19-1798; Rev 0; 12/00 Low Supply Current, Step-Up DC-DC Converters with True-Shutdown Features ♦ >95% Efficiency ________________________Applications True-Shutdown is a trademark of Maxim Integrated Products. ♦ True-Shutdown Circuitry Output Disconnects from Input in Shutdown No External Schottky Diode Needed ♦ 25µA Quiescent Supply Current ♦ Low-Noise Antiringing Feature ♦ LBI/LBO Comparator Enabled in Shutdown ♦ 2µA Shutdown Current ♦ 8-Pin µMAX Package Ordering Information PART TEMP. RANGE PIN-PACKAGE MAX1795 EUA -40°C to +85°C 8 µMAX MAX1796 EUA -40°C to +85°C 8 µMAX MAX1797 EUA -40°C to +85°C 8 µMAX Portable Digital Audio Players PDAs/Palmtops Wireless Handsets Portable Terminals Pin Configuration Typical Operating Circuit IN 0.7V TO 5.5V TOP VIEW LBI 1 FB 2 LBO 3 SHDN 4 8 MAX1795 MAX1796 MAX1797 µMAX BATT BATT 7 OUT 6 LX 5 GND LBI LX MAX1795 MAX1796 MAX1797 OUT OUT FB LBO OFF SHDN ON GND ________________________________________________________________ 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. MAX1795/MAX1796/MAX1797 General Description The MAX1795/MAX1796/MAX1797 are high efficiency step-up DC-DC converters intended for small portable hand-held devices. These devices feature Maxim’s True-Shutdown™ circuitry, which fully disconnects the output from the input in shutdown, improves efficiency, and eliminates costly external components. All three devices also feature Maxim’s proprietary LX-damping circuitry for reduced EMI in noise-sensitive applications. For additional in-system flexibility, a battery monitoring comparator (LBI/LBO) remains active even when the DC-DC converter is in shutdown. The input voltage range is +0.7V to VOUT, where VOUT can be set from +2V to +5.5V. Startup is guaranteed from +0.85V. The MAX1795/MAX1796/MAX1797 have a preset, pin-selectable 5V or 3.3V output. The output can also be adjusted to other voltages, using two external resistors. The three devices differ only in their current limits, allowing optimization of external components for different loads: The MAX1795, MAX1796, and MAX1797 have current limits of 0.25A, 0.5A, and 1A, respectively. All devices are packaged in a compact 8pin µMAX package that is only 1.09mm tall and half the size of an 8-pin SO. MAX1795/MAX1796/MAX1797 Low Supply Current, Step-Up DC-DC Converters with True-Shutdown ABSOLUTE MAXIMUM RATINGS OUT, LX, SHDN, LBI, LBO, BATT to GND................-0.3V to +6V FB .............................................................-0.3V to (VOUT + 0.3V) ILX, IOUT ..............................................................................±1.5A Output Short-Circuit Duration ...................................................5s Continuous Power Dissipation 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 (VBATT = +2V, OUT = FB (VOUT = +3.3V), SHDN = LBI = GND, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL Minimum Input Voltage Operating Voltage CONDITIONS MIN After startup VBATT Startup Voltage (Note 1) 1.0 TA = +25°C, RL = 3kΩ 0.85 VOUT 3.3 3.43 5.0 5.20 IOUT VFB VOUT = +2V to +5.5V Feedback Input Current IFB VFB = +1.24V Internal NFET, PFET OnResistance RDS(ON) VOUT = +3.3V, ILX = 100mA ILIM ILEAK 2 5.5 100 180 200 300 MAX1797 400 550 MAX1795 50 120 MAX1796 100 200 MAX1797 250 370 1.20 1.24 1.28 V 4 100 nA mA NFET 0.17 0.3 PFET 0.27 0.45 0.2 0.25 0.35 MAX1796 0.4 0.5 0.625 MAX1797 0.8 1.0 1.25 VLX = 0 and +5.5V, VOUT = +5.5V 100 VFB = +1.4V V MAX1796 MAX1795 RDAMP V MAX1795 Synchronous Rectifier Turn-Off Current Limit Operating Current into OUT (Note 2) mV/°C 4.80 Feedback Set-Point Voltage (Adjustable Mode) Damping Switch On-Resistance V 3.17 BATT = +2V, FB = GND (VOUT = +5.0V) LX Leakage Current V 1.0 FB = OUT BATT = +2V, FB = OUT (VOUT = +3.3V) LX Switch Current Limit (NFET only) 5.5 FB = GND 2.0 UNITS V -2.2 Adjustable Output Voltage Range Steady-State Output Current MAX 0.7 Startup Voltage Tempco Output Voltage TYP Ω A 0.2 µA 25 mA 200 400 Ω 25 45 µA _______________________________________________________________________________________ Low Supply Current, Step-Up DC-DC Converters with True-Shutdown (VBATT = +2V, OUT = FB (VOUT = +3.3V), SHDN = LBI = GND, TA = 0°C to +85°C, unless otherwise noted. Typical values are TA = +25°C.) PARAMETER SYMBOL Operating Current into BATT CONDITIONS MIN VFB = +1.4V, VLBI = +1V Shutdown Current into BATT tON VFB = +1V, if current limit not reached LX Switch Minimum Off-Time tOFF VFB = +1V LBI Threshold Voltage Falling VLBI MAX UNITS 2 4 µA 2 4 µA 3 4 5 µs 0.8 1 1.2 µs SHDN = BATT, VLBI = +1V LX Switch MaxImum On-Time TYP VBATT = +2V 0.8 0.85 0.90 VBATT = LBI 0.875 0.925 0.975 LBI Hysteresis 25 LBI Input Current ILBI VLBI = +0.8V 9 V mV 100 VBATT = VLBI = +0.975V, sinking 20µA (50Ω typ) 0.1 VBATT = VLBI = +1.1V, sinking 100µA (25Ω typ) 0.1 LBO Low Output Voltage nA V LBO Off-Leakage Current VLBO = +5.5V 1 0.2 ✕ VBATT VIL SHDN Input Voltage 0.8 ✕ VBATT VIH Shutdown Input Current 100 VSHDN = 0 and +5.5V 100 nA V nA ELECTRICAL CHARACTERISTICS (VBATT = +2V, OUT = FB (VOUT = +3.3V), SHDN = LBI = GND, TA = -40°C to +85°C, unless otherwise noted.) (Note 3) PARAMETER Operating Voltage SYMBOL VBATT Output Voltage VOUT CONDITIONS MAX UNITS V Note 1 1.0 5.5 FB = OUT 3.13 3.47 FB = GND 4.75 5.25 2.0 5.5 Adjustable Output Voltage Range FB = OUT (VOUT = +3.3V) Steady-State Output Current (Note 1) MIN IOUT FB = GND (VOUT = +5.0V) Feedback Set-Point Voltage (Adjustable Mode) VFB VOUT = +2V to +5.5V Feedback Input Current IFB VFB = +1.25V MAX1795 100 MAX1796 200 MAX1797 400 MAX1795 60 MAX1796 125 MAX1797 250 1.19 V V mA 1.29 V 100 nA _______________________________________________________________________________________ 3 MAX1795/MAX1796/MAX1797 ELECTRICAL CHARACTERISTICS (continued) MAX1795/MAX1796/MAX1797 Low Supply Current, Step-Up DC-DC Converters with True-Shutdown ELECTRICAL CHARACTERISTICS (continued) (VBATT = +2V, OUT = FB (VOUT = +3.3V), SHDN = LBI = GND, TA = -40°C to +85°C, unless otherwise noted.) (Note 3) PARAMETER Internal NFET, PFET OnResistance LX Switch Current Limit (NFET only) LX Leakage Current Damping Switch On-Resistance SYMBOL RDS(ON) ILIM ILEAK CONDITIONS VOUT = +3.3V, ILX = 100mA MIN MAX NFET 0.3 PFET 0.45 MAX1795 0.19 MAX1796 0.35 0.7 MAX1797 0.8 1.32 UNITS 0.37 100 400 Ω 45 µA Operating Current into OUT (Note 2) VFB = +1.4V Operating Current into BATT VFB = +1.4V, VLBI = +1V 4 µA Shutdown Current into BATT SHDN = BATT, VLBI = +1V 4 µA 5.25 µs µs LX Switch Maximum On-Time tON VFB = +1V, if current limit not reached LX Switch Minimum Off-Time tOFF VFB = +1V 0.7 1.3 VBATT = +2V 0.8 0.90 VBATT = LBI 0.875 0.975 LBI Threshold Voltage VLBI LBI Input Current ILBI 2.75 VLBI = +0.8V 100 VBATT = VLBI = +0.975V, sinking 20µA (50Ω typ) 0.1 VBATT = VLBI = +1.1V, sinking 100µA (25Ω typ) 0.1 VLBO = +5.5V 100 LBO Off-Leakage Current 0.2 ✕ VBATT VIL SHDN Input Voltage 0.8 ✕ VBATT VIH Shutdown Input Current V nA V LBO Low Output Voltage VSHDN = 0 and +5.5V 100 Note 1: Operating Voltage: Since the regulator is bootstrapped to the output, once started it will operate down to a 0.7V input. Note 2: Device is bootstrapped (power to IC comes from OUT). This correlates directly with the actual battery supply current. Note 3: Specifications to -40°C are guaranteed by design, not production tested. 4 A µA VLX = 0 and +5.5V, VOUT = +5.5V RDAMP Ω _______________________________________________________________________________________ nA V nA Low Supply Current, Step-Up DC-DC Converters with True-Shutdown VBATT = +1.2V 60 50 40 VBATT = +1.2V 60 50 40 80 50 40 30 20 10 10 0 0 100 10 L = 10µH 0 1000 0.1 1 10 100 1000 0.1 1 10 100 1000 LOAD CURRENT (mA) LOAD CURRENT (mA) LOAD CURRENT (mA) MAX1796 EFFICIENCY vs. LOAD CURRENT (+3.3V) MAX1797 EFFICIENCY vs. LOAD CURRENT (+5V) MAX1797 EFFICIENCY vs. LOAD CURRENT (+3.3V) 60 50 40 100 70 VBATT = +2.4V 60 VBATT = +1.2V 50 40 80 70 60 40 30 30 20 20 10 10 10 0 0 10 100 1000 VBATT = +1.2V 50 20 1 VBATT = +2.4V 90 30 0 0.1 1 10 100 1000 0.1 1 10 100 LOAD CURRENT (mA) LOAD CURRENT (mA) LOAD CURRENT (mA) NO-LOAD BATTERY CURRENT vs. INPUT BATT VOLTAGE STARTUP VOLTAGE vs. LOAD CURRENT SHUTDOWN THRESHOLD vs. INPUT BATT VOLTAGE 200 150 VOUT = +5V 3.0 2.5 VOUT = +3.3V 2.0 1.5 1.0 50 VOUT = +3.3V 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 BATT VOLTAGE (V) 2.5 2.0 1.5 1.0 0.5 0.5 0 0 0 1000 MAX1795/96/97 toc09 3.5 EFFICIENCY (%) 250 3.0 SHUTDOWN THRESHOLD (V) 4.0 MAX1795/96/97 toc07 300 MAX1795/96/97 toc08 0.1 MAX1795/96/97 toc06 80 EFFICIENCY (%) VBATT = +1.2V VBATT = +3.6V 90 EFFICIENCY (%) 80 70 100 MAX1795/96/97 toc05 VBATT = +2.4V 90 100 VBATT = +1.2V 60 20 10 VBATT = +2.4V 70 20 1 VBATT = +3.6V 90 30 100 EFFICIENCY (%) 70 100 30 0.1 BATTERY CURRENT (µA) 80 EFFICIENCY (%) VBATT = +2.4V 90 MAX1795/96/97 toc04 EFFICIENCY (%) 80 VBATT = +2.4V MAX1795/96/97 toc03 90 MAX1796 EFFICIENCY vs. LOAD CURRENT (+5V) EFFICIENCY (%) VBATT = +3.6V 70 100 MAX1795/96/97 toc01 100 MAX1795 EFFICIENCY vs. LOAD CURRENT (+3.3V) MAX1795/96/97 toc02 MAX1795 EFFICIENCY vs. LOAD CURRENT (+5V) 0.1 1 10 100 LOAD CURRENT (mA) 1000 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 BATT VOLTAGE (V) _______________________________________________________________________________________ 5 MAX1795/MAX1796/MAX1797 Typical Operating Characteristics (L = 22µH, CIN = 47µF, COUT = 47µF, TA = +25°C, unless otherwise noted.) Typical Operating Characteristics (continued) (L = 22µH, CIN = 47µF, COUT = 47µF, TA = +25°C, unless otherwise noted.) INCREASING VLBI 0.875 DECREASING VLBI 0.825 200 INCREASING VLBI 0.900 0.875 0.850 DECREASING VLBI -40 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 -15 10 35 60 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 BATT VOLTAGE (V) 300 VOUT = +3.3V 200 VOUT = +5.0V 1000 800 LOAD CURRENT (mA) LOAD CURRENT (mA) 400 VOUT = +3.3V 600 400 VOUT = +5.0V 200 100 0 0 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 BATT VOLTAGE (V) BATT VOLTAGE (V) OUT LEAKAGE CURRENT vs. OUTPUT VOLTAGE HEAVY-LOAD SWITCHING WAVEFORMS MAX1795/96/97 toc16 MAX1795/96/97 toc15 OUT LEAKAGE CURRENT (µA) 0 MAX1797 MAXIMUM OUTPUT CURRENT vs. BATT INPUT VOLTAGE MAX1795/96/97 toc13 500 SHDN = BATT VOUT = +5V VBATT = +2.4V VLX 5V/div 1.2 IINDUCTOR 500mA/div 0.8 0.4 0 VOUT (AC-COUPLED) 100mV/div OUT BIASED WITH EXTERNAL VOLTAGE SOURCE 4.00µs/div 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 OUTPUT VOLTAGE (V) 6 VOUT = +5.0V 100 0 85 MAX1796 MAXIMUM OUTPUT CURRENT vs. BATT INPUT VOLTAGE 1.6 150 TEMPERATURE (°C) BATT VOLTAGE (V) 2.0 VOUT = +3.3V 50 0.825 0.800 0.800 MAX1795/96/97 toc12 0.925 MAX1795/96/97 toc14 0.850 VBATT = +3.6V LOAD CURRENT (mA) 0.900 250 MAX1795/96/97 toc12 0.925 0.950 LOW-BATTERY INPUT THRESHOLD (V) MAX1795/96/97 toc10 0.950 MAX1795 MAXIMUM OUTPUT CURRENT vs. BATT INPUT VOLTAGE LOW-BATTERY INPUT THRESHOLD vs. TEMPERATURE LOW-BATTERY INPUT THRESHOLD vs. INPUT BATT VOLTAGE LOW-BATTERY INPUT THRESHOLD (V) MAX1795/MAX1796/MAX1797 Low Supply Current, Step-Up DC-DC Converters with True-Shutdown VIN = +3.6V VOUT = +5.0V ILOAD = 400mA _______________________________________________________________________________________ Low Supply Current, Step-Up DC-DC Converters with True-Shutdown LINE-TRANSIENT RESPONSE LIGHT-LOAD SWITCHING WAVEFORMS MAX1795/96/97 toc18 MAX1795/96/97 toc17 VLX 5V/div VBATT +2.7V TO +3V IINDUCTOR 500mA/div VOUT (AC-COUPLED) 20mV/div VOUT (AC-COUPLED) 100mV/div 10µs/div VBATT = +2.7V TO +3V VOUT = +5.0V NO LOAD 20µs/div VBATT = +3.6V VOUT = +5.0V ILOAD = 40mA LOAD-TRANSIENT RESPONSE STARTUP-SHUTDOWN WAVEFORMS MAX1795/96/97 toc19 MAX1795/96/97 toc20 VSHDN 5V/div IOUT 100mA/div VOUT 2V/div VOUT 100mV/div IINDUCTOR 500mA/div 40µs/div VBATT = +2.4V VOUT = +3.3V ILOAD = 0 TO 325mA 2ms/div VBATT = +2.4V VOUT = +5.0V ILOAD = 200mA _______________________________________________________________________________________ 7 MAX1795/MAX1796/MAX1797 Typical Operating Characteristics (continued) (L = 22µH, CIN = 47µF, COUT = 47µF, TA = +25°C, unless otherwise noted.) MAX1795/MAX1796/MAX1797 Low Supply Current, Step-Up DC-DC Converters with True-Shutdown Pin Description PIN NAME FUNCTION 1 LBI Low-Battery Comparator Input. Internally set to trip at +0.85V. This function remains operational in shutdown. 2 FB Dual-Mode™ Feedback Input. Connect to GND for preset 5.0V output. Connect to OUT for preset 3.3V output. Connect a resistive voltage-divider from OUT to GND to adjust the output voltage from 2V to 5.5V. 3 LBO Low-Battery Comparator Output, Open-Drain Output. LBO is low when VLBI < 0.85V. This function remains operational in shutdown. 4 SHDN 5 GND 6 LX Shutdown Input. If SHDN is high, the device is in shutdown mode, OUT is high impedance, and LBI/LBO are still operational. Connect shutdown to GND for normal operation. Ground Inductor Connection 7 OUT Power Output. OUT provides bootstrap power to the IC. 8 BATT Battery Input and Damping Switch Connection Detailed Description The MAX1795/MAX1796/MAX1797 compact step-up DC-DC converters start up with voltages as low as 0.85V and operate with an input voltage down to +0.7V. Consuming only 25µA of quiescent current, these devices have an internal synchronous rectifier that reduces cost by eliminating the need for an external diode and improves overall efficiency by minimizing losses in the circuit (see Synchronous Rectification section for details). The internal N-channel MOSFET power switch resistance is typically 0.17Ω, which minimizes losses. The LX switch current limits of the MAX1795/ MAX1796/MAX1797 are 0.25A, 0.5A, and 1A, respectively. All three devices offer Maxim’s proprietary TrueShutdown circuitry, which disconnects the output from the input in shutdown and puts the output in a high impedance state. These devices also feature Maxim’s proprietary LX-damping circuitry, which reduces EMI in noise-sensitive applications. For additional in-system flexibility, the LBI/LBO comparator remains active in shutdown. (Figure 1 is a typical application circuit). Control Scheme A unique minimum-off-time, current-limited control scheme is the key to the MAX1795/MAX1796/ MAX1797s’ low operating current and high efficiency over a wide load range. The architecture combines the high output power and efficiency of a pulse-width-modulation (PWM) device with the ultra-low quiescent cur- VIN 22µH BATT LX SHDN VOUT = 3.3V VIN LBO MAX1795 MAX1796 MAX1797 OUT COUT* GND LBI FB *SEE TABLE 1 FOR COMPONENT VALUES. Figure 1. Typical Application Circuit rent of a traditional pulse-skipping controller (Figure 2). Switching frequency depends upon the load current and input voltage, and can range up to 500kHz. Unlike conventional pulse-skipping DC-DC converters (where ripple amplitude varies with input voltage), ripple in these devices does not exceed the product of the switch current limit and the filter-capacitor equivalent series resistance (ESR). Dual Mode is a trademark of Maxim Integrated Products. 8 47µF 1M _______________________________________________________________________________________ Low Supply Current, Step-Up DC-DC Converters with True-Shutdown MAX1795/MAX1796/MAX1797 BATT R1 1M SHDN ZEROCROSSING AMPLIFIER MAX1795 MAX1796 MAX1797 LBO LBI + _ S Q + OUT OUT R + _ 47µF BODY DIODE CONTROL 0.85V S BATT Q BATT R 22µH TIMER BLOCK 47µF S Q START TON MAX LX R TOFF MAX OUT R2 FB R3 FB SELECT REFERENCE + _ + _ CURRENT-LIMIT AMPLIFIER ERROR AMPLIFIER + _ GND Figure 2. Functional Diagram Synchronous Rectification The internal synchronous rectifier eliminates the need for an external Schottky diode, reducing cost and board space. During the cycle off-time, the P-channel MOSFET turns on and shunts the MOSFET body diode. As a result, the synchronous rectifier significantly improves efficiency without the addition of an external component. Conversion efficiency can be as high as 95%, as shown in the Typical Operating Characteristics. Shutdown The device enters shutdown when V SHDN is high, reducing supply current to less than 2µA. During shutdown, the synchronous rectifier disconnects the output from the input, eliminating the DC conduction path that normally exists with traditional boost converters in shutdown mode. In shutdown, OUT becomes a high-imped- ance node. The LBI/LBO comparator remains active in shutdown. As shown in Figure 1, the MAX1795/MAX1796/MAX1797 may be automatically shut down when the input voltage drops below a preset threshold by connecting LBO to SHDN (see Low-Battery Detection section). BATT/Damping Switch The MAX1795/MAX1796/MAX1797 each contain an internal damping switch to minimize ringing at LX. The damping switch connects a resistor across the inductor when the inductor’s energy is depleted (Figure 3). Normally, when the energy in the inductor is insufficient to supply current to the output, the capacitance and inductance at LX form a resonant circuit that causes ringing. The ringing continues until the energy is dissipated through the series resistance of the inductor. The damping switch supplies a path to quickly dissi- _______________________________________________________________________________________ 9 MAX1795/MAX1796/MAX1797 Low Supply Current, Step-Up DC-DC Converters with True-Shutdown pate this energy, minimizing the ringing at LX. Damping LX ringing does not reduce VOUT ripple, but does reduce EMI (Figures 3, 4, and 5). VIN Setting the Output Voltage BATT MAX1795 MAX1796 MAX1797 R1 200Ω VOUT can be set to 3.3V or 5.0V by connecting the FB pin to GND (5V) or OUT (3.3V). To adjust the output voltage, connect a resistive voltage-divider from OUT to FB to GND (Figure 6). Choose a value less than 250kΩ for R2. 22µH DAMPING SWITCH LX VIN VOUT OUT 47µF 47µF BATT LX Figure 3. Simplified Diagram of Inductor Damping Switch R3 OUT SHDN LBI R4 MAX1795 MAX1796 MAX1797 GND OUTPUT 2V TO 5.5V 47µF 1M R1 LOW-BATTERY OUTPUT LBO FB R2 VLX 1V/div Figure 6. Setting an Adjustable Output Use the following equation to calculate R1: R1 = R2 [(VOUT / VFB) - 1] where VFB = +1.245V, and VOUT may range from +2V to +5.5V. 2µs/div Figure 4. LX Ringing for Conventional Step-Up Converter (without Damping Switch) VLX 1V/div 2µs/div Low-Battery Detection The MAX1795/MAX1796/MAX1797 each contain an onchip comparator for low-battery detection. If the voltage at LBI is above 0.85V, LBO (an open-drain output) sinks current to GND. If the voltage at LBI is below 0.85V, LBO goes high impedance. The LBI/LBO function remains active even when the part is in shutdown. Connect a resistive voltage-divider to LBI from BATT to GND. The low-battery monitor threshold is set by two resistors, R3 and R4 (Figure 6). Since the LBI bias current is typically 2nA, large resistor values (R4 up to 250kΩ) can be used to minimize loading of the input supply. Calculate R3 using the following equation: R3 = R4[(VTRIP / 0.85V) - 1] Figure 5. LX Waveform with Damping Switch 10 ______________________________________________________________________________________ Low Supply Current, Step-Up DC-DC Converters with True-Shutdown Connect a pullup resistor of 100kΩ or greater from LBO to OUT for a logic output. LBO is an open-drain output and can be pulled as high as 6V regardless of the voltage at OUT. When LBI is below the threshold, the LBO output is high impedance. If the low-battery comparator is not used, ground LBI and LBO. Applications Information Inductor Selection An inductor value of 22µH performs well in most applications. The MAX1795/MAX1796/MAX1797 will also work with inductors in the 10µH to 47µH range. Smaller inductance values typically offer a smaller physical size for a given series resistance, allowing the smallest overall circuit dimensions, but have lower output current capability. Circuits using larger inductance values exhibit higher output current capability, but are physically larger for the same series resistance and current rating. The inductor’s incremental saturation current rating should be greater than the peak switch-current limit, which is 0.25A for the MAX1795, 0.5A for the MAX1796, and 1A for the MAX1797. However, it is generally acceptable to bias the inductor into saturation by as much as 20% although this will slightly reduce efficiency. Table 1 lists some suggested components for typical applications. The inductor’s DC resistance significantly affects efficiency. Calculate the maximum output current (IOUT(MAX)) as follows, using inductor ripple current (IRIP) and duty cycle (D): VOUT + ILIM L t OFF VOUT + ILIM − IRIP = D = × (RPFET + LESR ) − VBATT (RPFET + LESR ) + 2 IRIP (RPFET + LESR ) − VBATT 2 I IOUT(MAX) = ILIM − RIP (1 − D) 2 where: IRIP = Inductor ripple current (A) VOUT = Output voltage (V) ILIM = Device current limit (0.25A, 0.5A, or 1A) RPFET = On-resistance of P-channel MOSFET (Ω) (typ 0.27Ω) LESR = ESR of Inductor (Ω) (typ 0.095Ω) VBATT = Input voltage (V) L = Inductor value in µH tOFF = LX switch’s off-time (µs) (typ 1µs) D = Duty cycle RNFET = On-resistance of N-channel MOSFET (Ω) (typ 0.17Ω) IOUT(MAX) = Maximum output current (A) Capacitor Selection Table 1 lists suggested tantalum or polymer capacitor values for typical applications. The ESR of both input bypass and output filter capacitors affects efficiency and output ripple. Output voltage ripple is the product of the peak inductor current and the output capacitor ESR. High-frequency output noise can be reduced by connecting a 0.1µF ceramic capacitor in parallel with the output filter capacitor. (See Table 2 for a list of suggested component suppliers.) PC Board Layout and Grounding Careful printed circuit layout is important for minimizing ground bounce and noise. Keep the IC’s GND pin and the ground leads of the input and output filter capacitors less than 0.2in (5mm) apart. In addition, keep all connections to the FB and LX pins as short as possible. In particular, when using external feedback resistors, locate them as close to FB as possible. To maximize output power and efficiency and minimize output ripple voltage, use a ground plane and solder the IC’s GND pin directly to the ground plane. I VOUT + ILIM − RIP (RPFET − RNFET + LESR ) 2 and ______________________________________________________________________________________ 11 MAX1795/MAX1796/MAX1797 VTRIP is the input voltage where the low-battery detector output goes high impedance. For single-cell applications, LBI may be connected to the battery. When V BATT <1.0V>, the LBI threshold increases to 0.925V (see Typical Operating Characteristics). MAX1795/MAX1796/MAX1797 Low Supply Current, Step-Up DC-DC Converters with True-Shutdown Table 1. Suggested Components for Typical Applications COMPONENT COMPONENT VALUE (MAX1797, 1A CURRENT LIMIT) COMPONENT VALUE (MAX1796, 0.5A CURRENT LIMIT) COMPONENT VALUE (MAX1795, 0.25A CURRENT LIMIT) Sumida CR32-220, 22µH Sumida CDRH6D28-220, 22µH Sumida CDRH4D28-220, 22µH Sumida CR32-100, 10µH Murata CQH3C100K34, 10µH Inductor Murata CQH4N100K(J)04, 10µH Coilcraft DS3316P-223, 22µH Coilcraft DS1608C-223, 22µH Coilcraft DS1608C-223, 22µH Coilcraft DS1608C-103, 10µH Input Capacitor Sanyo POSCAP 6TPA47M, 47µF Sanyo POSCAP 6TPA47M, 47µF Sanyo POSCAP 6TPA47M, 47µF AVX TPSD476M016R0150, 47µF AVX TPSD226M016R0150, 22µF AVX TPSD106M016R0150, 10µF Taiyo Yuden UMK316BI150KH, 0.1µF Taiyo Yuden UMK316BI150KH, 0.1µF Taiyo Yuden UMK316BI150KH, 0.1µF Output Capacitor Chip Information Table 2. Component Suppliers COMPANY PHONE FAX AVX USA 803-946-0690 USA 803-626-3123 Coilcraft USA 847-639-6400 USA 847-639-1238469 Coiltronics USA 561-241-7876 USA 561-241-9339 Murata USA 814-237-1431 1-800-831-9172 USA 814-238-0490 Nihon USA 805-867-2555 Japan 81-3-34947411 USA 805-867-2556 Japan 81-3-34947414 Sanyo US A 619-661-6835 Ja pan 81- 7- 20706306 US A 619-661-1055 Ja pan 81- 7- 20701174 Sprague USA 603-224-1961 USA 603-224-1430 Sumida USA 647-956-0666 Japan 81-3-36075111 US A 647-956-0702 Ja pan 81- 3- 36075144 Taiyo Yuden USA 408-573-4150 USA 408-573-4159 12 TRANSISTOR COUNT: 1100 PROCESS: BiCMOS ______________________________________________________________________________________ Low Supply Current, Step-Up DC-DC Converters with True-Shutdown 8LUMAXD.EPS 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. Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 _____________________13 © 2000 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products. MAX1795/MAX1796/MAX1797 Package Information