MAX1606EUA

19-1744; Rev 0; 7/00 28V Internal Switch LCD Bias Supply with True Shutdown 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. o Adjustable Output Voltage up to 28V o 20mA at 20V from a Single Li+ Battery o True Shutdown (Output Disconnected from Input) o Output Short-Circuit Protection o 88% Efficiency o Up to 500kHz Switching Frequency o Selectable Inductor Current Limit (125mA, 250mA, or 500mA) o 0.1µA Shutdown Current o 8-Pin µMAX Package Features MAX1606 ________________________Applications LCD Bias Generators Cellular or Cordless Phones Palmtop Computers Personal Digital Assistants (PDAs) Organizers Handy Terminals PART MAX1606EUA Ordering Information TEMP. RANGE -40°C to +85°C PIN-PACKAGE 8 µMAX Typical Operating Circuit Pin Configuration VIN = 0.8V TO 5.5V SW BATT MAX1606 LX VOUT = VIN TO 28V TOP VIEW BATT FB VCC GND 1 2 3 4 8 SW SHDN LIM LX VCC = 2.4V TO 5.5V MAX1606 7 6 5 VCC LIM FB ON OFF SHDN GND µMAX ________________________________________________________________ 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. 28V Internal Switch LCD Bias Supply with True Shutdown MAX1606 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 BATT Input Voltage Range VCC Undervoltage Lockout VCC Quiescent Supply Current VCC Shutdown Supply Current BATT Input Supply Current BATT Shutdown Supply Current VCC Line Regulation BATT Line Regulation Load Regulation Efficiency Feedback Set Point Feedback Input Bias Current INDUCTOR CONNECTIONS (LX, SW) LX Voltage Range LX Switch Current Limit VLX LIM = VCC ILX(MAX) LIM = floating LIM = GND LX On-Resistance LX Leakage Current Maximum LX On-Time Minimum LX Off-Time SW Leakage Current tON tOFF VFB > 1.1V VFB < 0.8V (soft-start) SW = GND, VBATT = 5.5V RLX VCC = 5V, ILX = 100mA VCC = 3.3V, ILX = 100mA VLX = 28V 10 0.8 3.9 13 1.0 5.0 0.40 0.20 0.10 0.50 0.25 0.125 0.8 1 2 2 16 1.2 6.0 1 28 0.56 0.285 0.15 Ω µA µs µs µA A V VFB IFB VFB = 1.3V ∆VLNR ∆VLNR ∆VLDR IBATT SYMBOL VCC VBATT VUVLO ICC (Note 1) (Note 1) VCC falling, 30mV typical hysteresis VFB = 1.3V SHDN = GND VFB = 1.3V SHDN = GND VOUT = 18V, ILOAD = 1mA, VBATT = 3.6V, VCC = VLIM = 2.4V to 5.5V VOUT = 18V, ILOAD = 1mA, VCC = VLIM = 3.3V, VBATT = 0.8V to 5.5V VOUT = 18V, VCC = VBATT = VLIM = 3.3V, ILOAD = 0mA to 20mA L1 = 100µH, VBATT = 3.6V, ILOAD = 10mA 1.225 CONDITIONS MIN 2.4 0.8 2.0 2.2 160 0.1 20 0.1 0.1 0.05 0.05 88 1.25 5 1.275 100 TYP MAX 5.5 5.5 2.37 320 1 40 1 UNITS V V V µA µA µA µA %/V %/V %/mA % V nA 2 _______________________________________________________________________________________ 28V Internal Switch LCD Bias Supply with True Shutdown ELECTRICAL CHARACTERISTICS (continued) (VCC = 3.3V, VCC = BATT = SHDN, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER CONTROL INPUTS (SHDN, LIM) VIH SHDN Input Threshold VIL SHDN Input Bias Current LIM Input Low Level LIM Input Float Level LIM Input High Level LIM Input Bias Current ILIM ISHDN 2.4V ≤ VCC ≤ 5.5V VCC = 5.5V, V SHDN = 0 to 5.5V 2.4V ≤ VCC ≤ 5.5V 2.4V ≤ VCC ≤ 5.5V, ILIM = ±0.5µA 2.4V ≤ VCC ≤ 5.5V SHDN = VCC, LIM = GND or VCC SHDN = GND VCC = 2.5V, VBATT = 1.5V RDS(ON) tSS VCC = 2.5V, VBATT = 1.5V, ISW = 100mA VCC = 2.5V, VBATT = 1.5V, RSW = 50Ω to GND 0.65 (VCC / 2) - 0.25V VCC - 0.4V -2 0.1 0.85 0.25 0.3 2 1 1.5 0.4 -1 2.4V ≤ VCC ≤ 5.5V 0.8 × VCC 0.2 × VCC 1 0.4 (VCC / 2) + 0.25V SYMBOL CONDITIONS MIN TYP MAX UNITS MAX1606 V µA V V V µA PMOS ISOLATION SWITCH (BATT to SW) PMOS Current Limit PMOS On-Resistance Soft-Start Time A Ω ms ELECTRICAL CHARACTERISTICS (VCC = 3.3V, VCC = BATT = SHDN, TA = -40°C to +85°C, unless otherwise noted.) (Note 2) PARAMETER VCC Supply Voltage BATT Input Voltage Range VCC Undervoltage Lockout VCC Quiescent Supply Current VCC Shutdown Supply Current BATT Input Supply Current BATT Shutdown Supply Current Feedback Set Point Feedback Input Bias Current INDUCTOR CONNECTIONS (LX, SW) LX Voltage Range LX Switch Current Limit LX On-Resistance LX Leakage Current Maximum LX On-Time tON VLX LIM = VCC ILX(MAX) RLX LIM = floating LIM = GND VCC = 3.3V, ILX = 100mA VLX = 28V 9 0.35 0.18 0.08 28 0.58 0.30 0.17 2 2 17 Ω µA µs A V VFB IFB VFB = 1.3V IBATT SYMBOL VCC VBATT VUVLO ICC (Note 1) (Note 1) VCC falling, 30mV typical hysteresis VFB = 1.3V SHDN = GND VFB = 1.3V SHDN = GND 1.215 CONDITIONS MIN 2.4 0.8 2.0 TYP MAX 5.5 5.5 2.37 360 1 40 1 1.285 100 UNITS V V V µA µA µA µA V nA _______________________________________________________________________________________ 3 28V Internal Switch LCD Bias Supply with True Shutdown MAX1606 ELECTRICAL CHARACTERISTICS (continued) (VCC = 3.3V, VCC = BATT = SHDN, TA = -40°C to +85°C, unless otherwise noted.) (Note 2) PARAMETER Minimum LX Off-Time SW Leakage Current CONTROL INPUTS (SHDN, LIM) VIH SHDN Input Threshold VIL SHDN Input Bias Current LIM Input Low Level LIM Input Float Level LIM Input High Level LIM Input Bias Current ILIM I SHDN 2.4V ≤ VCC ≤ 5.5V VCC = 5.5V, V SHDN = 0 to 5.5V 2.4V ≤ VCC ≤ 5.5V 2.4V ≤ VCC ≤ 5.5V, ILIM = ±0.5µA 2.4V ≤ VCC ≤ 5.5V SHDN = VCC, LIM = GND or VCC SHDN = GND VCC = 2.5V, VBATT = 1.5V RDS(ON) VCC = 2.5V, VBATT = 1.5V, ISW = 100mA 0.65 (VCC / 2) - 0.25V VCC - 0.4V -2 2 1 1.5 0.4 -1 2.4V ≤ VCC ≤ 5.5V 0.8 × VCC 0.2 × VCC 1 0.4 (VCC / 2) + 0.25V SYMBOL tOFF VFB > 1.1V VFB < 0.8V (soft-start) SW = GND, VBATT = 5.5V CONDITIONS MIN 0.75 3.8 TYP MAX 1.25 6.0 1 UNITS µs µA V µA V V V µA PMOS ISOLATION SWITCH (BATT to SW) PMOS Current Limit PMOS On-Resistance A 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.) OUTPUT VOLTAGE vs. SUPPLY VOLTAGE MAX1606 toc01 OUTPUT VOLTAGE vs. BATTERY VOLTAGE MAX1606 toc02 OUTPUT VOLTAGE vs. LOAD CURRENT MAX1606 toc03 18.2 18.1 OUTPUT VOLTAGE (V) IOUT = 5mA 18.0 17.9 17.8 17.7 17.6 2.0 2.5 3.0 3.5 4.0 4.5 5.0 IOUT = 1mA 18.2 18.1 OUTPUT VOLTAGE (V) IOUT = 5mA 18.0 17.9 17.8 17.7 17.6 0 1 2 3 VBATT (V) 4 5 6 IOUT = 1mA 18.2 18.1 OUTPUT VOLTAGE (V) LIM = GND 18.0 17.9 LIM = OPEN 17.8 17.7 17.6 0 5 10 15 20 LIM = VCC 5.5 25 VCC (V) LOAD CURRENT (mA) 4 _______________________________________________________________________________________ 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.) EFFICIENCY vs. LOAD CURRENT (L1 = 10µH) MAX1606 toc05 MAX1606 toc06 MAX1606 EFFICIENCY vs. SUPPLY VOLTAGE MAX1606 toc04 EFFICIENCY vs. BATTERY VOLTAGE 100 90 80 IOUT = 5mA 100 90 100 IOUT = 5mA 90 EFFICIENCY (%) 80 70 IOUT = 1mA 60 50 40 2.0 2.5 3.0 3.5 4.0 4.5 5.0 L1 = 10µH, LIM = VCC L1 = 100µH, LIM = OPEN LIM = VCC EFFICIENCY (%) 80 70 60 LIM = GND L1 = 10µH, LIM = VCC L1 = 100µH, LIM = OPEN 0 1 2 3 VBATT (V) 4 5 6 50 40 0 5 10 15 20 25 LOAD CURRENT (mA) LIM = OPEN EFFICIENCY (%) 70 60 50 40 30 20 IOUT = 1mA 5.5 VCC (V) EFFICIENCY vs. LOAD CURRENT (L1 = 47µH) MAX1607 toc07 EFFICIENCY vs. LOAD CURRENT (L1 = 100µH) MAX1607 toc08 PEAK INDUCTOR CURRENT LIMIT vs. SUPPLY VOLTAGE VBATT = 3.6V 500 CURRENT LIMIT (mA) 400 300 200 100 0 LIM = OPEN LIM = GND LIM = VCC MAX1606 toc09 100 90 EFFICIENCY (%) 80 70 60 50 40 0 5 10 15 20 25 LIM = GND LIM = OPEN 100 LIM = OPEN 90 EFFICIENCY (%) 80 LIM = GND 70 60 50 40 LIM = VCC 600 LIM = VCC 30 0 5 10 15 20 25 30 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 LOAD CURRENT (mA) LOAD CURRENT (mA) VCC (V) PEAK INDUCTOR CURRENT LIMIT vs. BATTERY VOLTAGE VCC = 3.3V 500 CURRENT LIMIT (mA) 400 ICC (mA) 300 200 100 0 0.5 1.5 2.5 3.5 4.5 5.5 VBATT (V) LIM = GND LIM = OPEN LIM = VCC MAX1606 toc10 SUPPLY CURRENT vs. SUPPLY VOLTAGE MAX1606 toc11 SUPPLY CURRENT vs. LOAD CURRENT 3.5 3.0 ICC (mA) 2.5 2.0 1.5 1.0 0.5 0 LIM = VCC (500mA) LIM = OPEN (250mA) LIM = GND (125mA) MAX1606 toc12 600 180 160 140 120 100 80 60 40 20 0 0 1 2 3 VCC (V) 4 5 4.0 0 5 10 15 20 25 LOAD CURRENT (mA) _______________________________________________________________________________________ 5 28V Internal Switch LCD Bias Supply with True Shutdown 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.) LINE TRANSIENT MAX1606 toc13 LOAD TRANSIENT MAX1606 toc14 SHUTDOWN WAVEFORM MAX1606 toc15 4V 2V A A 0 20V B 10V 0 500mA C C 0 400µs/div A: VSHDN, 2V/div B: VOUT, 10V/div, ROUT = 1.8kΩ C: IL1, 500mA/div B 6V 4V 2V A 10mA 0 18.1V 18V 18.1V 18V 17.9V 100µs/div A: VBATT = VCC = 2.4V TO 5.5V, 2V/div B: VOUT = 18V, ROUT = 3.6kΩ, 100mV/div B 17.9V 500mA 0 40µs/div A: IOUT = 1mA TO 10mA, 10mA/div B: VOUT = 18V, 100mV/div C: IL1, 500mA/div Pin Description PIN 1 NAME BATT FUNCTION 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. IC Supply Voltage, 2.4V to 5.5V. Bypass VCC to GND with a 1µF or greater capacitor. Ground Inductor Switching Connection. Internally connected to the drain of a 28V N-channel MOSFET. LX is high impedance in shutdown. Inductor Current-Limit Selection. Connect LIM to VCC for 500mA, leave LIM floating for 250mA, or connect LIM to GND for 125mA. 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. 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. 2 3 4 5 6 FB VCC GND LX LIM 7 SHDN 8 SW 6 _______________________________________________________________________________________ 28V Internal Switch LCD Bias Supply with True Shutdown MAX1606 VIN = 0.8V TO 5.5V BATT SW C2 L1 10µH MAX1606 LX D1 VOUT = VIN TO 28V VCC = 2.4V TO 5.5V VCC LIM LOGIC CONTROL COUT N C1 CURRENT LIMIT SHUTDOWN LOGIC ON OFF ERROR AMPLIFIER SHDN ILIM FB CFF R1 R2 1.25V Figure 1. Functional Diagram VCC (2.4V TO 5.5V) VCC MAX1606 LIM NO CONNECTION LIM VCC (2.4V TO 5.5V) VCC MAX1606 LIM VCC (2.4V TO 5.5V) VCC MAX1606 GND IPEAK = 500mA GND IPEAK = 250mA GND 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 _______________________________________________________________________________________ 28V Internal Switch LCD Bias Supply with True Shutdown MAX1606 L1 10µH VBATT = 0.8V TO 5.5V C2 10µF VCC = 2.4V TO 5.5V C1 1µF ON OFF SW BATT MAX1606 VCC LIM FB LX D1 CFF 10pF VOUT = 18V R1 1MΩ COUT 1µF 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). R2 75k GND SHDN Figure 3. Typical Application Circuit Design Procedure 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. 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). 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 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 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) VIN = 0.8V TO 5.5V C5 10µF VCC = 2.4V TO 5.5V C6 1µF LIM D2 ON OFF SHDN GND D1 C2 1µF SW BATT MAX1606 VCC FB C1 0.1µF VNEG = -19V LX R3 1Ω C1 1nF R1 240k R2 16.5k C4 0.01µF D3 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. 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. 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. 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.