19-1794 Rev 0; 10/00 Compact, High-Efficiency, Dual-Output Step-Up DC-DC Converter The MAX1817’s main regulator supplies 125mA at either a preset 3.3V or an adjustable 2.5V to 5.5V output voltage with up to 88% efficiency. A 0.1µA shutdown state also minimizes battery drain. The MAX1817’s secondary step-up converter provides the LCD bias voltage and is adjustable up to +28V. Other features include a fast switching frequency to reduce the size of external components and a low quiescent current to maximize battery life. Both outputs can be independently shut down for improved flexibility. The MAX1817 is supplied in a compact 10-pin µMAX package. The MAX1817 evaluation kit (MAX1817EVKIT) is available to speed up design. Features ♦ Dual Step-Up Converter in a Tiny 10-Pin µMAX Package ♦ Main Output Up to 125mA Load Current Fixed 3.3V or Adjustable 2.5V to 5.5V Up to 88% Efficiency Internal Switch ♦ LCD Output Up to 28V for LCD Bias Internal Switch ♦ Input Voltage Range +1.5V to +5.5V ♦ Minimal External Components Required ♦ 0.1µA Logic-Controlled Shutdown ♦ Low 15µA Quiescent Supply Current Ordering Information ________________________Applications Organizers/Translators PDAs PART TEMP. RANGE PIN-PACKAGE MAX1817EUB -40°C to +85°C 10 µMAX MP3 Players GPS Receivers Typical Operating Circuit +1.5V TO +5.5V Pin Configuration TOP VIEW FB 1 LCD LCD ON/OFF 10 OUT 2 MAX1817 9 LX ONLCD 3 8 GND LX LXLCD FBLCD 4 7 LXLCD ONLCD FBLCD AGND 5 6 N.C. µMAX MAX1817 MAIN ON/OFF ON ON AGND OUT FB GND MAIN ________________________________________________________________ Maxim Integrated Products 1 For price, delivery, and to place orders, please contact Maxim Distribution at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com. MAX1817 General Description The MAX1817 is a compact, high-efficiency, dual-output step-up converter for portable devices that provides both the main logic supply and the LCD bias. The device operates from an input voltage of +1.5V to +5.5V, allowing the use of 2- or 3-cell alkaline batteries, or 1-cell lithium-ion (Li+) batteries. MAX1817 Compact, High-Efficiency, Dual-Output Step-Up DC-DC Converter ABSOLUTE MAXIMUM RATINGS OUT to GND .............................................................-0.3V to +6V ON, ONLCD, FB, FBLCD, LX to GND ......-0.3V to (VOUT + 0.3V) LXLCD to GND .......................................................-0.3V to +30V AGND to GND .......................................................-0.3V to +0.3V Continuous Power Dissipation (TA = +70°C) 10-Pin µMAX (derate 5.6mW/°C above +70°C) ...........444mW LXLCD, LX Maximum Current ........................................0.5ARMS 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 (VON = VONLCD = VOUT = +3.3V, FB = GND, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER CONDITIONS MIN TYP MAX UNITS 5.5 V GENERAL Input Voltage Range Startup Voltage 1.5 RLOAD = 35Ω 1.5 RLOAD = ∞, VFB = 1.35V 1.2 1.55 5 10 V Quiescent Current from OUT (Main Only) VFB = VFBLCD = 1.35V, VONLCD = 0 Quiescent Current from OUT VFB = VFBLCD = 1.35V 15 30 µA Shutdown Quiescent Current VON = VONLCD = 0 0.1 1 µA 2.2 2.4 µA MAIN OUTPUT OUT Undervoltage Lockout Fixed-Mode Output Voltage VOUT rising, VFB = 1.35V 1.95 2.15 VFB ≤ 45mV 3.14 3.3 3.47 V 1.20 1.25 1.30 V 50 nA 105 mV 5.5 V Adjustable-Mode FB Regulation Voltage FB Input Bias Current TM FB Dual Mode VFB = 1.35V Threshold 45 Output Voltage Adjustment Range 2.5 Maximum LX On-Time VFB = 0.5V Zero Crossing Comparator Threshold (VLX - VOUT) 75 2.4 5 7.5 µs 0 20 40 mV 22 45 70 µs Zero Crossing Comparator Backup Timer VFB = +0.5V Line Regulation IOUT = 100mA, VIN = +2V to +3V 1.6 % Load Regulation VIN = +2.5V, ILOAD = 10mA to 100mA 1.6 % LX On-Resistance VOUT = 3.3V, ILX = 100mA 0.35 0.65 Ω 0.75 1.05 A LX Current Limit 0.5 Dual Mode is a trademark of Maxim Integrated Products. 2 V VOUT falling, VFB = 1.35V _______________________________________________________________________________________ Compact, High-Efficiency, Dual-Output Step-Up DC-DC Converter (VON = VONLCD = VOUT = +3.3V, FB = GND, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER CONDITIONS LX Leakage Current VON = GND, VLX = 5.5V ON Input Low Voltage 1.8V < VOUT < 5.5V, VFB = 0.5V ON Input High Voltage 1.8V < VOUT < 5.5V, VFB = 0.5V MIN TYP 0.1 MAX UNITS 1 µA 400 mV 1.6 V ON Input Bias Current 1 µA LCD OUTPUT LXLCD Voltage LXLCD On-Resistance VOUT = 3.3V, ILXLCD = 100mA LXLCD Current Limit LXLCD Leakage Current 0.28 VLXLCD = 28V, VONLCD = 0 FBLCD Regulation Voltage 1.20 28 V 1.1 2.0 Ω 0.5 0.7 A 0.1 1 µA 1.25 1.30 V 50 nA FBLCD Input Bias Current VFBLCD = 1.35V LCD Line Regulation ILOAD = 5mA, VIN = +2V to +3V 0.1 % LCD Load Regulation ILOAD = 1mA to 5mA, VIN = +2.5V 0.5 % Maximum LXLCD On-Time Minimum LXLCD Off-Time 4 9 14 VFBLCD ≥ 1.2V 0.5 1 1.5 VFBLCD ≤ 0.7V 2.4 5 7.5 ONLCD Input Low Voltage 2.5V < VOUT < 5.5V ONLCD Input High Voltage 2.5V < VOUT < 5.5V µs 400 mV 1 µA 1.6 ONLCD Input Bias Current µs V ELECTRICAL CHARACTERISTICS (VON = VONLCD = VOUT = +3.3V, FB = GND, TA = -40°C to +85°C, unless otherwise noted.) (Note 1) PARAMETER CONDITIONS MIN MAX UNITS 1.5 5.5 V GENERAL Input Voltage Range Startup Voltage RLOAD = ∞, VFB = 1.35V 1.7 V Quiescent Current from OUT (Main Only) VFB = VFBLCD = 1.35V, VONLCD = 0 10 µA Quiescent Current from OUT VFB = VFBLCD = 1.35V 30 µA Shutdown Quiescent Current VON = VONLCD = 0 1 µA MAIN OUTPUT OUT Undervoltage Lockout Fixed-Mode Output Voltage VOUT rising, VFB = 1.35V V VOUT falling, VFB = 1.35V 1.95 VFB ≤ 45mV 3.14 3.47 V 1.20 1.30 V 50 nA Adjustable-Mode FB Regulation Voltage FB Input Bias Current 2.4 VFB = 1.35V _______________________________________________________________________________________ 3 MAX1817 ELECTRICAL CHARACTERISTICS (continued) MAX1817 Compact, High-Efficiency, Dual-Output Step-Up DC-DC Converter ELECTRICAL CHARACTERISTICS (continued) (VON = VONLCD = VOUT = +3.3V, FB = GND, TA = -40°C to +85°C, unless otherwise noted.) (Note 1) MIN MAX UNITS FB Dual Mode Threshold PARAMETER 45 105 mV Output Voltage Adjustment Range 2.5 5.5 V 2.4 7.5 µs 0 40 mV 22 70 µs 0.65 Ω 1.05 A Maximum LX On-Time CONDITIONS VFB = 0.5V Zero Crossing Comparator Threshold (VLX - VOUT) Zero Crossing Comparator Backup Timer VFB = 0.5V LX On-Resistance VOUT = 3.3V, ILX = 100mA LX Current Limit 0.42 LX Leakage Current VON = GND, VLX = 5.5V ON Input Low Voltage 1.8V < VOUT < 5.5V, VFB = 0.5V ON Input High Voltage 1.8V < VOUT < 5.5V, VFB = 0.5V 1 µA 400 mV 1.6 ON Input Bias Current V 1 µA 28 V 2 Ω LCD OUTPUT LXLCD Voltage LXLCD On-Resistance VOUT = 3.3V, ILXLCD = 100mA LXLCD Current Limit LXLCD Leakage Current 0.25 VLXLCD = 28V, VONLCD = 0 FBLCD Regulation Voltage FBLCD Input Bias Current 1.20 µA V 70 nA 4 14 µs VFBLCD ≥ 1.2V 0.5 1.5 VFBLCD ≤ 0.7V 2.2 7.5 ONLCD Input Low Voltage 2.5V < VOUT < 5.5V ONLCD Input High Voltage 2.5V < VOUT < 5.5V Note 1: Specifications to -40°C are guaranteed by design and not production tested. _______________________________________________________________________________________ µs 400 mV 1 µA 1.6 ONLCD Input Bias Current 4 A 1 1.30 VFBLCD = 1.35V Maximum LXLCD On-Time Minimum LXLCD Off-Time 0.7 V Compact, High-Efficiency, Dual-Output Step-Up DC-DC Converter LCD OUTPUT EFFICIENCY vs. LOAD CURRENT 65 50 45 40 1 10 100 25 0.01 1000 0.1 1 10 80 60 0 1.0 100 1.1 1.2 1.3 1.4 1.5 1.6 1.7 STARTUP VOLTAGE (V) LOAD CURRENT (mA) LOAD CURRENT (mA) NO-LOAD SUPPLY CURRENT vs. INPUT VOLTAGE NO-LOAD SUPPLY CURRENT vs. INPUT VOLTAGE (LCD OFF) 18 450 MAX1817 toc04 16 14 VOUT = 3.3V VLCD = 18V, NO LOAD R1 = 1MΩ, R2 = 75kΩ 400 SUPPLY CURRENT (µA) 0.1 100 20 VOUT = 3.3V, NO LOAD 30 CIRCUIT OF FIGURE 2 120 40 35 E: VIN = 1.8V, VOUT = 3.3V 70 A: VIN = +2.4V, VLCD = 12V B: VIN = +2.4V, VLCD = 18V C: VIN = +2.4V, VLCD = 24V D: VIN = +1.8V, VLCD = 12V E: VIN = +1.8V, VLCD = 18V F: VIN = +1.8V, VLCD = 24V 55 RESISTIVE LOAD LCD OFF 140 12 10 8 6 MAX1817-05 75 A LOAD CURRENT (mA) D: VIN = 1.8V, VOUT = 5V C: VIN = 2.4V, VOUT = 3.3V B D F E 60 EFFICIENCY (%) 80 C 160 MAX1817-02 70 85 SUPPLY CURRENT (µA) EFFICIENCY (%) 90 B: VIN = 2.4V, VOUT = 5V A: VIN = 3.3V, VOUT = 5V STARTUP VOLTAGE vs. LOAD CURRENT 75 MAX1817-01 95 MAX1817 toc03 MAIN OUTPUT EFFICIENCY vs. LOAD CURRENT 350 300 250 200 150 4 100 2 50 0 0 1.0 1.5 2.0 2.5 3.0 3.5 0 4.0 1 2 3 4 INPUT VOLTAGE (V) INPUT VOLTAGE (V) MAIN CONVERTER SWITCHING WAVEFORM LCD CONVERTER SWITCHING WAVEFORM A A 0 0 B B C C 10µs/div A: ILX, 500mA/div B: VOUT, 50mV/div, AC-COUPLED C: VLX, 5V/div VIN = 2.4V, VOUT = 3.3V, ILOAD,OUT = 50mA, VONLCD = 0 6 MAX1817-07 MAX1817-06 0 5 0 4µs/div A: ILXLCD, 500mA/div B: VLCD, 100mV/div, AC-COUPLED C: VLXLCD, 10V/div VIN = 2.4V, VOUT = 3.3V, ILOAD,OUT = 0, VLCD = 18V, ILOAD,LCD = 5mA _______________________________________________________________________________________ 5 MAX1817 Typical Operating Characteristics (Circuit of Figure 3, TA = +25°C, unless otherwise noted.) MAX1817 Compact, High-Efficiency, Dual-Output Step-Up DC-DC Converter Typical Operating Characteristics (continued) (Circuit of Figure 3, TA = +25°C, unless otherwise specified) MAIN LOAD TRANSIENT RESPONSE LCD LOAD TRANSIENT RESPONSE MAX1817-08 MAX1817-09 A A B B 0 0 400µs/div A: VOUT, 100mV/div, AC-COUPLED B: ILOAD, OUT, 50mA/div VIN = 2.4V, VOUT = 3.3V 200µs/div A: VLCD, 50mV/div, AC-COUPLED B: ILOAD, OUT, 10mA/div VIN = 2.4V, VOUT = 3.3V (NO LOAD), VLCD = 18V MAIN LINE TRANSIENT RESPONSE LCD LINE TRANSIENT RESPONSE MAX1817-10 MAX1817-11 A 2.4V B 1.8V 400µs/div A: VOUT, 100mV/div, AC-COUPLED B: VIN, 1V/div A 2.4V B 1.8V 200µs/div A: VLCD, 100mV/div, AC-COUPLED B: VIN, 1V/div VOUT = 3.3V (NO LOAD), VLCD = 18V, ILOAD,LCD = 2mA VOUT = 3.3V, ILOAD,MAIN = 20mA, VONLCD = 0 MAIN OUTPUT TURN-ON/TURN-OFF RESPONSE LCD OUTPUT TURN-ON/TURN-OFF RESPONSE MAX1817-12 MAX1817-13 A 0 A 0 B 0 B 0 C C 0 100µs/div A: VOUT, 2V/div B: IIN, 500mA/div C: VON, 5V/div VIN = 2.4V, RLOAD,MAIN = 165Ω, VONLCD = 0 6 0 400µs/div A: VLCD, 10V/div B: IIN, 200mA/div C: VONLCD, 5V/div VIN = 2.4V, VOUT = 3.3V (NO LOAD), RLOAD,LCD = 9kΩ _______________________________________________________________________________________ Compact, High-Efficiency, Dual-Output Step-Up DC-DC Converter PIN NAME FUNCTION 1 FB Main Output Feedback Input. Connect FB to GND for fixed 3.3V main output. For other output voltages, use a resistive voltage-divider to set the output voltage. The feedback regulation voltage is 1.25V at FB. 2 ON Main Step-Up Converter On/Off Control. Connect ON to OUT for automatic startup. Connect ON to GND to put the IC into shutdown mode. 3 ONLCD LCD Output On/Off Control. Connect ONLCD to OUT to enable the LCD output. Connect ONLCD to GND to disable the LCD output. The main output must be ≥2.4V to enable the LCD output. 4 FBLCD LCD Output Feedback Input. Use a resistive voltage-divider from the LCD output to FBLCD to set the voltage. The feedback regulation voltage is 1.25V at FBLCD. 5 AGND Analog Ground. Connect AGND to GND as close to the IC as possible. 6 N.C. 7 LXLCD 8 GND 9 LX Main Output Switching Node. Drain of the internal N-channel MOSFET that drives the main output. Connect an external inductor and rectifier to LX. 10 OUT Main Step-Up Converter Output. OUT is used to measure the output voltage in fixed mode (FB = GND) and is the internal bias supply input to the IC. When shut down (ON = ONLCD = GND), OUT is high impedance, drawing 1µA (max). No Connection. Not internally connected. LCD Output Switching Node. Drain of the internal N-channel MOSFET that drives the LCD output. Connect an external inductor and rectifier to LXLCD. Power Ground. Connect GND to AGND as close to the IC as possible. ________________Detailed Description The MAX1817 dual step-up converter is designed to supply the main power and LCD bias for low-power, hand-held devices. The MAX1817’s main step-up converter includes a 0.35Ω N-channel power MOSFET switch and provides a fixed 3.3V or adjustable 2.5V to 5.5V output at up to 125mA from an input as low as 1.5V. The MAX1817’s LCD bias step-up converter includes a high-voltage 1.1Ω power MOSFET switch to support as much as 5mA at 28V (Figure 1). During startup, the MAX1817 extends the LCD MOSFET switch minimum off-time, limiting surge current. Both converters require an inductor and external rectifier. The MAX1817 runs in bootstrap mode, powering the IC from the main step-up converter’s output. Independent logic-controlled shutdown for the main and LCD stepup converters reduces quiescent current to 0.1µA. Main Step-Up Converter The MAX1817 main step-up converter runs from a +1.5V to +5.5V input voltage and produces a fixed 3.3V or adjustable 2.5V to 5.5V output voltage as well as biasing the internal control circuitry. The MAX1817 switches only as often as is required to supply sufficient power to the load. This allows the converter to operate at lower frequencies at light loads, improving efficiency. The control scheme maintains regulation when the error amplifier senses the output voltage is below the feedback threshold, turning on the internal N-channel MOSFET and initiating an on-time. The on-time is terminated when the 0.75A current limit is reached or when the maximum on-time is reached. The N-channel MOSFET remains off until the inductor current drops to 0, forcing discontinuous inductor current. At the end of a cycle, the error comparator waits for the voltage at FB to drop below the regulation threshold, at which time another cycle is initiated. The main step-up converter uses a startup oscillator to allow it to start from an input voltage as low as +1.2V. This is necessary since the control circuitry is powered from the step-up converter output (OUT). When the voltage at OUT is below the OUT undervoltage lockout, a fixed 50% duty cycle drives the internal N-channel MOSFET, forcing the main output voltage to rise. Once _______________________________________________________________________________________ 7 MAX1817 Pin Description MAX1817 Compact, High-Efficiency, Dual-Output Step-Up DC-DC Converter VIN DUAL-MODE FEEDBACK OUT ZEROCROSSING DETECTOR ERROR COMPARATOR MAIN FB LX MAIN LXLCD LCD CONTROL LOGIC MAIN 1.25V AGND 75mV ON ON OFF STARTUP CURRENT LIMIT MAIN SHUTDOWN LOGIC MAIN GND UNDERVOLTAGE LOCKOUT ON ONLCD OFF SHUTDOWN LOGIC LCD CONTROL LOGIC LCD BIAS LCD 1.25V AGND ERROR COMPARATOR LCD CURRENT LIMIT LCD MAX1817 GND AGND FBLCD GND Figure 1. MAX1817 Simplified Functional Diagram the output voltage rises above the undervoltage threshold, the control circuitry is enabled, allowing proper regulation of the output voltage. LCD Step-Up Converter The MAX1817’s LCD step-up converter generates an LCD bias voltage up to 28V by use of a 500mA, 1.1Ω internal N-channel switching MOSFET (Figure 1). The LCD step-up converter control circuitry is powered from the main step-up converter output (OUT), so the voltage at OUT must be above the OUT undervoltage lockout voltage for the LCD step-up converter to operate. 8 During startup, the MAX1817 extends the minimum offtime to 5µs for VFBLCD voltages <0.9V, limiting initial surge current. The LCD step-up converter features an independent shutdown control, ONLCD. The LCD step-up converter features a minimum-offtime, current-limited control scheme. A pair of oneshots that set a minimum off-time and a maximum ontime governs the duty cycle. The switching frequency can be up to 500kHz and depends upon the load, and input and output voltages. _______________________________________________________________________________________ Compact, High-Efficiency, Dual-Output Step-Up DC-DC Converter L2 10µH LXLCD ON OFF OFF LCD ON MAIN ONLCD 4.7pF C4 R1 1M OFF R3 300k C3 22µF MAIN 5V FB GND LX LXLCD LCD ON ONLCD FBLCD MAIN ON MAX1817 OUT ON FBLCD ON MAX1817 OUT D1 D2 LCD 18V C2 1µF R2 75k AGND L1 10µH L2 10µH D1 D2 LX C1 10µF VIN L1 10µH R4 100k Figure 2. Setting Main Output Voltage Using External Resistors Low-Voltage Startup The MAX1817’s internal circuitry is powered from OUT. The main step-up converter has a low-voltage startup circuit to control main DC-DC converter operation until V OUT exceeds the 2.2V (typ) undervoltage lockout threshold. The minimum startup voltage is a function of load current (see Typical Operating Characteristics). The MAX1817 main converter typically starts up into a 35Ω load with input voltages down to +1.5V, allowing startup with two alkaline cells even in deep discharge. Shutdown: ON and ONLCD The MAX1817 features independent shutdown control of the main and LCD step-up converters. With both converters shut down, supply current is reduced to 0.1µA. A logic low at ON shuts down the main step-up converter, and LX enters a high-impedance state. However, the main output remains connected to the input through the inductor and output rectifier, holding VOUT to one diode drop below the input voltage when the main converter is shut down. If the input voltage is sufficiently high to drive VOUT above the undervoltage lockout voltage, the LCD step-up converter operates. A logic low at ONLCD shuts down the LCD step-up converter, and LXLCD enters a high-impedance state. The LCD output remains connected to the input through the inductor and output rectifier, holding it to one diode drop below the input. OFF MAX1817 C1 10µF VIN 4.7pF C4 R1 1M LCD 18V C2 1µF R2 75k FB AGND C3 22µF MAIN 3.3V GND Figure 3. Typical Application Circuit ___________________Design Procedure Setting the Main Output Voltage The main step-up converter feedback input (FB) features Dual Mode operation. With FB grounded, the main output voltage is preset to 3.3V. It can also be adjusted from 2.5V to 5.5V with external resistors R3 and R4 as shown in Figure 2. To set the output voltage externally, select resistor R4 from 10kΩ to 100kΩ. Calculate R3 using: R3 = R4 [(VOUT / VFB) – 1] where VFB = 1.25V, and VOUT can range from 2.5V to 5.5V. Setting the LCD Output Voltage Set the LCD output voltage with two external resistors R1 and R2 as shown in Figure 3. Since the input leakage current at FBLCD has a maximum of 50nA, large resistors can be used without significant accuracy loss. Begin by selecting R2 in the 10kΩ to 100kΩ range, and calculate R1 using the following equation: R1 = R2 [(VLCD / VFBLCD ) – 1] where VFBLCD = 1.25V, and VLCD can range from VIN to 28V. _______________________________________________________________________________________ 9 MAX1817 Compact, High-Efficiency, Dual-Output Step-Up DC-DC Converter L1,10µH VIN C6 0.1µF R3 1 C1 10µF D3* D4* -19V VLCD C5 1µF L2 10µH D2 LX D1** LXLCD ONLCD ON C2 0.1µF FBLCD OUT FB AGND C4 10pF R1 240k MAX1817 MAIN R2 16.5k C3 22µF GND *D3, D4 = CENTRAL SEMICONDUCTOR CMPD7000 DUAL **D1 = CENTRAL SEMICONDUCTOR CMSD4448 (1N4148) Figure 4. Negative Voltage for LCD Bias Using a Charge Pump to Make Negative LCD Output Voltage The MAX1817 can generate a negative LCD output by adding a diode-capacitor charge-pump circuit (D3, D4, and C6) to the LXLCD pin as shown in Figure 4. FBLCD is driven through a resistive voltage-divider from the positive output, which is not loaded, allowing a very small capacitor value at C2. For best stability and lowest ripple, the time constant of the R1 + R2 series combination and C2 should be near that of C5 and the effective load resistance. Output load regulation of the negative output degrades compared to the standard positive output circuit and may rise at very light loads. If this is not acceptable, reduce the resistance of R1 and R2, while maintaining their ratio, to effectively preload the output with a few hundred µA. This is why the R1 and R2 values shown in Figure 4 are lower than typical values for a positive-output design. When loaded, the magnitude of the negative output voltage is slightly lower (closer to ground by approximately a diode forward voltage) than the voltage on C2. Applications Information Inductor Selection The MAX1817’s high switching frequency allows the use of small surface-mount inductors. The 10µH values 10 shown in Figure 3 are recommended for most applications, although values between 4.7µH and 47µH are suitable. Smaller inductance values typically offer a smaller physical size for a given series resistance, allowing the smallest overall circuit dimensions. Larger inductance values exhibit higher output current capability, but larger physical dimensions. Circuits using larger inductance values may start up at lower input voltages and exhibit less ripple, but they may 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 most inductors into saturation by as much as 20%, although this may slightly reduce efficiency. For best efficiency, select inductors with resistance no greater than the internal N-channel FET resistance in each step-up converter. For maximum output current, choose L such that: L < [(VIN ✕ tON) / IPEAK] where tON is the maximum switch on-time (5µs for main step-up converter) or 9µs for LCD step-up converter) and IPEAK is the switch peak current limit (0.75A for the main step-up converter, or 0.5A for the LCD step-up converter). With this inductor value, the maximum output current the main converter is able to deliver is given by: IOUT(MAX) = 0.5 ✕ IPEAK / (1 + tON / tOFF) where tON / tOFF = (VOUT + VD - VIN) / (VIN - VON), VIN and VOUT are the input and output voltages, VD is the Schottky diode drop (0.3V typ), and VON = IPEAK ✕ RON, where RON is the switch on-resistance. For VIN = 1.5V and VOUT = 3.3V, with a minimum IPEAK value of 0.5A, and VON(MAX) given by (0.5) ✕ (0.65) = 0.325V, the available output current that the converter can provide is at least 90mA. For larger inductor values, IPEAK is determined by: IPEAK = [(VIN ✕ tON) / L] External Rectifiers The high maximum switching frequency of the MAX1817 requires a high-speed rectifier. Schottky diodes such as the Motorola MBR0530 or the Nihon EP05Q03L are recommended. To maintain high efficiency, the average current rating of the Schottky diode should be greater than the peak switching current. A junction diode such as the Central Semiconductor CMPD4448 can be used for the LCD output with little ______________________________________________________________________________________ Compact, High-Efficiency, Dual-Output Step-Up DC-DC Converter Input Bypass Capacitor The input supplies high currents to the inductors and requires local bulk bypassing close to the inductors. A low equivalent series resistance (ESR) input capacitor connected in parallel with the battery will reduce peak battery currents and input-reflected noise. Battery bypassing is especially helpful at low input voltages and with high-impedance batteries (such as alkaline types). Benefits include improved efficiency and lower useful end-of-life voltage for the battery. A single 10µF low-ESR surface-mount capacitor is sufficient for most applications. Output Bypass Capacitors For most applications, use a small surface-mount 22µF or greater ceramic capacitor on the main converter output, and a 1µF or greater ceramic capacitor on the LCD output. For small ceramic capacitors, the output ripple voltage is dominated by the capacitance value. If tantalum or electrolytic capacitors are used, the ESR of the capacitors dominates the output ripple voltage. Decreasing the ESR reduces the output ripple voltage and the peak-to-peak transient voltage. (main converter) from 2.5V to 5.5V may require a larger value LCD feed-forward capacitor to prevent multipulsing of the LCD converter. Larger feed-forward capacitors slightly degrade load regulation, so choose the smallest value capacitor that provides stability. Layout Considerations The MAX1817’s high-frequency operation makes PC board layout important for optimal performance. Use separate analog and power ground planes. Connect the two planes together at a single point as close as possible to the IC. Use surface-mount components where possible. If leaded components are used, minimize lead lengths to reduce stray capacitance and keep the components close to the IC to minimize trace resistance. Where an external voltage-divider is used to set output voltage, the traces from FB or FBLCD to the feedback resistors should be extremely short (less than 0.2in or 5mm) to minimize coupling from LX and LXLCD. Refer to the MAX1817 evaluation kit for a full PC board example. ____________________Chip Information TRANSISTOR COUNT: 2785 PROCESS: BiCMOS LCD Compensation The MAX1817’s LCD step-up converter feedback requires a small 4.7pF feed-forward capacitor for the typical application circuit. Circuits with adjustable VOUT ______________________________________________________________________________________ 11 MAX1817 loss in efficiency. Choose a reverse breakdown voltage greater than the output voltage. Compact, High-Efficiency, Dual-Output Step-Up DC-DC Converter 10LUMAX.EPS MAX1817 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. 12 ____________________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.