19-1744; Rev 0; 7/00 28V Internal Switch LCD Bias Supply with True Shutdown Features ♦ Adjustable Output Voltage up to 28V ♦ 20mA at 20V from a Single Li+ Battery ♦ True Shutdown (Output Disconnected from Input) ♦ Output Short-Circuit Protection ♦ 88% Efficiency ♦ Up to 500kHz Switching Frequency ♦ Selectable Inductor Current Limit (125mA, 250mA, or 500mA) ♦ 0.1µA Shutdown Current ♦ 8-Pin µMAX Package Ordering Information ________________________Applications LCD Bias Generators Cellular or Cordless Phones PART TEMP. RANGE PIN-PACKAGE MAX1606EUA -40°C to +85°C 8 µMAX Palmtop Computers Personal Digital Assistants (PDAs) Organizers Handy Terminals Pin Configuration Typical Operating Circuit VIN = 0.8V TO 5.5V SW LX BATT MAX1606 VCC = 2.4V TO 5.5V VCC FB LIM ON VOUT = VIN TO 28V TOP VIEW BATT 1 8 FB 2 7 SHDN VCC 3 6 LIM GND 4 5 LX MAX1606 SW µMAX OFF SHDN GND ________________________________________________________________ Maxim Integrated Products 1 For free samples and the latest literature, visit www.maxim-ic.com or phone 1-800-998-8800. For small orders, phone 1-800-835-8769. MAX1606 General Description The MAX1606 is a step-up DC-DC converter that contains a 0.5A internal power switch and a 0.5A output isolation switch in an 8-pin µMAX package. The IC operates from a 2.4V to 5.5V supply voltage but can boost battery voltages as low as 0.8V up to 28V. The MAX1606 uses a unique control scheme that provides high efficiency over a wide range of load conditions. An internal 0.5A MOSFET reduces external component count, and a high switching frequency (up to 500kHz) allows for tiny surface-mount components. The current limit can be set to 500mA, 250mA, or 125mA, allowing the user to reduce the output ripple and component size in low-current applications. Additional features include a low quiescent supply current and a true shutdown mode that saves power by disconnecting the output from the input. The MAX1606 is ideal for small LCD panels with low current requirements but can also be used in other applications. A MAX1606 evaluation kit is available to help speed up design time. MAX1606 28V Internal Switch LCD Bias Supply with True Shutdown ABSOLUTE MAXIMUM RATINGS VCC, FB, BATT, SW to GND .....................................-0.3V to +6V BATT to SW ..............................................................-0.3V to +6V SHDN, LIM to GND.....................................-0.3V to (VCC + 0.3V) LX to GND ..............................................................-0.3V to +30V Current into LX or BATT..............................................600mARMS Current out of SW .......................................................600mARMS Output Short-Circuit Duration ........................................Indefinite Continuous Power Dissipation (TA = +70°C) 8-Pin µMAX (derate 4.1mW/°C above +70°C) .............330mW Operating Temperature Range ...........................-40°C to +85°C Junction Temperature ......................................................+150°C Storage Temperature Range .............................-65°C to +150°C Lead Temperature (soldering, 10s) .................................+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 (VCC = 3.3V, VCC = BATT = SHDN, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER VCC Supply Voltage SYMBOL CONDITIONS MIN VCC (Note 1) BATT Input Voltage Range VBATT (Note 1) 0.8 VCC Undervoltage Lockout VUVLO VCC falling, 30mV typical hysteresis 2.0 VCC Quiescent Supply Current MAX UNITS 5.5 V 5.5 V 2.2 2.37 V 320 µA µA ICC VFB = 1.3V 160 SHDN = GND 0.1 1 IBATT VFB = 1.3V 20 40 µA SHDN = GND 0.1 1 µA VCC Shutdown Supply Current BATT Input Supply Current TYP 2.4 BATT Shutdown Supply Current VCC Line Regulation ∆VLNR VOUT = 18V, ILOAD = 1mA, VBATT = 3.6V, VCC = VLIM = 2.4V to 5.5V 0.1 %/V BATT Line Regulation ∆VLNR VOUT = 18V, ILOAD = 1mA, VCC = VLIM = 3.3V, VBATT = 0.8V to 5.5V 0.05 %/V Load Regulation ∆VLDR VOUT = 18V, VCC = VBATT = VLIM = 3.3V, ILOAD = 0mA to 20mA 0.05 %/mA L1 = 100µH, VBATT = 3.6V, ILOAD = 10mA 88 % Efficiency Feedback Set Point VFB Feedback Input Bias Current IFB 1.225 VFB = 1.3V 1.25 1.275 V 5 100 nA 28 V INDUCTOR CONNECTIONS (LX, SW) LX Voltage Range LX Switch Current Limit LX On-Resistance VLX ILX(MAX) RLX LX Leakage Current Maximum LX On-Time Minimum LX Off-Time SW Leakage Current 2 LIM = VCC 0.40 0.50 0.56 LIM = floating 0.20 0.25 0.285 LIM = GND 0.10 0.125 0.15 VCC = 5V, ILX = 100mA 0.8 VCC = 3.3V, ILX = 100mA Ω 1 2 2 µA 10 13 16 µs VFB > 1.1V 0.8 1.0 1.2 VFB < 0.8V (soft-start) 3.9 5.0 6.0 VLX = 28V tON tOFF A SW = GND, VBATT = 5.5V _______________________________________________________________________________________ 1 µs µA 28V Internal Switch LCD Bias Supply with True Shutdown (VCC = 3.3V, VCC = BATT = SHDN, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS CONTROL INPUTS (SHDN, LIM) VIH 2.4V ≤ VCC ≤ 5.5V SHDN Input Threshold VIL SHDN Input Bias Current ISHDN 0.8 × VCC 0.2 × VCC 2.4V ≤ VCC ≤ 5.5V VCC = 5.5V, V SHDN = 0 to 5.5V -1 LIM Input Low Level 2.4V ≤ VCC ≤ 5.5V LIM Input Float Level 2.4V ≤ VCC ≤ 5.5V, ILIM = ±0.5µA (VCC / 2) - 0.25V LIM Input High Level 2.4V ≤ VCC ≤ 5.5V VCC - 0.4V LIM Input Bias Current ILIM SHDN = VCC, LIM = GND or VCC 1 µA 0.4 V (VCC / 2) + 0.25V V V -2 SHDN = GND V 2 µA 0.1 1 0.85 1.5 A VCC = 2.5V, VBATT = 1.5V, ISW = 100mA 0.25 0.4 Ω VCC = 2.5V, VBATT = 1.5V, RSW = 50Ω to GND 0.3 PMOS ISOLATION SWITCH (BATT to SW) PMOS Current Limit PMOS On-Resistance VCC = 2.5V, VBATT = 1.5V RDS(ON) Soft-Start Time tSS 0.65 ms ELECTRICAL CHARACTERISTICS (VCC = 3.3V, VCC = BATT = SHDN, TA = -40°C to +85°C, unless otherwise noted.) (Note 2) PARAMETER MAX UNITS VCC (Note 1) 2.4 5.5 V BATT Input Voltage Range VBATT (Note 1) 0.8 5.5 V VCC Undervoltage Lockout VUVLO VCC falling, 30mV typical hysteresis 2.0 2.37 V VCC Supply Voltage SYMBOL VCC Quiescent Supply Current ICC VCC Shutdown Supply Current BATT Input Supply Current IBATT CONDITIONS MIN 360 µA SHDN = GND 1 µA VFB = 1.3V 40 µA 1 µA VFB = 1.3V SHDN = GND BATT Shutdown Supply Current Feedback Set Point VFB Feedback Input Bias Current IFB TYP 1.215 VFB = 1.3V 1.285 V 100 nA 28 V INDUCTOR CONNECTIONS (LX, SW) LX Voltage Range VLX LX Switch Current Limit LX On-Resistance ILX(MAX) RLX LX Leakage Current Maximum LX On-Time LIM = VCC 0.35 0.58 LIM = floating 0.18 0.30 LIM = GND 0.08 0.17 VCC = 3.3V, ILX = 100mA VLX = 28V tON 9 A 2 Ω 2 µA 17 µs _______________________________________________________________________________________ 3 MAX1606 ELECTRICAL CHARACTERISTICS (continued) ELECTRICAL CHARACTERISTICS (continued) (VCC = 3.3V, VCC = BATT = SHDN, TA = -40°C to +85°C, unless otherwise noted.) (Note 2) PARAMETER SYMBOL Minimum LX Off-Time tOFF SW Leakage Current CONDITIONS MIN TYP MAX VFB > 1.1V 0.75 1.25 VFB < 0.8V (soft-start) 3.8 6.0 SW = GND, VBATT = 5.5V UNITS µs µA 1 CONTROL INPUTS (SHDN, LIM) VIH 2.4V ≤ VCC ≤ 5.5V VIL 2.4V ≤ VCC ≤ 5.5V 0.8 × VCC SHDN Input Threshold SHDN Input Bias Current I SHDN VCC = 5.5V, V SHDN = 0 to 5.5V -1 2.4V ≤ VCC ≤ 5.5V LIM Input Float Level 2.4V ≤ VCC ≤ 5.5V, ILIM = ±0.5µA (VCC / 2) - 0.25V LIM Input High Level 2.4V ≤ VCC ≤ 5.5V VCC - 0.4V LIM Input Bias Current SHDN = VCC, LIM = GND or VCC ILIM V 0.2 × VCC LIM Input Low Level 1 µA 0.4 V (VCC / 2) + 0.25V V V -2 2 SHDN = GND µA 1 PMOS ISOLATION SWITCH (BATT to SW) PMOS Current Limit VCC = 2.5V, VBATT = 1.5V PMOS On-Resistance RDS(ON) 0.65 VCC = 2.5V, VBATT = 1.5V, ISW = 100mA 1.5 A 0.4 Ω Note 1: The MAX1606 requires a supply voltage between +2.4V and +5.5V; however, the input voltage (VBATT) used to power the inductor can vary from +0.8V to 5.5V. Note 2: Specifications to -40°C are guaranteed by design and not production tested. Typical Operating Characteristics (VCC = 3.3V, VBATT = 3.6V, L1 = 10µH, SHDN = LIM = VCC, VOUT(NOM) = 18V (Figure 3), TA = +25°C, unless otherwise noted.) IOUT = 1mA 17.8 17.7 18.0 IOUT = 1mA 17.9 17.8 2.0 2.5 3.0 3.5 4.0 VCC (V) 4.5 5.0 5.5 LIM = VCC 18.1 LIM = GND 18.0 17.9 LIM = OPEN 17.8 17.7 17.7 17.6 4 IOUT = 5mA OUTPUT VOLTAGE (V) 18.0 17.9 18.1 OUTPUT VOLTAGE (V) IOUT = 5mA 18.2 MAX1606 toc02 MAX1606 toc01 18.1 OUTPUT VOLTAGE vs. LOAD CURRENT OUTPUT VOLTAGE vs. BATTERY VOLTAGE 18.2 MAX1606 toc03 OUTPUT VOLTAGE vs. SUPPLY VOLTAGE 18.2 OUTPUT VOLTAGE (V) MAX1606 28V Internal Switch LCD Bias Supply with True Shutdown 17.6 17.6 0 1 2 3 VBATT (V) 4 5 6 0 5 10 15 LOAD CURRENT (mA) _______________________________________________________________________________________ 20 25 28V Internal Switch LCD Bias Supply with True Shutdown EFFICIENCY vs. SUPPLY VOLTAGE 100 MAX1606 toc06 90 IOUT = 5mA 90 90 70 IOUT = 1mA 60 LIM = VCC EFFICIENCY (%) 80 EFFICIENCY (%) 70 60 IOUT = 1mA 50 80 70 LIM = OPEN 60 40 50 L1 = 10µH, LIM = VCC L1 = 100µH, LIM = OPEN 40 20 2.5 3.0 3.5 4.0 4.5 5.0 40 0 1 2 3 4 5 6 0 15 20 25 LOAD CURRENT (mA) EFFICIENCY vs. LOAD CURRENT (L1 = 47µH) EFFICIENCY vs. LOAD CURRENT (L1 = 100µH) PEAK INDUCTOR CURRENT LIMIT vs. SUPPLY VOLTAGE 70 90 EFFICIENCY (%) 80 LIM = OPEN LIM = GND 60 50 600 80 LIM = GND LIM = VCC 70 VBATT = 3.6V 60 50 40 10 15 20 25 30 400 300 LIM = OPEN 200 LIM = GND 100 40 5 LIM = VCC 500 CURRENT LIMIT (mA) LIM = VCC 100 MAX1607 toc08 LIM = OPEN 0 0 5 10 15 20 25 30 2.0 2.5 3.0 3.5 4.0 4.5 LOAD CURRENT (mA) LOAD CURRENT (mA) VCC (V) PEAK INDUCTOR CURRENT LIMIT vs. BATTERY VOLTAGE SUPPLY CURRENT vs. SUPPLY VOLTAGE SUPPLY CURRENT vs. LOAD CURRENT LIM = VCC 500 180 160 140 3.0 ICC (mA) ICC (mA) LIM = OPEN 300 100 80 LIM = GND 100 1.5 2.5 3.5 VBATT (V) 4.5 5.5 2.0 40 1.0 20 0.5 0 0 LIM = OPEN (250mA) 2.5 1.5 60 200 5.5 LIM = GND (125mA) 3.5 120 400 5.0 4.0 MAX1606 toc11 VCC = 3.3V MAX1606 toc10 600 0.5 10 VBATT (V) 90 0 5 VCC (V) 100 EFFICIENCY (%) 5.5 MAX1607 toc07 2.0 CURRENT LIMIT (mA) LIM = GND 50 L1 = 10µH, LIM = VCC L1 = 100µH, LIM = OPEN 30 MAX1606 toc12 EFFICIENCY (%) 80 MAX1606 toc09 IOUT = 5mA MAX1606 toc05 100 MAX1606 toc04 100 EFFICIENCY vs. LOAD CURRENT (L1 = 10µH) EFFICIENCY vs. BATTERY VOLTAGE LIM = VCC (500mA) 0 0 1 2 3 VCC (V) 4 5 0 5 10 15 20 25 LOAD CURRENT (mA) _______________________________________________________________________________________ 5 MAX1606 Typical Operating Characteristics (continued) (VCC = 3.3V, VBATT = 3.6V, L1 = 10µH, SHDN = LIM = VCC, VOUT(NOM) = 18V (Figure 3), TA = +25°C, unless otherwise noted.) MAX1606 28V Internal Switch LCD Bias Supply with True Shutdown Typical Operating Characteristics (continued) (VCC = 3.3V, VBATT = 3.6V, L1 = 10µH, SHDN = LIM = VCC, VOUT(NOM) = 18V (Figure 3), TA = +25°C, unless otherwise noted.) SHUTDOWN WAVEFORM LOAD TRANSIENT LINE TRANSIENT MAX1606 toc15 MAX1606 toc14 MAX1606 toc13 4V 2V 10mA 6V A 4V 2V A 0 18.1V 20V B 18V B 18V 500mA C 0 400µs/div A: VSHDN, 2V/div B: VOUT, 10V/div, ROUT = 1.8kΩ C: IL1, 500mA/div 40µs/div 100µs/div A: IOUT = 1mA TO 10mA, 10mA/div B: VOUT = 18V, 100mV/div C: IL1, 500mA/div A: VBATT = VCC = 2.4V TO 5.5V, 2V/div B: VOUT = 18V, ROUT = 3.6kΩ, 100mV/div B 500mA C 0 17.9V 10V 0 17.9V 18.1V A 0 Pin Description 6 PIN NAME FUNCTION 1 BATT 2 FB 3 VCC IC Supply Voltage, 2.4V to 5.5V. Bypass VCC to GND with a 1µF or greater capacitor. 4 GND Ground 5 LX Inductor Switching Connection. Internally connected to the drain of a 28V N-channel MOSFET. LX is high impedance in shutdown. 6 LIM Inductor Current-Limit Selection. Connect LIM to VCC for 500mA, leave LIM floating for 250mA, or connect LIM to GND for 125mA. 7 SHDN Active-Low Shutdown Input. A logic low shuts down the device and reduces the supply current to 0.1µA. When shutdown, the MAX1606 isolates the output from the input by turning off the Pchannel MOSFET between BATT and SW. Connect SHDN to VCC for normal operation. 8 SW Inductor Supply Voltage, 0.8V to 5.5V. Internally connected to the source of a P-channel MOSFET used to isolate the output from the input during shutdown. Bypass with a 10µF or greater capacitor. Feedback Input. Connect to a resistive divider network between the output and GND to set the output voltage between VBATT and 28V. The feedback threshold is 1.25V. Isolation Switch Output, Inductor Connection. Internally connected to the drain of a P-channel MOSFET used to isolate the output from the input during shutdown. _______________________________________________________________________________________ 28V Internal Switch LCD Bias Supply with True Shutdown BATT MAX1606 VIN = 0.8V TO 5.5V SW C2 L1 10µH MAX1606 VCC = 2.4V TO 5.5V N LIM C1 CURRENT LIMIT SHUTDOWN LOGIC ON VOUT = VIN TO 28V COUT LOGIC CONTROL VCC D1 LX CFF ILIM R1 FB SHDN OFF ERROR AMPLIFIER R2 1.25V Figure 1. Functional Diagram VCC (2.4V TO 5.5V) VCC (2.4V TO 5.5V) VCC (2.4V TO 5.5V) VCC VCC MAX1606 LIM VCC MAX1606 NO CONNECTION GND MAX1606 LIM LIM GND GND IPEAK = 500mA IPEAK = 250mA IPEAK = 125mA Figure 2. Setting the Peak Inductor Current Limit Detailed Description The MAX1606 step-up DC-DC converter operates from a 2.4V to 5.5V supply and converts voltages as low as 0.8V up to 28V. The device includes an internal switching MOSFET with a 0.8Ω on-resistance and selectable current limit (Figure 1) and consumes 160µA of supply current. During startup, the MAX1606 extends the minimum off-time, limiting initial battery surge current. The MAX1606 uses a P-channel MOSFET to isolate the output from the input during true shutdown mode. This isolation switch also includes short-circuit current limiting, which protects the inductor and diode during a short-circuit fault. Control Scheme The MAX1606 features a minimum off-time, current-limited control scheme. The duty cycle is governed by a pair of one-shots that set a minimum off-time and a maximum on-time. The switching frequency can be up to 500kHz and depends upon the load and input voltage. The peak current limit of the internal N-channel MOSFET is pin selectable and may be set at 125mA, 250mA, or 500mA (Figure 2). _______________________________________________________________________________________ 7 MAX1606 28V Internal Switch LCD Bias Supply with True Shutdown L1 10µH VBATT = 0.8V TO 5.5V SW D1 C2 10µF CFF 10pF MAX1606 VCC = 2.4V TO 5.5V C1 1µF VOUT = 18V LX BATT VCC R1 1MΩ FB LIM R2 75k ON OFF SHDN COUT 1µF GND output rectifier, holding the output voltage to one diode drop below VIN when the converter is shutdown and allowing the output to draw power from the input. The MAX1606 features true shutdown, which uses an internal P-channel MOSFET to disconnect the output from the input when the MAX1606 is shutdown. This eliminates power drawn from the input during shutdown. Separate/Same Power for VBATT and VCC Separate voltage sources can supply the inductor (VBATT) and the IC (VCC). Since the chip bias is provided by a logic supply (2.4V to 5.5V), this allows the output power to be sourced directly from low-voltage batteries (0.8V to 5.5V). Conversely, VBATT and VCC can also be supplied from one supply if it remains within VCC’s operating limits (2.4V to 5.5V). Figure 3. Typical Application Circuit Design Procedure Setting the Output Voltage (FB) Adjust the output voltage by connecting a voltagedivider from the output (VOUT) to FB (Figure 3). Select R2 between 10kΩ and 200kΩ. Calculate R1 with the following equation: R1 = R2 [(VOUT / VFB) – 1] where VFB = 1.25V and VOUT may range from VBATT to 28V. The input bias current of FB has a maximum value of 100nA, which allows large-value resistors to be used. For less than 1% error, the current through R2 should be greater than 100 times the feedback input bias current (IFB). Current-Limit Select Pin (LIM) The MAX1606 allows a selectable inductor current limit of 125mA, 250mA, or 500mA (Figure 2). This allows flexibility in designing for higher current applications or for smaller, compact designs. The lower current limit allows the use of a physically smaller inductor in spacesensitive, low-power applications. Connect LIM to VCC for 500mA, leave floating for 250mA, or connect to GND for 125mA. Shutdown (SHDN) Pull SHDN low to enter shutdown. During shutdown the supply current drops to 0.1µA, the output is disconnected from the input, and LX enters a high-impedance state. The capacitance and load at the output determine the rate at which VOUT decays. SHDN can be pulled as high as 6V, regardless of the input and output voltages. With the typical step-up converter circuit, the output remains connected to the input through the inductor and 8 Inductor Selection Smaller inductance values typically offer smaller physical size for a given series resistance or saturation current. Circuits using larger inductance values may start up at lower input voltages and exhibit less ripple, but also provide reduced output power. This occurs when the inductance is sufficiently large to prevent the maximum current limit from being reached before the maximum on-time expires. The inductor’s saturation current rating should be greater than the peak switching current. However, it is generally acceptable to bias the inductor into saturation by as much as 20%, although this will slightly reduce efficiency. Picking the Current Limit The peak LX current limit (ILX(MAX)) required for the application may be calculated from the following equation: ILX(MAX) ≥ ( ) VOUT − VBATT(MIN) × t OFF(MIN) VOUT × IOUT(MAX) + VBATT(MIN) 2×L where tOFF(MIN) = 0.8µs, and VBATT(MIN) is the minimum voltage used to supply the inductor. The set current limit must be greater than this calculated value. Select the appropriate current limit by connecting LIM to VCC, GND, or leaving it unconnected (see CurrentLimit Select Pin and Figure 2). Diode Selection The high switching frequency of 500kHz requires a highspeed rectifier. Schottky diodes, such as the Motorola MBRS0530 or the Nihon EP05Q03L, are recommended. To maintain high efficiency, the average current rating of the Schottky diode should be greater than the peak _______________________________________________________________________________________ 28V Internal Switch LCD Bias Supply with True Shutdown MAX1606 L1 10µH D1, D2 = CENTRAL SEMICONDUCTOR CMPD7000 (DUAL) D3 = CENTRAL SEMICONDUCTOR CMSD4448 (1N4148) SW VIN = 0.8V TO 5.5V D3 LX BATT C5 10µF VCC = 2.4V TO 5.5V VCC R1 240k C1 1nF R3 1Ω MAX1606 R2 16.5k C4 0.01µF FB C1 0.1µF C6 1µF VNEG = -19V LIM D2 ON SHDN GND OFF D1 C2 1µF Figure 4. Negative Voltage for LCD Bias switching current. Choose a reverse breakdown voltage greater than the output voltage. Capacitors For most applications, use a small 1µF ceramic surface-mount output capacitor. For small ceramic capacitors, the output ripple voltage is dominated by the capacitance value. If tantalum or electrolytic capacitors are used, the higher ESR increases the output ripple voltage. Decreasing the ESR reduces the output ripple voltage and the peak-to-peak transient voltage. Surface-mount capacitors are generally preferred because they lack the inductance and resistance of their through-hole equivalents. Two inputs, VCC and VBATT, require bypass capacitors. Bypass VCC with a 1µF ceramic capacitor as close to the IC as possible. The BATT input supplies high currents to the inductor and requires local bulk bypassing close to the inductor. A 10µF low-ESR surface-mount capacitor is sufficient for most applications. A feed-forward capacitor connected from the output to FB improves stability over a wide range of battery voltages. A 10pF capacitor is sufficient for most applications. Larger values (up to 47pF) may be needed with lower current-limit settings (LIM = GND or open) and low input voltages, or with nonoptimum PC board layouts. Note that increasing CFF may slightly affect load regulation. PC Board Layout and Grounding Careful printed circuit layout is important for minimizing ground bounce and noise. Keep the MAX1606’s ground pin and the ground leads of the input and output capacitors less than 0.2in (5mm) apart. In addition, keep all connections to FB and LX as short as possible. In particular, external feedback resistors should be as close to FB as possible. To minimize output voltage ripple, and to maximize output power and efficiency, use a ground plane and solder GND directly to the ground plane. Refer to the MAX1606EVKIT evaluation kit for a layout example. Applications Information Negative Voltage for LCD Bias The MAX1606 can also generate a negative output by adding a diode-capacitor charge-pump circuit (D1, D2, and C3) to the LX pin as shown in Figure 4. Feedback is still connected to the positive output, which is not loaded, allowing a very small capacitor value at C4. For best stability and lowest ripple, the time constant of the R1-R2 series combination and C4 should be near or less than that of C2 and the effective load resistance. Output load regulation of the negative output is somewhat looser than with the standard positive output circuit, and may rise at very light loads due to coupling through the capacitance of D2. If this is objectionable, reduce the resistance of R1 and R2, while maintaining their ratio, to effectively preload the output with a few hundred microamps. This is why the R1-R2 values shown in Figure 4 are about four-times lower than typical values used for a positive-output design. When loaded, the negative output voltage will be slightly lower (closer to ground by approximately a diode forward voltage) than the inverse of the voltage on C4. Chip Information TRANSISTOR COUNT: 3883 _______________________________________________________________________________________ 9 28V Internal Switch LCD Bias Supply with True Shutdown MAX1606 Package Information Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time. 10 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 © 2000 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.