.215 1.228 1.241 TA = -40°C to +85°C 1.200 90 VFB = +1.25V VIN = +5.5V 2 VIN = +2.7V 1.3 ppm/°C mA ±50 VMODE IFB 1.215 TYP TA = +25°C ITC VFB TA = +25°C IOUT = 0 to 0.5mA FB = GND, CTLIN = 0.1% to 100% duty cycle, IOUT = 0 CTLIN to VOUT Gain TC Output Temperature Coefficient (Note 3) MIN IIN Reference Output Voltage Maximum Output Voltage CONDITIONS V mV/°C µA 1.256 V 122 150 mV 5 50 nA _______________________________________________________________________________________ V ECB and LCD Display Bias Supply with Accurate Output Voltage and Temperature Compensation MAX1729 ELECTRICAL CHARACTERISTICS (VIN = +3V, CTLIN = IN, FB = GND, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note 1) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS CTLIN Low Voltage VIL VIN = +2.7V to +5.5V 0.4 V CTLIN Bias Current IIHL VIN = +5.5V, CTLIN = GND or IN ±1 COMP Impedance RCOMP µA kΩ CTLIN Minimum Pulse Width for Shutdown tOFF CTLIN Minimum Pulse Width for VOUT Control tCTLIN VFB = 0, Internal Feedback Mode 33 VFB = +1.25V, External Feedback Mode 60 150 Ω 1250 2400 µs (Note 4) COMP Rise/Fall Time tR/tF VFB = +1.25V Switch On-Resistance RON ILX = 30mA Switch Off-Leakage Current ILX(OFF) VLX = 18V LX to PS Diode Forward Voltage VLX-PS IDIODE = 30mA PFM On-Time Constant K PS to OUT Voltage (Note 5) Note 1: Note 2: Note 3: Note 4: Note 5: TA = +25°C TA = -40°C to +85°C 700 25 ns 20 ns VIN = +2.7V 2.5 VIN = +5.5V 1.5 6 5 0.4 5.0 Ω 0.1 1 µA 700 970 mV 8 10 0.6 11 1.0 V-µs V Specifications to -40°C are guaranteed by design, not production tested. When VIN is below this level, the boost and LDO outputs are disabled. Guaranteed by design. Minimum time to hold CTLIN low to invoke shutdown. If CTLIN is held low for less than tOFF, device does not enter shutdown. Switching regulator regulates this voltage to keep LDO from dropping out. _______________________________________________________________________________________ 3 Typical Operating Characteristics (Circuit of Figure 2, TA = +25°C, unless otherwise noted.) VIN = 5.5V VIN = 5.5V 0.30 0.20 20 10 10 0.10 0 0.05 0.1 1 10 0.01 0.1 1 OUTPUT CURRENT (mA) MAXIMUM OUTPUT CURRENT vs. SUPPLY VOLTAGE PS TO OUT (LDO) POWER-SUPPLY REJECTION RATIO -20 5 PSRR (dB) VOUT = 9.4V 4 3 2 -40 -60 VOUT = 16.4V -80 1 0 2.5 3.5 4.0 4.5 5.0 5.5 6.0 3.5 4.0 5.0 1.5 1.0 0 1 10 100 1k 10k 2.5 100k 3.0 3.5 4.0 4.5 5.0 OUTPUT VOLTAGE vs. DUTY CYCLE START-UP DELAY FROM SHUTDOWN DELAY TO SHUTDOWN MAX1729 toc 07 12 VCTLIN 5V/div VCTLIN 5V/div VREF 1V/div VREF 1V/div 10 5.5 8 VTC 1V/div VTC 1V/div 6 4 VOUT 10V/div 2 VIN = 5V IO = 0.5mA 0 0 10 20 30 40 50 60 70 80 90 100 20ms/div 6.0 MAX1729 toc 09a INPUT VOLTAGE (V) 14 6.0 2.0 FREQUENCY (Hz) 16 5.5 2.5 SUPPLY VOLTAGE (V) 18 VOUT 10V/div VIN = 5V IO = 0.5mA 500ms/div DUTY CYCLE (%) 4 4.5 0.5 MAX1729 toc 08 3.0 3.0 3.0 -100 2.5 VOUT = 2.5V SHUTDOWN SUPPLY CURRENT MAX1729 toc 05 VOUT = +2.5V 6 VOUT = 9.4V SUPPLY VOLTAGE (V) 0 MAX1729 toc 04 7 0.15 10 OUTPUT CURRENT (mA) SHUTDOWN CURRENT (µA) 0.01 VOUT = 16.4V 0.25 20 0 MAXIMUM OUTPUT CURRENT (mA) VIN = 2.7V 30 0.35 SUPPLY CURRENT (mA) EFFICIENCY (%) EFFICIENCY (%) 30 40 MAX1729 toc 03 50 VIN = 2.7V 40 0.40 MAX1729 toc 02 50 NO-LOAD SUPPLY CURRENT vs. SUPPLY VOLTAGE 60 MAX1729 toc 01 60 EFFICIENCY vs. OUTPUT CURRENT VOUT = 16.4 (CTLIN = IN) MAX1729 toc 06 EFFICIENCY vs. OUTPUT CURRENT VOUT = 9.4 (CTLIN AT 50% DUTY CYCLE) OUTPUT VOLTAGE (V) MAX1729 ECB and LCD Display Bias Supply with Accurate Output Voltage and Temperature Compensation _______________________________________________________________________________________ ECB and LCD Display Bias Supply with Accurate Output Voltage and Temperature Compensation IL 20mA/div OUTPUT RIPPLE 10mV/div OUTPUT RIPPLE 10mV/div VTC 1V/div VOUT 10V/div VIN = 5V IO = 0.5mA IL 20mA/div VLX 5V/div VLX 5V/div 2µs/div 5ms/div 2µs/div LOAD-TRANSIENT RESPONSE MAX1729 toc 13 LINE-TRANSIENT RESPONSE VOUT 20mV/div CTLIN = IN IOUT = 0.5mA 16.404 VOUT 20mV/div MAX1729 toc 14 VREF 1V/div MAX1729 toc 10 MAX1729 toc 09b VCTLIN 5V/div SWITCHING WAVEFORMS MEDIUM LOAD MAX1729 toc 11 SWITCHING WAVEFORMS HEAVY LOAD DELAY TO SHUTDOWN 16.400 500µA 5V VIN 1V/div 4V IOUT 250µA/div 20µA 1ms/div 2ms/div _______________________________________________________________________________________ 5 MAX1729 Typical Operating Characteristics (continued) (Circuit of Figure 2, TA = +25°C, unless otherwise noted.) ECB and LCD Display Bias Supply with Accurate Output Voltage and Temperature Compensation MAX1729 Pin Description PIN NAME FUNCTION 1 IN Supply Input. Bypass with 0.1µF capacitor to ground. Connect to supply side of inductor (L1). 2 TC Temperature-Sensor Output. Bypass to GND with a 1000pF capacitor. 3 REF Reference Voltage Output. Bypass to GND with a 0.1µF capacitor. 4 COMP 5 FB 6 CTLIN 7 OUT 8 PS Output of boost converter and input to LDO. Bypass to GND with a 0.068µF capacitor. 9 LX Drain of the internal MOSFET Switch 10 GND Compensation Pin. In internal feedback mode (Figure 2), bypass with a 1µF capacitor. In external feedback mode, COMP is a buffered inverse version of CTLIN (Figure 3). Feedback and Mode Control Input. Connect to GND for internal feedback mode operation. Control Input. Drive low for more than 1.2ms to put the device into shutdown. Bypass to GND with a 1.0µF capacitor. Ground Detailed Description The MAX1729 is designed to provide bias voltage for ECB or LCD displays. It is composed of a step-up DC-DC converter followed by a linear regulator (Figure 1), a combination that provides step-up/stepdown voltage conversion while minimizing output ripple. The device allows you to adjust a display’s color or contrast by dynamically adjusting the MAX1729’s output voltage using a PWM control signal. In internal feedback mode, the output voltage is adjustable between +2.5V and +16V. In external feedback mode, the output voltage is adjustable, and its range is set by a resistor network that is programmed to match the output voltage range of LCD/ECB displays needing a maximum output up to +18V. Boost Converter The MAX1729’s DC-DC boost converter is implemented with an on-chip N-channel MOSFET, a diode, and an error comparator. The IC’s unique PFM control system varies the on-time and off-time of the switch based on the 6 boost converter’s input and output voltage values, as follows: t ON = t OFF ≥ K VIN K VPS − VIN where K is typically 8V-µs. This timing maintains discontinuous conduction and sets the peak inductor current (IPEAK) to: K IPEAK = L where L is the inductance of L1 (Figures 2, 3, and 4). When the error comparator detects that the drop across the linear regulator (VPS - VOUT) is less than approximately 0.6V, the internal switch is turned on (tON initiates) and current through the inductor ramps to IPEAK. At the end of tON, the switch is turned off for at least tOFF, allowing the _______________________________________________________________________________________ ECB and LCD Display Bias Supply with Accurate Output Voltage and Temperature Compensation TC REF TEMPERATURE SENSOR BOOST CONVERTER VOLTAGE REFERENCE LDO LINEAR REGULATOR PS Control Signal OUT MAX1729 FB FEEDBACK CONTROL 122mV An externally generated PWM control signal on CTLIN controls VOUT in internal feedback mode and influences VOUT in external feedback mode. In either mode, if CTLIN is held low for longer than 1.24ms, the MAX1729 enters shutdown mode, decreasing the supply current below 2µA. Shutdown mode limits the minimum duty cycle and frequency that may be used to keep the device active. CTLIN frequencies between 2kHz and 12kHz are recommended. Internal Feedback Mode SHUTDOWN CONTROL VREF CTLIN GND COMP GND NOTE: SWITCH STATES SHOWN FOR INTERNAL FEEDBACK MODE. Figure 1. Internal Block Diagram inductor current to ramp down and VPS to increase. If, at the end of tOFF, VPS - VOUT is still too low, then another tON is initiated immediately. Otherwise, the boost converter remains idle in a low-quiescent-current state until VPS - VOUT drops again and the error comparator initiates another cycle. Linear Regulator The PNP low-dropout linear regulator of the MAX1729 regulates the boost-converter output to the desired output voltage. The boost converter’s regulation circuitry holds the linear regulator’s input voltage (VPS) approximately 0.6V above the output voltage to keep the regulator out of dropout, thereby enhancing ripple rejection. The linear regulator incorporates short-circuit protection, which limits the output current to approximately 6mA. Temperature Sensor Output The MAX1729 generates a temperature sensor voltage (VTC) that varies at 16.5mV/°C (typ) and is nominally In internal feedback mode, the signal at CTLIN is inversely buffered, level-shifted, and output at COMP through a resistor. Internal resistance (33kΩ typical) and C6 then filter the signal before it is used by the internal feedback network to set VOUT. If temperature compensation is used, the temperature sensor output voltage is read by an ADC and used to adjust the duty cycle of the PWM control signal. See the Designing for Internal Feedback Mode section for more information. External Feedback Mode In external feedback mode, the output voltage of the MAX1729 is controlled by the duty cycle of the PWM control signal and an external resistor network, as shown in Figure 3. In this mode, the signal at CTLIN is inverted, level-shifted, and presented directly to COMP. R3, R4, and C6 filter the signal, before it is summed into the feedback node. Design Procedure Designing for Internal Feedback Mode For a 3kHz PWM control signal use a 1µF low-leakage ceramic capacitor for C6. For applications requiring a higher-frequency PWM control signal, reduce the value of C6 to between 1µF and 0.22µF for frequencies between 3kHz and 12kHz. Higher C6 values reduce output ripple. In Figure 2, VOUT is governed by the following equation: VOUT = VOUT(MIN) + Duty Cycle ⋅ Gain where V OUT(MIN) is 2.45V and Gain is nominally 13.95V/100%, as listed in the Electrical Characteristics. _______________________________________________________________________________________ 7 MAX1729 LX IN equal to the reference voltage at room temperature. TC is capable of sinking or sourcing 50µA. This output is used to compensate for ECB color or LCD contrast variations caused by changes in temperature. It may be read with an ADC and used to modify an external PWM control signal or, in external feedback mode, summed directly into the feedback-resistor network. MAX1729 ECB and LCD Display Bias Supply with Accurate Output Voltage and Temperature Compensation To use a DC control signal to adjust the output voltage, use the circuit shown in Figure 4. In this configuration, VOUT is governed by the following equation: VOUT ≈ 24.67VFB - 22.71VCOMP The impedance looking into COMP is nominally 33kΩ. A source output impedance of less than 500Ω is recommended. Also, ensure V OUT ≤ 18V by keeping VCOMP above 0.6V. External Component Value Formulas 1) Given the maximum output voltage needed (VMAX), choose the maximum feedback current and solve for R1 (10µA to 30µA is recommended for maximum feedback current) as follows: V - VFB R1 = MAX IFB Designing for External Feedback Mode L1 To solve for VOUT in external feedback mode, assume the current into the FB pin is zero and the voltage at FB is 1.228V. Then take the sum of the currents into FB and solve for VOUT: VIN C1 0.1µF 1 1 1 1 VOUT = R1 + + + V R1 R2 R 3 + R4 R5 FB C3 0.1µF R1 R1 − VCOMP − V R 3 + R4 R5 TC PWM GENERATOR 220µH IN LX PS GND REF MAX1729 C2 0.068µF VOUT OUT C4 1µF R1 CTLIN FB COMP TC R5 Using the following formulas, calculate the external component values required for MAX1729 operation in external feedback mode, as shown in Figure 3. An example follows the formulas. R4 R3 R2 C5 1000pF C6 1µF Figure 3. External Feedback Mode L1 L1 VIN C1 0.1µF C3 0.1µF DIGITAL PWM CONTROLLER 220µH IN LX PS GND REF MAX1729 CTLIN ADC TC C5 1000pF Figure 2. Internal Feedback Mode 8 VIN OUT C2 0.068µF C4 1µF VOUT 220µH C1 0.1µF C3 0.1µF IN LX PS GND REF MAX1729 CTLIN FB COMP C6 1µF C5 1000pF TC OUT C2 0.068µF C4 1µF VOUT FB COMP Figure 4. Using a DC Control Signal _______________________________________________________________________________________ DC CONTROL INPUT VCOMP ECB and LCD Display Bias Supply with Accurate Output Voltage and Temperature Compensation R1 R3 = 1/2 VFB VMAX – VMIN R4 = R3 3) For first-order temperature compensation, calculate R5 as shown below. (If temperature compensation is not used, leave R5 open.) R1 R5 = 16.5mV/ °C Tempco where Tempco is the negative temperature coefficient needed to compensate the ECB or LCD display for changes in temperature. 4) Solve for VCOMP. The duty cycle used here corresponds to the duty cycle that yields the maximum output voltage, not including first-order temperature compensation. R4 VCOMP =VFB 1 – Duty Cycle ⋅ R3 + R4 where a 90% duty cycle corresponds to Duty Cycle = 0.9. 5) Use the results from the above calculations to solve for R2. (For applications not utilizing temperature compensation, use 1 / R5 = 0.) 1 1 VOUT VCOMP VFB = + + R2 VFB R1 R3 R5 1 1 1 − + + R1 R3 R5 External Component Value Example The example application requires the output voltage to adjust between 5V and 10V, using the circuit shown in Figure 3. The device in our example needs a temperature coefficient of 33mV/°C, which yields the following results. 1) VMAX = 10V and IFB = 29.24µA is within the limits and yields a reasonable resistor value, therefore: R1 = 10V − 1.228V = 300kΩ 29.24µA 2) VMAX = 10V and VMIN = 5V, therefore: MAX1729 2) Given the maximum output voltage (VMAX) and minimum output voltage (VMIN), calculate values for R3 and R4 as follows: 300kΩ R3 = 1/2 1.228 = 36,840Ω 5V with R3 = 36.7kΩ, then VMIN = 5.019V. Let R4 = R3 = 36.7kΩ. 3) Tempco = 33mV/°C, therefore: 300kΩ R5 = 16.5mV / °C = 150kΩ 33mV/ °C 4) If external circuitry limits the duty cycle to 90%, the following equation is true: 0.9 VCOMP = 1.228 1 − = 0.6754V 2 5) Solving for R2: V V 1 V 1 = OUT + COMP + FB R2 R3 R5 V R1 FB 1 1 1 1 − + + = R1 R3 R5 56560 With R2 = 56kΩ, a duty cycle of 87.4% generates a VOUT of 10V. Component Selection Inductors Use a 220µH inductor to maximize output current (2.5mA typical). Use an inductor with DC resistance less than 10Ω and a saturation current exceeding 35mA. For lower peak inductor current, use a 470µH inductor with DC resistance less than 20Ω and a saturation current over 18mA. This limits output current to typically less than 1mA. See Table 1 for a list of recommended inductors. The inductor should be connected from the battery to the LX pin, as close to the IC as possible. Capacitors The equivalent series resistance (ESR) of output capacitor C2 directly affects output ripple. To minimize output ripple, use a low-ESR capacitor. A physically smaller capacitor, such as a common ceramic capacitor, minimizes board space and cost while creating an output ripple that’s acceptable in most applications. Refer to Table 2 for recommended capacitor values. _______________________________________________________________________________________ 9 MAX1729 ECB and LCD Display Bias Supply with Accurate Output Voltage and Temperature Compensation Table 1. Recommended Inductors SUPPLIER PART INDUCTANCE (µH) DC RESISTANCE (Ω) SATURATION CURRENT (mA) MAX HEIGHT (mm) Murata LQH3C221K04M00 220 8.4 70 2.2 Panasonic ELT3KN115B 470 19 40 1.6 Table 2. Recommended Capacitor Values CAPACITOR CAPACITANCE (µF) C1 0.1 C2 0.068 C3 0.1 C4 1 C5 1000pF C6* 1 *Use a low-leakage capacitor. Applications Information PC Board Layout Considerations Proper PC board layout minimizes output ripple and increases efficiency. For best results, use a ground plane, minimize the space between C1, C2, and GND of the MAX1729, and place the inductor as close to LX and IN as possible. For an example of proper PC board layout, refer to the MAX1729 Evaluation Kit. Chip Information TRANSISTOR COUNT: 1154 10 ______________________________________________________________________________________ ECB and LCD Display Bias Supply with Accurate Output Voltage and Temperature Compensation 10LUMAXB.EPS ______________________________________________________________________________________ 11 MAX1729 Package Information MAX1729 ECB and LCD Display Bias Supply with Accurate Output Voltage and Temperature Compensation NOTES 12 ______________________________________________________________________________________