19-1327; Rev 1; 2/98 KIT ATION EVALU E L B A AVAIL DAC-Controlled Boost/Inverter LCD Bias Supply with Internal Switch ____________________________Features ♦ Internal 500mA, 28V N-Channel Switch (no external FET required) ♦ Adjustable Output Voltage to +27.5V or -27.5V ♦ 6-Bit DAC-Controlled Output Voltage ♦ Up to 90% Efficiency ♦ Small 16-Pin QSOP Package (Same size as 8-pin SO) ♦ Power-OK Indicator ♦ 65µA Quiescent Current ♦ 1.5µA Shutdown Current ♦ Up to 300kHz Switching Frequency Ordering Information PART 0°C to +70°C MAX686EEE -40°C to +85°C Pin Configuration PIN-PACKAGE Dice* 16 QSOP *Dice are specified at TA = +25°C, DC parameters only. Functional Diagram appears at end of data sheet. Applications Positive or Negative LCD Bias Personal Digital Assistants Notebook Computers Portable Data-Collection Terminals Palmtop Computers Varactor Tuning Diode Bias TEMP. RANGE MAX686C/D Typical Operating Circuit VCC = 2.7V TO 5.5V 0.1µF MBR0530L 22µH VIN = 0.8V TO 27.5V VOUT R2 VCC LX VDD DACOUT TOP VIEW R3 LCDON 16 LX PGND 1 UP 2 15 N.C. DN 3 14 LCDON POL 4 MAX686 12 VCC ISET 6 11 POK 10 FB DACOUT 8 UP 9 MAX686 R1 DAC CONTROL 13 GND VDD 5 SHDN 7 FB ON/OFF DN POK SHDN REF POL GND ISET PGND 0.1µF REF QSOP ________________________________________________________________ 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 408-737-7600 ext. 3468. MAX686 General Description The MAX686 DAC-controlled boost/inverter IC converts a positive input voltage to a positive or negative LCD bias voltage up to +27.5V or -27.5V. The device features an internal N-channel MOSFET switch, programmable current limiting, and an internal 6-bit digital-toanalog converter (DAC) for digital adjustment of the output voltage. It comes in a small 16-pin QSOP package (same size as an 8-pin SO). The MAX686 uses a current-limited, pulse-frequencymodulation (PFM) control scheme to provide high efficiency over a wide range of load conditions. Its high switching frequency (up to 300kHz) allows the use of small external components. An LCDON output allows the LCD bias voltage to be automatically disabled when the display logic voltage is removed, protecting the display. The MAX686 has a +2.7V to +5.5V input voltage range for the IC, and a +0.8V to +27.5V input voltage range for the inductor. Typical quiescent supply current is 65µA. Shutdown current is 1.5µA. The MAX686 offers high-level integration to save space, reduce power consumption, and increase battery life, making it an excellent choice for battery-powered portable equipment. The MAX629 is similar to the MAX686, except that it does not contain a built-in DAC. Both devices have evaluation kits to facilitate designs. MAX686 DAC-Controlled Boost/Inverter LCD Bias Supply with Internal Switch ABSOLUTE MAXIMUM RATINGS Voltage VCC, ISET, POK, POL, SHDN, UP, DN, VDD to GND ...........................................-0.3V to +6V FB, REF, DACOUT to GND.......................-0.3V to (VCC + 0.3V) PGND to GND .....................................................-0.3V to +0.3V LX, LCDON to GND..............................................-0.3V to +30V Current LX (sinking) .....................................................................600mA LCDON (sinking)...............................................................10mA Continuous Power Dissipation (TA = +70°C) QSOP (derate 8.30mW/°C above +70°C) ......................667mW Operating Temperature Ranges MAX686C/D ..........................................................0°C to +70°C MAX686EEE.......................................................-40°C to +85°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 (VCC = VDD = VIN = +5V, CREF = 0.1µF, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER Supply Voltage (Note 1) Input Voltage SYMBOL CONDITIONS VCC, VDD VIN Voltage applied to L1 MIN MAX UNITS 2.7 TYP 5.5 V 0.8 VOUT V POL = GND, VFB = 1.3V, IDACOUT = 0mA 65 125 µA Shutdown Current ISHDN SHDN = GND 1.3 4 µA VCC Undervoltage Lockout VLOCK Rising or falling 2.5 2.65 V Supply Current ICC + IDD 2.10 VCC Undervoltage Lockout Hysteresis VCC DAC Reset Threshold 100 0.5 VRESET 1.5 mV 2.1 V Line Regulation Boost configuration, VOUT = 27.5V, ILOAD = 5mA, VCC = VDD = 2.7V to 5.5V 0.1 %/V Load Regulation Boost configuration, VOUT = 27.5V, ILOAD = 0mA to 5mA 0.01 %/mA LX LX Voltage Range LX Switch Current Limit ILX LX On-Resistance RLX LX Leakage Current Maximum LX On-Time Minimum LX Off-Time 2 28 VLX ILXLEAK ISET = VCC 0.42 0.50 0.58 ISET = GND 0.21 0.25 0.29 VCC = VDD = 5V, ILX = 100mA 0.6 1.2 VCC = VDD = 3.3V, ILX = 100mA 0.8 1.6 VLX = 28V tOFF Ω µA µs 8 10 12 0.8 1 1.2 POL = VCC, VFB < 0.15V 2.8 3.5 4.2 POL = GND, VFB < 0.8V 4 5 6 POL = VCC, VFB > 0.4V 4 5 6 _______________________________________________________________________________________ A 1.5 POL = GND, VFB > 1.2V tON V µs DAC-Controlled Boost/Inverter LCD Bias Supply with Internal Switch (VCC = VDD = VIN = +5V, CREF = 0.1µF, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS 1.225 1.250 1.275 V 1 10 REFERENCE AND FB INPUT REF Output Voltage VREF VCC = VDD = 2.7V to 5.5V, no load IREF = 0µA to 25µA, CREF = 0.1µF REF Load Regulation FB Set Point VFB FB Input Bias Current IFB 1.225 1.250 1.275 V POL = VCC -15 0 15 mV ±50 nA 1.100 1.125 IPOK POK Hysteresis LCDON Sink Current I LCDON LCDON Leakage Current mV POL = GND POWER OK COMPARATOR, LCDON OUTPUT POK Threshold VPOK VPOK rising POK Input Current 1.5 IREF = 0µA to 50µA, CREF = 0.47µF V LCDON = 0.4V, VPOK = 1.25V 2 V LCDON = 28V, VPOK = GND 1.150 V ±50 nA 12 mV 6 mA 0.02 1 µA VREF VREF + 0.015 V DAC OUTPUT (Notes 2, 3) Full-Scale Output Voltage VFS -50µA < IDACOUT < 0µA VREF 0.015 Zero-Scale Output Voltage VZS 0µA < IDACOUT < 20µA 0 15 6 Resolution mV bits Mid-Scale Accuracy MSA Mid-scale = VREF x 32/63 -2 2 % Differential Nonlinearity DNL Guaranteed monotonic -1 1 LSB kΩ 0.7 V ±1 µA Output Resistance in Shutdown 1.5 RDACOUT LOGIC INPUTS: POL, ISET, UP, DN, SHDN Input Low Level VIL 2.7V < VCC = VDD < 5.5V Input High Level 2.7V < VCC = VDD < 5.5V VIH 2.4 V Input Bias Current IBIAS Pulse Width High tPWH UP, DN, TA = +25°C 1 µs Pulse Width Low tPWL UP, DN, TA = +25°C 1 µs Pulse Separation tPWS UP, DN, TA = +25°C 1 µs _______________________________________________________________________________________ 3 MAX686 ELECTRICAL CHARACTERISTICS (continued) MAX686 DAC-Controlled Boost/Inverter LCD Bias Supply with Internal Switch ELECTRICAL CHARACTERISTICS (VCC = VDD = VIN = +5V, CREF = 0.1µF, TA = -40°C to +85°C, unless otherwise noted.) (Note 4) PARAMETER Supply Voltage (Note 1) Input Voltage Supply Current SYMBOL CONDITIONS VCC, VDD VIN ICC + IDD Voltage applied to L1 ISHDN SHDN = GND VCC Undervoltage Lockout VLOCK Rising or falling VLX LX Switch Current Limit ILX LX On-Resistance RLX LX Leakage Current Maximum LX On-Time Minimum LX Off-Time REFERENCE AND FB INPUT REF Output Voltage ILXLEAK VREF REF Load Regulation VFB FB Input Bias Current IFB 2.7 5.5 V 0.8 VOUT V 125 µA 2.10 µA V 28 V 0.4 0.6 0.2 0.3 A VCC = VDD = 5V, ILX = 100mA 1.2 VCC = VDD = 3.3V, ILX = 100mA 1.6 VLX = 28V 1.5 µA µs Ω 7.5 12.5 POL = GND, VFB > 1.2V 0.7 1.3 POL = VCC, VFB < 0.15V 2.8 4.2 POL = GND, VFB < 0.8V 3.8 6.2 POL = VCC, VFB > 0.4V 3.8 6.2 VCC = VDD = 2.7V to 5.5V, no load 1.22 1.28 V 10 mV 1.28 V 15 mV ±50 nA POL = GND 1.22 POL = VCC -15 1.05 IPOK I LCDON 4 2.65 ISET = GND POWER OK COMPARATOR, LCDON OUTPUT POK Threshold VPOK VPOK rising LCDON Sink Current UNITS IREF = 0µA to 25µA, CREF = 0.1µF FB Set Point POK Input Current MAX ISET = VCC tON tOFF TYP POL = GND, VFB = 1.3V, IDACOUT = 0mA Shutdown Current LX LX Voltage Range MIN V LCDON = 0.4V, VPOK = 1.25V µs 1.20 V ±50 nA 2 mA DAC OUTPUT (Notes 2, 3) Full-Scale Output Voltage VFS -50µA < IDACOUT < 0µA VREF 0.02 VREF + 0.02 V Zero-Scale Output Voltage VZS 0µA < IDACOUT < 20µA 0 15 mV MSA Mid-scale = VREF x 32/63 -3 6 Resolution Mid-Scale Accuracy LOGIC INPUTS: POL, ISET, UP, DN, SHDN Input Low Level VIL 2.7V < VCC = VDD < 5.5V Input High Level 2.7V < VCC = VDD < 5.5V Input Bias Current VIH Bits 3 % 0.7 V ±1 µA 2.4 IBIAS V Note 1: The MAX686 requires a supply voltage at VCC = VDD between +2.7V and +5.5V; however, the voltage that supplies the inductor can vary from +0.8V to +27.5V, depending on circuit operating conditions. Note 2: The DAC output is set to its midpoint value at power-on. Note 3: The DAC setting is guaranteed to remain valid as long as VCC is greater than the VCC DAC Reset Threshold. Note 4: Specifications to -40°C are guaranteed by design, not production tested. 4 _______________________________________________________________________________________ DAC-Controlled Boost/Inverter LCD Bias Supply with Internal Switch E D F A: VIN = 12V, ISET = VCC B: VIN = 12V, ISET = GND C: VIN = 5V, ISET = VCC D: VIN = 5V, ISET = GND E: VIN = 3V, ISET = GND F: VIN = 3V, ISET = VCC 70 65 1 10 100 1 D C E 70 F C: VIN = 5V, ISET = GND D: VIN = 5V, ISET = VCC E: VIN = 3V, ISET = GND F: VIN = 3V, ISET = VCC 65 60 1000 0.1 1 10 100 LOAD CURRENT (mA) EFFICIENCY vs. LOAD CURRENT (VOUT = -18V) MAXIMUM OUTPUT CURRENT vs. INPUT VOLTAGE (VOUT = +12V, +24V) MAXIMUM OUTPUT CURRENT vs. INPUT VOLTAGE (VOUT = -12V, -18V) E 70 F 65 A: VIN = 9V, ISET = GND B: VIN = 9V, ISET = VCC C: VIN = 5V, ISET = GND D: VIN = 5V, ISET = VCC E: VIN = 3V, ISET = GND F: VIN = 3V, ISET = VCC 60 55 50 0.1 1 10 B C D 10 A: VOUT = 12V, ISET = VCC B: VOUT = 12V, ISET = GND C: VOUT = 24V, ISET = VCC D: VOUT = 24V, ISET = GND 1 2 VCC = VIN = VDD INPUT CURRENT = ICC + IDD VOUT = 18V, NO LOAD 3 4 INPUT VOLTAGE (V) 5 6 8 10 12 14 A: VOUT = -12V, ISET = VCC B: VOUT = -18V, ISET = VCC C: VOUT = -12V, ISET = GND D: VOUT = -18V, ISET = GND 0 2 4 6 8 10 12 14 16 18 INPUT VOLTAGE (V) OUTPUT VOLTAGE RIPPLE VIN = VCC = 5V CREF = 0.1µF 1.250 VOUT 50mV/div AC-COUPLED ISET = GND 1.249 1.248 1.247 VOUT 50mV/div AC-COUPLED 1.246 1.245 ISET = VCC VOUT = 24V ILOAD = 5mA 1.244 6 D MAX686 TOC09 1.251 REFERENCE VOLTAGE (V) 10 2 4 1.252 MAX686 TOC07 100 1 C 10 REFERENCE VOLTAGE vs. LOAD CURRENT INPUT CURRENT vs. INPUT VOLTAGE 1 B&D B INPUT VOLTAGE (V) LOAD CURRENT (mA) 1000 A&C A 1 0 MAX686 TOC06 MAX686 TOC05 A 100 100 100 OUTPUT CURRENT (mA) OUTPUT CURRENT (mA) A D MAX686 TOC08 C 1000 MAX686 TOC04 80 INPUT CURRENT (µA) 100 B 75 LOAD CURRENT (mA) B 0 10 80 LOAD CURRENT (mA) 85 75 0.1 A: VIN = 9V, ISET = GND B: VIN = 9V, ISET = VCC A A: VIN = 9V, ISET = VCC B: VIN = 9V, ISET = GND C: VIN = 5V, ISET = VCC D: VIN = 5V, ISET = GND E: VIN = 3V, ISET = GND F: VIN = 3V, ISET = VCC 65 85 F 75 60 0.1 B D 80 70 60 EFFICIENCY (%) E 85 80 75 90 EFFICIENCY (%) EFFICIENCY (%) 85 A C MAX686 TOC02 B C EFFICIENCY vs. LOAD CURRENT (VOUT = -12V) EFFICIENCY (%) A 90 95 MAX686 TOC01 95 EFFICIENCY vs. LOAD CURRENT (VOUT = +12V) MAX686 TOC03 EFFICIENCY vs. LOAD CURRENT (VOUT = +24V) MAX686 __________________________________________Typical Operating Characteristics (Circuits of Figures 1 and 2, VCC = VDD = VIN = +5V, L1 = 22µH, SHDN = VCC, CREF = 0.1µF, TA = +25°C, unless otherwise noted.) 0 20 40 60 80 100 120 140 20µs/div LOAD CURRENT (µA) _______________________________________________________________________________________ 5 MAX686 DAC-Controlled Boost/Inverter LCD Bias Supply with Internal Switch _____________________________Typical Operating Characteristics (continued) (Circuits of Figures 1 and 2, VCC = VDD = VIN = +5V, L1 = 22µH, SHDN = VCC, CREF = 0.1µF, TA = +25°C, unless otherwise noted.) LINE-TRANSIENT RESPONSE (ISET = VCC) LINE-TRANSIENT RESPONSE (ISET = GND) MAX686 TOC10 MAX686 TOC10A VCC = VDD =VIN 5V 5V 3V 3V VOUT 50mV/div AC-COUPLED VOUT 50mV/div AC-COUPLED VOUT = 24V ILOAD = 5mA VCC = VDD = VIN 5ms/div 5ms/div LOAD-TRANSIENT RESPONSE (ISET = GND) LOAD-TRANSIENT RESPONSE (ISET = VCC) MAX686 TOC12 MAX686 TOC11 VOUT = 24V VOUT = 24V 5mA 5mA IOUT 100µA IOUT 100µA VOUT 50mV/div AC-COUPLED VOUT 20mV/div AC-COUPLED 2ms/div 2ms/div POWER-UP RESPONSE (POSITIVE CONFIGURATION) POWER-DOWN RESPONSE (POSITIVE CONFIGURATION) MAX686 TOC13B MAX686 TOC13A ISET = VCC RL = 4.7kΩ SHDN 2V/div SHDN 2V/div 18.7V 18.7V VOUT 5V/div VOUT 5V/div ISET = VCC RL = 4.7kΩ 500µs/div 6 5V 5V 5ms/div _______________________________________________________________________________________ DAC-Controlled Boost/Inverter LCD Bias Supply with Internal Switch POWER-DOWN RESPONSE (NEGATIVE CONFIGURATION) POWER-UP RESPONSE (NEGATIVE CONFIGURATION) MAX686 TOC14B MAX686 TOC14A ISET = VCC RL = 4.7kΩ SHDN 5V/div SHDN 5V/div 0V 0V VOUT 5V/div VOUT 5V/div -16.8V -16.8V ISET = VCC RL = 4.7kΩ 20ms/div 500µs Pin Description PIN NAME FUNCTION 1 PGND 2 UP Increment Output Voltage Input. Increments the DAC on each rising edge such that |VOUT| increases. 3 DN Decrement Output Voltage Input. Decrements the DAC on each rising edge such that |VOUT| decreases. 4 POL Polarity Input. Changes polarity and threshold of FB to allow regulation of either positive or negative output voltages. POL also changes the polarity of the DAC output such that increasing codes always increases the magnitude of the output voltage. Set POL = GND for positive output voltage, or set POL = VCC for negative output voltage. 5 VDD Gate-Drive Supply for Internal MOSFET. Connect to VCC. 6 ISET Set LX Current Limit. Sets the peak current limit for the internal switch. Connect to VCC for 500mA current limit. Connect to GND for 250mA current limit. 7 SHDN Shutdown Input. A logic low on SHDN places the MAX686 in shutdown mode. Connect to VCC for normal operation. 8 DACOUT 9 REF 10 FB 11 POK Power-OK Sense Input/Power-OK Comparator Input. When the voltage applied to POK is greater than 1.125V, LCDON is low. Connect to a resistive voltage divider monitoring V IN or VOUT. 12 VCC IC Power-Supply Input 13 GND Ground 14 LCDON 15 N.C. 16 LX Power Ground. Connect to GND. DAC Output Voltage Reference Output. Bypass with a 0.1µF ceramic capacitor to GND. Feedback Input. Connect to an external voltage divider to set the MAX686 output voltage. See the section Setting the Output Voltage with the DAC. Power-OK Comparator Open-Drain Output. Connect to external switch to turn LCD power on or off. See the section Controlling the LCD Using POK and LCDON. No Connection. Not internally connected. Drain of Internal 28V, 500mA N-Channel Switch _______________________________________________________________________________________ 7 MAX686 _____________________________Typical Operating Characteristics (continued) (Circuits of Figures 1 and 2, VCC = VDD = VIN = +5V, L1 = 22µH, SHDN = VCC, CREF = 0.1µF, TA = +25°C, unless otherwise noted.) MAX686 DAC-Controlled Boost/Inverter LCD Bias Supply with Internal Switch D1 MBR0530L L1 22µH VOUT VIN = 0.8V TO 27.5V 15µF 4.7µF VCC = 2.7V TO 5.5V VCC LX VDD DACOUT R2 CF 22pF R3 0.1µF LCDON FB MAX686 UP R1 DAC CONTROL ON/OFF DN POK SHDN REF POL ISET GND 0.1µF PGND Figure 1. Boost Configuration: Positive Output Voltage L1 22µH R4 2Ω VIN = 0.8V TO 27.5V 15µF 2.2µF VCC = 2.7V TO 5.5V 0.1µF VCC LX VDD DACOUT POL LCDON R3 CF 100pF FB UP MAX686 D1 MBRO530L D2 MBRO530L R1 R2 DAC CONTROL ON/OFF DN REF SHDN POK NEGATIVE OUTPUT VOLTAGE VIN ≤ |VOUT| ≤ 27.5V ISET GND PGND 0.1µF 2.2µF Figure 2. Negative Output Voltage Application Circuit 8 _______________________________________________________________________________________ 0.1µF DAC-Controlled Boost/Inverter LCD Bias Supply with Internal Switch MAX686 D2 MBR0530L R4 2Ω L1 22µH VIN = 0.8V TO 27.5V 15µF 2.2µF VCC = 2.7V TO 5.5V 0.1µF VCC LX VDD DACOUT POL LCDON D1 MBR0530L R3 CF 470pF FB UP R1 MAX686 DAC CONTROL DN ON/OFF NEGATIVE OUTPUT VOLTAGE REF POK SHDN |VOUT| ≤ (27.5V - VIN) ISET GND PGND R2 0.1µF 2.2µF Figure 3. Alternative Negative Output Voltage Application Circuit Detailed Description The MAX686 is a step-up converter that contains an internal N-channel MOSFET switch to convert a +0.8V to +27.5V battery voltage to a higher positive or a negative voltage. Figure 1 shows the MAX686 configured to produce a positive output voltage. Figure 2 shows the MAX686 configured with one additional diode and capacitor to produce a negative output voltage. Figure 3 shows an alternative method for developing negative output voltages. Set the output voltage with an external resistor-divider network. Adjust the output voltage with the internal digital-to-analog converter (DAC). The MAX686’s current-limited pulse-frequency-modulation (PFM) control scheme has programmable current limiting and provides high efficiency over a wide range of load conditions. Boost Control Scheme (POL = GND) A combination of peak current limiting and a pair of oneshots controls the MAX686 switching. During the oncycle, the internal switch closes, and current through the inductor ramps up until either the fixed 10µs maximum on-time expires (at low input voltages) or the switch peak current limit is reached. The peak current limit is selectable to either 500mA (ISET = V CC) or 250mA (ISET = GND) (see the section Setting the Peak Inductor Current Limit). After the on-cycle terminates, the switch turns off, and the inductor charges the output capacitor through the diode. If the output is out of regulation after the minimum off-time has transpired, another on-cycle begins. If the output is within regulation when the minimum offtime transpires, the off-cycle extends until the output falls out of regulation, at which point an on-cycle starts. The MAX686 regulates the voltage on FB (V FB ) to 1.25V. When the output is well below regulation (VFB is less than 1V and the switch current limit is exceeded), the MAX686 operates in initial power-up mode, and the minimum off-time increases to 5µs to provide soft-start. The switching frequency, which depends on the load, the input voltage, and the output voltage, can be as high as 300kHz. Inverting Control Scheme (POL = VCC) In inverting operation, the MAX686 regulates the voltage on FB (VFB) to 0V, and the error amplifier’s polarity is reversed. The minimum off-time changes to 3.5µs for negative output voltages. When the output is well below regulation (VFB is 0.25V or more and the switch current limit is exceeded), initial power-up is assumed, and the minimum off-time increases to 5µs to provide soft-start. _______________________________________________________________________________________ 9 MAX686 DAC-Controlled Boost/Inverter LCD Bias Supply with Internal Switch Power-OK Comparator POK is the input to the power-OK comparator. The comparator drives an internal N-channel MOSFET. The MOSFET’s open-drain output, LCDON, can drive an external PNP transistor or P-channel MOSFET, switching a positive VOUT to the LCD (Figures 6 and 7). When the voltage at POK exceeds 1.125V (power OK), LCDON goes low, turning on the external PNP transistor. When the voltage at POK drops below 1.125V (power not OK), the external PNP transistor turns off, cutting off power to the LCD display. This feature ensures that the LCD display is not damaged due to improper voltage levels. During shutdown or undervoltage lockout, LCDON is high impedance. Shutdown Mode When SHDN is low, the MAX686 enters shutdown mode, in which the control circuit, POK comparator, DAC output buffer, reference, and internal biasing circuitry turn off. The DAC setting is stored as long as VCC remains above the DAC reset threshold. Supply current drops to 1.5µA. SHDN is a logic-level input; connect it to VCC for normal operation. The output voltage in shutdown mode depends on the output voltage polarity. In the positive output voltage configuration (Figure 1), the output is directly connected to the input through the diode (D1) and the inductor (L1). When the device is in shutdown mode, the output voltage falls to one diode drop below the input voltage, and any load connected to the output may still conduct current. In the negative output voltage configuration (Figures 2 and 3), there is no DC path between the input and the output, and the output falls to GND in shutdown mode. Internal DAC The MAX686 contains an internal 6-bit counter and DAC to control the output voltage digitally (see the section Setting the Output Voltage with the DAC). The UP and DN input pins drive an internal up/down counter that directly controls the DAC. To increase the magnitude of VOUT in the boost configuration, apply a rising edge to UP. This decreases the DAC output voltage one step and correspondingly increases V OUT. Conversely, to decrease the magnitude of VOUT, apply a rising edge to DN. This increases the DAC output voltage one step and correspondingly decreases VOUT. The UP and DN control direction reverses for a negative output to maintain the same control direction of the absolute magnitude of the output voltage. Upon power-up, the DAC code internally goes to mid-scale. The DAC’s internal counter does not roll over once it reaches full scale or zero. Therefore, additional rising 10 edges to make the counter roll over are ignored, preventing unexpected undervoltages or overvoltages. Internal Reference The MAX626’s 1.25V internal reference is accurate to ±2% over temperature. It can source up to 50µA of current and should be bypassed with at least a 0.1µF capacitor. See the Bypass Capacitors section. Design Procedure Setting the Output Voltage with the DAC For either positive or negative output voltage applications, set the MAX686’s output voltage using three external resistors (R1, R2, and R3) as shown in Figures 1, 2, and 3. Since the input bias current at FB has a 50nA maximum value, large resistors can be used in the feedback loop without a significant loss of accuracy. Select R1 to be in the 10kΩ to 220kΩ range and calculate R2 and R3 using the applicable equations from the following subsections. Setting the Minimum Positive Output Voltage The minimum output voltage is set with the resistordivider (R1-R2, Figure 1) from VOUT to FB. The minimum output voltage occurs when VDACOUT = VFB = 1.25V. Therefore, R3 has no effect on the minimum output voltage. Choose R1 to be 120kΩ so that the current in the divider is about 10µA. Then determine R2 as follows: R2 = R1 x (VOUT(MIN) - VFB) / VFB For example, if VOUT(MIN) = 12.5V: R2 = 120kΩ x (12.5 - 1.25) / (1.25) =1.08MΩ Mount R1 and R2 close to the FB pin to minimize parasitic capacitance. Setting the Maximum Positive Output Voltage The DAC is adjustable from 0V to 1.25V in 64 steps, and 1LSB = 1.25V / 63 = 19.8mV. Calculate R3 to adjust VOUT with DACOUT (Figure 1). For VOUT(MAX) = 25V and VOUT(MIN) = 12.5V, determine R3 as follows: R3 = R2 x (VFB) / (VOUT(MAX) - VOUT(MIN)) = 1.08MΩ x (1.25) / (25 - 12.5) = 108kΩ The general form for VOUT as a function of the DAC output (VDACOUT) is: VOUT = VOUT(MIN) + (VFB - VDACOUT) x R2 / R3 At power-up, the DAC resets to mid-scale where VDACOUT = 0.635V. Therefore, the output voltage after power-up is: VOUT(MID) = VOUT(MIN) + (1.25 - 0.635) x R2 / R3 = 18.65V ______________________________________________________________________________________ DAC-Controlled Boost/Inverter LCD Bias Supply with Internal Switch Setting the Minimum Negative Output Voltage For a negative output voltage, the FB threshold voltage (VFB) is 0V, and R1 is placed between FB and REF (Figures 2 and 3). Again, choose R1 to be 120kΩ so that the current in the divider is about 10µA. Then determine R2 as follows: R2 = R1 x |VOUT / VREF | For example, if VOUT(MIN) = -12.5V: R2 = 120kΩ x |(-12.5) / (1.25)| = 1.2MΩ Setting the Maximum Negative Output Voltage Assume VOUT(MAX) = -25V and VOUT(MIN) = -12.5V, then determine R3 and VOUT(MID) as follows: R3 = R2 x (VFB - VDACOUT(MAX)) / (VOUT(MAX) VOUT(MIN)) = 1.2MΩ x (0 - 1.25) / (-25 - -12.5) =120kΩ For a negative output voltage, VOUT = VOUT(MIN) + (VFB - VDACOUT) x R2 / R3. At power-up, the DAC resets to mid-scale where VDACOUT = 0.635V. Therefore, the output voltage after reset is: VOUT(MID) = -12.5 + (0 - 0.635) x (1.2M) / (120k) = -18.85V Note that for a negative output voltage, |VOUT| increases as VDACOUT increases. |VOUT(MAX)| corresponds to V DACOUT = 1.25V, and | V OUT(MIN)| corresponds to VDACOUT = 0V. Setting the Output Voltage without the DAC The MAX686 may be used without the DAC to control the output voltage. For either positive or negative output voltage applications, set the MAX686’s output voltage using only two external resistors (R1 and R2) as shown in Figure 1, 2, or 3. Since the input bias current at FB has a 50nA maximum value, large resistors can be used in the feedback loop without a significant loss of accuracy. Select R1 to be in the 10kΩ to 220kΩ range and calculate R2 using the applicable equations from the following subsections. Setting the Positive Output Voltage Use the circuit of Figure 1, connecting POL to GND and omitting R3. Connecting POL to GND sets the threshold voltage at FB to VREF. Choose the value of R1 in the 10kΩ to 220kΩ range and calculate R2 as follows: R2 = R1 x (VOUT / VREF -1) where VREF = 1.25V. Setting the Negative Output Voltage For negative output voltages, configure R1 and R2 as shown in Figures 2 and 3, connecting POL to VCC and omitting R3. Connecting POL to VCC sets the FB threshold voltage to GND for negative output voltages. Choose R1 in the 10kΩ to 220kΩ range and calculate R2 as follows: R2 = R1 x |VOUT|/ VREF where VREF = 1.25V. Figures 2 and 3 demonstrate two possible methods of generating a negative voltage with the MAX686. In Figure 3, D2 connects to the input supply (VIN). This connection features the best output ripple performance, but |VOUT| must be limited to values less than -27.5V - VIN. If the application requires a larger negative voltage, use the method of Figure 2, connecting D2 to GND. This method allows a maximum output voltage of -27.5V, but |VOUT| must be greater than VIN. Setting the Peak Inductor Current Limit External current-limit selection provides added control over the MAX686’s output performance. A higher current limit increases the amount of energy stored in the inductor during each cycle, which provides higher output current capability. For higher output current applications, choose the 500mA current-limit option by connecting ISET to VCC. When the load requires lower output current, the 250mA current limit provides several advantages. First, a smaller inductor saves board area and cost. Second, smaller energy transfers per cycle reduce output ripple for a given capacitor. Connecting ISET to GND selects the 250mA current-limit option. Connecting ISET to VCC selects the 500mA current-limit option. Refer to the Typical Operating Characteristics for efficiency and load current graphs at each ISET current setting. Selecting Inductors The MAX686’s high switching frequency allows for the use of a small inductor. The 22µH inductor shown in Figures 1, 2, and 3 is recommended for most applications, although values between 10µH and 47µH are acceptable. Use inductors with a ferrite core or equivalent; powder iron cores are not recommended for use with high switching frequencies. The inductor’s incremental saturation rating must exceed the selected current limit. For highest efficiency, use an inductor with a low DC resistance (under 200mΩ). See Table 1 for a list of inductor suppliers. ______________________________________________________________________________________ 11 MAX686 Note that for a positive output voltage, VOUT increases as V DACOUT decreases. V OUT(MAX) corresponds to V DACOUT = 0V, and V OUT(MIN) corresponds to VDACOUT = 1.25V. MAX686 DAC-Controlled Boost/Inverter LCD Bias Supply with Internal Switch MBR0530L 22µH VIN = 0.8V TO 27.5V VOUT 15µF R2 VIN = 0.8V TO 27.5V MBR0530L 22µH VOUT 15µF R2 CF CF LX VCC = 2.7V TO 5.5V VCC = 2.7V TO 5.5V 0.1µF LX VCC 0.1µF MAX686 R3 REF VCC R3 MAX686 RPOT 100k POTENTIOMETER DACOUT FB FB R1 R1 Figure 4. Feed-Forward Capacitor Figure 5. Using a Potentiometer to Adjust Output Voltage Selecting Capacitors Table 1. Component Suppliers SUPPLIER PHONE FAX AVX: TPS series (803) 946-0690 (803) 626-3123 Matsuo: 267 series (714) 969-2491 (714) 960-6492 Sprague 595D series (603) 224-1961 (603) 224-1430 Motorola: MBR0530L (602) 303-5454 (602) 994-6430 Nihon; EC11 FS1 series (805) 867-2555 (805) 867-2698 CAPACITORS DIODES INDUCTORS Coilcraft: DO1608 and DT1608 series (847) 639-6400 (847) 639-1469 Murata-Erie: LQH4 series (814) 237-1431 (814) 238-0490 Sumida: CD43, CD54, and CD74 series (847) 956-0666 (847) 956-0702 TDK: NLC565050 series (847) 390-4373 (847) 390-4428 Selecting Diodes The MAX686’s high switching frequency demands a high-speed rectifier. Schottky diodes, such as the 1N5818 or MBR0530L, are recommended. Make sure that the diode’s peak current rating exceeds the peak current set by ISET and that its breakdown voltage exceeds the output voltage. Schottky diodes are preferred due to their low forward voltage. However, ultrahigh-speed silicon rectifiers are also acceptable. Table 1 lists Schottky diode suppliers. 12 Output Filter Capacitors The primary selection criterion for the output filter capacitor is low equivalent series resistance (ESR). The product of the peak inductor current and the output filter capacitor’s ESR determines the amplitude of the high-frequency ripple seen on the output voltage. These requirements can be balanced by appropriately selecting the current limit, as discussed in the Setting the Peak Inductor Current Limit section. Table 1 lists some low-ESR capacitor suppliers. Bypass Capacitors Although the output current of many MAX686 applications may be relatively small, the input supply must be able to source current transients equal to the ISET current limit. The input bypass capacitor reduces the peak currents drawn from the voltage source and reduces noise caused by the MAX686’s switching action. The input source impedance determines the size of the capacitor required at the input (VIN). As with the output filter capacitor, low ESR is the primary consideration. A 15µF, low-ESR capacitor is adequate for most applications, although smaller bypass capacitors may also be acceptable in light-load applications. Bypass the IC separately with a 0.1µF ceramic capacitor placed as close as possible to the VCC and GND pins. Bypass REF to GND with a 0.1µF ceramic capacitor for REF currents up to 25µA. REF can source up to 50µA of current for external loads. For 25µA ≤ IREF ≤ 50µA, bypass REF with a 0.47µF capacitor. ______________________________________________________________________________________ DAC-Controlled Boost/Inverter LCD Bias Supply with Internal Switch R2 LX VCC VOUT R6 R2 ILCD LX VCC R3 DACOUT MBR0530L 22µH VIN = 0.8V TO 27.5V R7 VOUT R6 ILCD R3 DACOUT VOUTSW LCDON MAX686 MBR0530L 22µH VIN = 0.8V TO 27.5V R7 VOUTSW LCDON POSITIVE OUTPUT VOLTAGE FB POSITIVE OUTPUT VOLTAGE FB R4 POK MAX686 MAX686 POK R1 R5 GND R1 GND Figure 6. Using the POK for Input Voltage Monitoring Figure 7. Using the POK for Output Voltage Monitoring Feed-Forward Capacitors Parallel a feed-forward capacitor (CF) across R2 to compensate the feedback loop and ensure stability (Figure 4). Use values up to 100pF for most applications. Choose the lowest capacitor value that ensures stability; high capacitance values may degrade line regulation. one diode drop below the input voltage (VIN) in shutdown. LCDON is not needed for negative outputs, which already fall to 0V in shutdown. • An input-sensing cutoff for positive outputs. Connect POK to a voltage divider to sense the input voltage. The output switches on only when the input reaches the set level (Figure 6). • An output-sensing cutoff for positive outputs. Connect POK to the feedback voltage divider to sense the output voltage. The output switches on only when it reaches 90% of the set voltage (Figure 7). Applications Information Using a Potentiometer to Adjust the Output Voltage The output can be adjusted with a potentiometer instead of the DAC (Figure 5). Choose RPOT = 100kΩ and connect it between REF and GND. Connect R3 to the potentiometer’s wiper instead of to DACOUT. Use the same design equations for adjusting the output voltage with the DAC. Controlling the LCD Using POK and LCDON When the voltage at POK is greater than 1.125V (typical), the open-drain LCDON output pulls low. LCDON can withstand up to 27.5V to control an external PNP transistor to switch on the MAX686’s positive output (Figures 6 and 7). A PFET can also be used, but a resistor-divider must be used in conjunction with it, so that the PFET does not exceed its VGS rating. Three useful applications of this feature are as follows: • An off-switch driver to ensure that a positive boosted output goes to 0V during shutdown. Connect POK to SHDN. Without this switch, the positive output falls to For positive output voltage sensing, connect POK directly to FB to monitor the output voltage (Figure 7). The POK threshold is 10% less than the set voltage at FB. Therefore, when the output voltage drops 10% below its set value, the POK circuit turns off the external PNP transistor, disconnecting the load. For input voltage sensing, a resistor-divider (R4-R5, Figure 6) from VIN to POK controls the open-drain output LCDON, which pulls low when VPOK > 1.125V. Choose R5 = 100kΩ. For example, if the minimum battery voltage is 5.3V, then determine R4 as follows: R4 = R5 x [(VIN / VPOK) - 1] = 100k x [(5.3 / 1.125) -1] = 371kΩ LCDON typically drives a low-cost PNP transistor (such as a 2N2907 or equivalent), switching a positive VOUT to the LCD. Choose a PNP with low VCESAT at the required load current. R7 limits the base current in the PNP, and ______________________________________________________________________________________ 13 MAX686 DAC-Controlled Boost/Inverter LCD Bias Supply with Internal Switch MBR0530L 22µH VIN = 2.7V TO 5.5V 15µF R2 CF LX VDD R8 100Ω VOUT MAX686 VCC to the source through a 100Ω resistor (R8), and bypass VCC with a 1µF ceramic capacitor as shown in Figure 8. Since the supply current is very small, the voltage drop across R8 is insignificant and does not degrade performance. The RC isolates V CC from the switching noise created by the inductor and internal power switch. Although, in many cases, the MAX686 and the inductor are powered from the same source, it is often advantageous in battery-powered applications to power the MAX686 IC (VCC, VDD) from an available regulated supply and to power the inductor (VIN) directly from a battery. The MAX686 requires a +2.7V to +5.5V supply at VCC, but the inductor can be powered from voltages as low as 0.8V, significantly increasing usable battery life. R3 DACOUT VCC 1µF FB R1 Layout Considerations Figure 8. Using a Common Supply-Voltage Source R6 turns it off when LCDON goes high. R6 and R7 can be the same value. Choose R7 such that the minimum base current is greater than 1/50 of the collector current. For example, assume VOUT(MIN) = 12.5V and ILCD = 10mA and then determine R7 as follows: R7 ≤ 50 x (12.5 - 0.7) / 10mA = 59kΩ Remember that the LCD voltage, VOUTSW, is the regulated output voltage minus the drop across the PNP switch (300mV typ). Connecting VIN to VCC The MAX686 (VCC, VDD) and the inductor (VIN) can be powered from the same source as long as the +5.5V VCC maximum limit is not violated. To ensure stability, connect VIN and VDD directly to the source, connect 14 Proper PC board layout is essential due to high current levels and fast switching waveforms that radiate noise. It is recommended that initial prototyping be performed using the MAX686 evaluation kit or equivalent PC board-based design. Breadboards or proto-boards should never be used when prototyping switching regulators. Connect the GND pin, the input bypass-capacitor ground lead, and the output filter-capacitor ground lead to a single point (star ground configuration) to minimize ground noise and improve regulation. Also, minimize lead lengths to reduce stray capacitance, trace resistance, and radiated noise, with preference given to the feedback circuit, the ground circuit, and LX. Place R1 and R2 as close to the feedback pin as possible. Place the bypass capacitors as close to the pins as possible. Refer to the MAX686 evaluation kit data sheet for an example of proper board layout. ______________________________________________________________________________________ DAC-Controlled Boost/Inverter LCD Bias Supply with Internal Switch VCC VDD GND BIAS UP DN SHDN DIGITAL INTERFACE MAX686 6-BIT DAC POL REF 1.25V DACOUT BANDGAP REFERENCE LX 1.125V ERROR AMP FB ON-TIME/ OFF-TIME CONTROL ISET 1.125V POK POK COMPARATOR PGND CURRENT-LIMIT COMPARATOR LCDON ______________________________________________________________________________________ 15 MAX686 Functional Diagram QSOP.EPS MAX686 DAC-Controlled Boost/Inverter LCD Bias Supply with Internal Switch Chip Information TRANSISTOR COUNT: 1325 SUBSTRATE CONNECTED TO GND 16 ______________________________________________________________________________________