19-1810; Rev 1; 1/02 KIT ATION EVALU E L B A IL AVA Dual, High-Efficiency, Step-Down Converter with Backup Battery Switchover Features The MAX1774 is a complete power-supply solution for PDAs and other hand-held devices. It integrates two high-efficiency step-down converters, a boost converter for backup battery regulation, and four voltage detectors in a small 32-pin QFN or 28-pin QSOP package. The MAX1774 accepts inputs from +2.7V to +28V and provides an adjustable main output from 1.25V to 5.5V at over 2A. The secondary core converter delivers an adjustable voltage from 1V to 5V and can deliver up to 1.5A. Both the main and core regulators have separate shutdown inputs. When the AC adapter power is removed, an external Pchannel MOSFET switches input to the main battery. When the main battery is low, the backup step-up converter sustains the main output voltage. When the backup battery can no longer deliver the required load, the system shuts down safely to prevent damage. Four onboard voltage detectors monitor the status of the AC adapter power, main battery, and backup battery. The MAX1774 evaluation kit is available to help reduce design time. ♦ Dual, High-Efficiency, Synchronous-Rectified Step-Down Converters ________________________Applications ♦ Input Voltage Range from +2.7V to +28V ♦ Thin, Small (1mm High) QFN Package ♦ Step-Up Converter for Backup Battery ♦ Main Power Adjustable from +1.25V to +5.5V Over 2A Load Current Up to 95% Efficiency ♦ Core Power Adjustable from 1V to 5V Internal Switches Up to 1.5A Load Current Up to 91% Efficiency ♦ Automatic Main Battery Switchover ♦ 100% (max) Duty Cycle ♦ Up to 1.25MHz Switching Frequency ♦ Four Low-Voltage Detectors Hand-Held Computers PDAs Internet Access Tablets POS Terminals Subnotebooks ♦ 170µA Quiescent Current ♦ 8µA Shutdown Current ♦ Digital Soft-Start ♦ Independent Shutdown Inputs LBO INS N.C. GND 25 26 27 LXC 28 SHDNM N.C. 29 30 BKUP 32 GND 31 TOP VIEW SHDNC Pin Configurations MDRV 1 24 ACO PGNDC 2 23 INC PGND 3 22 GND NDRV 4 21 FBC CVL 5 20 CS- IN 6 19 CS+ PDRV 7 18 FBM CVH 8 17 N.C. MAX1774 Ordering Information PART MAX1774EEI MAX1774EMJ 11 12 13 14 15 16 BIN BKOFF ACI DBI LBI REF 9 10 LXB LXB2 32 7mm x 7mm QFN PIN-PACKAGE 28 QSOP 32 7mm x 7mm QFN Functional Diagram AC ADAPTER MAIN (+3.3V) MAIN BATTERY MAX1774 CORE (+1.8V) AC OK LOW MAIN BATTERY BACKUP BATTERY GND TEMP RANGE -40°C to +85°C -40°C to +85°C DEAD MAIN BATTERY GND Pin Configurations continued at end of data sheet. ________________________________________________________________ Maxim Integrated Products For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com. 1 MAX1774 General Description MAX1774 Dual, High-Efficiency, Step-Down Converter with Backup Battery Switchover ABSOLUTE MAXIMUM RATINGS IN, SHDNM, MDRV, DBI, LBI, ACI, CVH to GND .......................................................-0.3V to +30V IN to CVH, PDRV ......................................................-0.3V to +6V BIN to CS-.................................................................-0.3V to +6V LXB to GND ................................................-0.3V to (VBIN+ 0.7V) PDRV to GND..................................(VCVH - 0.3V) to (VIN + 0.3V) All Other Pins to GND...............................................-0.3V to +6V PGND to GND .......................................................-0.3V to +0.3V Continuous Power Dissipation 28-Pin QSOP (derate 10.8mW/°C above +70°C)........860mW 32-Pin QFN (derate 23.2mW/°C above +70°C) ........1860mW Operating Temperature .......................................-40°C to +85°C Storage Temperature.........................................-65°C to +150°C 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 (Figure 1, VIN = VINS +12V, VINC = VCS- = VCS+ = +3.3V, VCORE = +1.8V, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL CONDITIONS MAX UNITS 28 V 18 40 µA VFBM = +1.5V, VFBC = +1.5V, V SHDNM = V SHDNC = +3.3V 110 220 µA IINC VFBM = +1.5V, VFBC = +1.5V, V SHDNM = V SHDNC = +3.3V 60 105 µA IBIN VBIN = +3.3V, CS- open VFBM = +1.5V, V SHDNM = +3.3V, V BKOFF = +1.5V, SHDNC = GND 60 105 µA SHDNM = SHDNC = GND 8 40 µA 5.5 V 1.25 1.29 V Input Voltage VIN Input Quiescent Supply Current IIN VFBM = +1.5V, VFBC = +1.5V, V SHDNM = V SHDNC = +3.3V CS- Quiescent Supply Current ICS- Core Regulator Quiescent Supply Current Backup Mode BIN Quiescent Supply Current IN Shutdown Supply Current MIN TYP 2.7 MAIN REGULATOR Main Output Voltage Adjust Range 1.25 FBM Regulation Threshold VFBM FBM Input Current IFBM V(CS+ - CS-) = 0 to +60mV, VIN = +3.5V to +28V 1.21 VFBM = +1.3V -0.1 0.1 µA Current-Limit Threshold VCS+ - VCS- 60 80 100 mV Minimum Current-Limit Threshold VCS+ - VCS- 5 15 25 mV Valley Current Threshold VCS+ - VCS- 40 50 60 mV Zero Current Threshold VCS+ - VCS- 0 5 15 mV PDRV, NDRV Gate Drive Resistance VCS- = +3.3V, IPDRV, INDRV = 50mA 2 5.5 Ω CS- to CVL Switch Resistance ICVL = 50mA 4.5 9.5 PDRV, NDRV Dead Time 2 50 _______________________________________________________________________________________ Ω ns Dual, High-Efficiency, Step-Down Converter with Backup Battery Switchover (Figure 1, VIN = VINS = +12V, VINC = VCS- = VCS+ = +3.3V, VCORE = +1.8V, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS Maximum Duty Cycle 100 % Minimum On-Time 200 400 650 ns Minimum Off-Time 200 400 650 ns 5.5 V CORE REGULATOR Input Voltage Range VINC INC Undervoltage Lockout 2.6 VINC rising 2.40 2.47 2.55 VINC falling 2.30 2.37 2.45 Core Output Voltage Adjust Range 1.0 Maximum Core Load Current VCORE = 1.8V (Note 1) FBC Regulation Threshold VFBC FBC Input Current IFBC Dropout Voltage ILXC 1.5 VINC = +2.5 to +5.5V, I O U T C = 0 to 200mA 0.97 1.0 1.03 V VFBC = +1.3V -0.1 0.1 µA 0.1 0.25 V 10 µA 0.25 0.5 Ω VINC = +5.5V, V L X C = 0 to +5.5V -10 LXC P-Channel, N-Channel OnResistance LXC P-Channel Current Limit V 1 IOUTC = 400mA LXC Leakage Current 5.0 V ICLC A 1200 1800 3000 mA LXC P-Channel Minimum Current 100 250 400 mA LXC N-Channel Valley Current 900 1400 2400 mA LXC N-Channel Zero-Crossing Current 40 110 170 mA LXC Dead Time 50 ns Max Duty Cycle 100 Minimum On-Time 170 400 690 ns % Minimum Off-Time 170 400 690 ns 5.5 V BACKUP REGULATOR Backup Battery Input Voltage VBBATT LXB N-Channel On-Resistance 0.9 1.9 3.5 Ω 350 600 mA VLXB = +5.5V, VFBM = +1.3V 1 µA VBIN = +5.5V, CS- = BKOFF = SHDNC = SHDNM = GND 1 µA VCS- = +3.3V, ILXB = 50mA LXB Current Limit 200 LXB Leakage Current BIN Leakage Current IBIN BIN, CS- Switch Resistance VCS- = +3.3V, BKOFF = GND, SHDNM = CVL 7.5 15 Ω BIN Switch Zero-Crossing Threshold VBIN = +2.5V, BKOFF = SHDNC = SHDNM = CVL 17 35 mV 5.6 9.2 µs LXB Maximum On-Time Zero Crossing Detector Timeout 2.8 40 µs _______________________________________________________________________________________ 3 MAX1774 ELECTRICAL CHARACTERISTICS (continued) MAX1774 Dual, High-Efficiency, Step-Down Converter with Backup Battery Switchover ELECTRICAL CHARACTERISTICS (continued) (Figure 1, VIN = VINS = +12V, VINC = VCS- = VCS+ = +3.3V, VCORE = +1.8V, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX 1.23 1.25 UNITS REFERENCE Reference Voltage 1.27 V Reference Load Regulation VREF IREF = 0 to 50µA 10 mV Reference Line Regulation VCS- = +2.5V to +5.5V, IREF = 50µA 5 mV Reference Sink Current 10 µA CVL, CVH REGULATORS CVL Output Voltage VCVL CVL Switchover Threshold CVH Output Voltage CVH Switchover Threshold ICVL = 50mA, VCS- = 0 2.6 ICVL = 50mA, VCS- = +3.3V CS- rising, hysteresis = 100mV typical CVL Undervoltage Lockout 3.1 3.2 2.40 2.47 2.55 VIN = +4V, ICVH = 25mA VIN 3.4 VIN 2.8 VIN = +12V, ICVH = 50mA VIN 4.2 VIN 3.7 V V V VCVH VIN 2.8 VIN rising, hysteresis = 350mV typ 5.5 V VCVL rising 2.40 2.47 2.55 VCVL falling 2.30 2.37 2.45 V BKOFF rising 0.51 0.55 0.59 V BKOFF falling 0.46 0.50 0.54 V LOW-VOLTAGE COMPARATORS Backup Regulator Shutdown Threshold V BKOFF BKOFF Input Bias Current V BKOFF = +5.5V V 1 µA LBI Threshold VLBI VLBI falling, hysteresis = 50mV typical 1.17 1.20 1.23 V DBI Threshold VDBI VDBI falling, hysteresis = 50mV typical 1.17 1.20 1.23 V BKUP Low-Input Threshold 0.4 V LBI, DBI Input Leakage Current VLBI = VDBI = +1.3V 100 nA LBO, BKUP, ACO, MDRV Output Low ISINK = 1mA 0.4 V LBO, BKUP, ACO, MDRV Output Leakage Current VLBI = +1.3V, VACI = +12V, V ACO = V LBO = V BKUP = +5.5V, V MDRV = +28V 1.0 µA ACI Threshold VACI – VINS, ACI falling 0.35 V ACI Input Leakage Current VACI = +1.3V 100 nA INS Input Leakage Current VINS = +3.3V 10 µA 0.4 V 0.22 1.5 LOGIC INPUTS SHDNM, SHDNC Input Low Voltage SHDNM, SHDNC Input High Voltage 4 2.0 _______________________________________________________________________________________ V Dual, High-Efficiency, Step-Down Converter with Backup Battery Switchover (Figure 1, VIN = VINS = +12V, VINC = VCS- = VCS+ = +3.3V, VCORE = +1.8V, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL SHDNM, SHDNC Input Low Current CONDITIONS SHDNM = SHDNC = GND SHDNC Input High Current V SHDNC = +5.5V SHDNM Input High Current V SHDNM = +5V MIN TYP -1 2 MAX UNITS 1 µA 5 µA 25 µA ELECTRICAL CHARACTERISTICS (Figure 1, VIN = VINS = +12V, VINC = VCS- = VCS+ = +3.3V, VCORE = +1.8V, TA = -40°C to +85°C, unless otherwise noted.) (Note 2) PARAMETER SYMBOL CONDITIONS MIN MAX UNITS 2.7 28 V Input Voltage VIN Input Quiescent Supply Current IIN VFBM = +1.5V, VFBC = +1.5V, V SHDNM = V SHDNC = +3.3V 40 µA CS- Quiescent Supply Current ICS- VFBM = +1.5V, VFBC = +1.5V, V SHDNM = V SHDNC = +3.3V 220 µA Core Regulator Quiescent Supply Current IINC VFBM = +1.5V, VFBC = +1.5V, V SHDNM = V SHDNC = +3.3V 105 µA Backup Mode BIN Quiescent Supply Current IBIN VBIN = +3.3V, CS- open VFBM = +1.5V, V SHDNM = +3.3V, V BKOFF = +1.5V, SHDNC = GND 110 µA SHDNM = SHDNC = GND 40 µA 1.25 5.5 V IN Shutdown Supply Current MAIN REGULATOR Main Output Voltage Adjust Range FBM Regulation Threshold VFBM V(CS+ - CS-) = 0 to +60mV, VIN = +3.5V to +28V 1.21 1.29 V FBM Input Current IFBM VFBM = +1.3V -0.1 0.1 µA Current-Limit Threshold VCS+ - VCS- 60 100 mV Minimum Current-Limit Threshold VCS+ - VCS- 5 25 mV Valley Current Threshold VCS+ - VCS- 40 60 mV Zero Current Threshold VCS+ - VCS- 0 15 mV PDRV, NDRV Gate Drive Resistance VCS- = +3.3V, IPDRV, INDRV = 50mA 5.5 Ω CS- to CVL Switch Resistance ICVL = 50mA 9.5 Ω Maximum Duty Cycle 100 % Minimum On-Time 200 650 ns Minimum Off-Time 200 650 ns _______________________________________________________________________________________ 5 MAX1774 ELECTRICAL CHARACTERISTICS (continued) MAX1774 Dual, High-Efficiency, Step-Down Converter with Backup Battery Switchover ELECTRICAL CHARACTERISTICS (continued) (Figure 1, VIN = VINS = +12V, VINC = VCS+ = VCS- = +3.3V, VCORE = +1.8V, TA = -40°C to +85°C, unless otherwise noted.) (Note 2) PARAMETER SYMBOL CONDITIONS MIN MAX UNITS V CORE REGULATOR Input Voltage Range VINC INC Undervoltage Lockout 2.6 5.5 VINC rising 2.39 2.55 VINC falling 2.29 2.45 1.0 5.0 Core Output Voltage Adjust Range Maximum Core Load Current FBC Regulation Threshold VFBC FBC Input Current IFBC Dropout Voltage LXC Leakage Current VCORE = 1.8V (Note 1) 1 VINC = +2.5 to +5.5V, IOUTC = 0 to 200mA 0.97 VFBC = +1.3V -0.1 IOUTC = 400mA ILXC VINC = +5.5V, VLXC = 0 to +5.5V -10 LXC P-Channel, N-Channel On-Resistance V V A 1.03 V 0.1 µA 0.25 V 10 µA 0.5 Ω LXC P-Channel Current Limit 1200 3010 mA LXC P-Channel Minimum Current 100 420 mA LXC N-Channel Valley Current 880 2450 mA LXC N-Channel Zero-Crossing Current 40 170 mA Max Duty Cycle 100 Minimum On-Time 160 700 ns Minimum Off-Time 170 690 ns 0.9 5.5 V 3.5 Ω 600 mA % BACKUP REGULATOR Backup Battery Input Voltage VBBATT LXB N-Channel On Resistance VCS- = +3.3V, ILXB = 50mA LXB Current Limit 200 LXB Leakage Current VLXB = +5.5V, VFBM = +1.3V 1 µA BIN Leakage Current VBIN = +5.5V, CS- = BKOFF = SHDNC = SHDNM = GND 1 µA BIN, CS- Switch Resistance VCS- = +3.3V, BKOFF = GND, SHDNC = CVL 15 Ω BIN Switch Zero-Crossing Threshold VBIN = +2.5V, BKOFF = SHDNC = SHDNM = CVL 35 mV 2.8 9.2 µs 1.220 IBIN LXB Maximum On-Time REFERENCE Reference Voltage 1.275 V Reference Load Regulation IREF = 0 to 50µA 10 mV Reference Line Regulation VCS- = +2.5V to +5.5V, IREF = 50µA 5 mV Reference Sink Current 6 VREF 10 _______________________________________________________________________________________ µA Dual, High-Efficiency, Step-Down Converter with Backup Battery Switchover (Figure 1, VIN = VINS = +12V, VINC = VCS+ = VCS- = +3.3V, VCORE = +1.8V, TA = -40°C to +85°C, unless otherwise noted.) (Note 2) PARAMETER SYMBOL CONDITIONS MIN MAX UNITS ICVL = 50mA, VCS- = 0 2.6 3.1 V VCS- rising, hysteresis = 100mV typical 2.40 2.55 V CVL, CVH REGULATORS CVL Output Voltage VCVL CVL Switchover Threshold VIN = +4V, ICVH = 25mA CVH Output Voltage VIN - 2.8 VCVH V VIN = +12V, ICVH = 50mA CVL Undervoltage Lockout VIN - 3.65 VCVL rising 2.40 2.57 VCVL falling 2.30 2.47 V BKOFF rising 0.51 0.59 V BKOFF falling 0.46 0.54 V LOW-VOLTAGE COMPARATORS Backup Regulator Shutdown Threshold V BKOFF BKOFF Input Bias Current V BKOFF = +5.5V V 1 µA LBI Threshold VLBI VLBI falling, hysteresis = 50mV typical 1.17 1.23 V DBI Threshold VDBI VDBI falling, hysteresis = 50mV typical 1.17 1.23 V BKUP Low-Input Threshold 0.4 V LBI, DBI Input Leakage Current VLBI, VDBI = +28V 100 nA LBO, BKUP, ACO, MDRV Output Low ISINK = 1mA 0.4 V LBO, BKUP, ACO, MDRV Output Leakage Current VLBI = +1.3V, VACI = VIN = +12V, VACO = V LBO = V BKUP = +5.5V, V MDRV = +28V 1.0 µA ACI Threshold VACI - VINS, ACI falling 0.5 V ACI Input Leakage Current VACI = +1.3V 100 nA MAIN Input Leakage Current LOGIC INPUTS VINS = +3.3V 10 µA 0.4 V SHDNM, SHDNC Input Low Voltage SHDNM, SHDNC Input High Voltage SHDNM, SHDNC Input Low Current 2.0 SHDNM = SHDNC = GND -1 V 1 µA SHDNC Input High Current V SHDNC = +5.5V 5 µA SHDNM Input High Current V SHDNM = +28V 25 µA Note 1: This parameter is guaranteed based on the LXC P-channel current limit and the LXC N-channel valley current. Note 2: Specifications to -40°C are guaranteed by design and not production tested. _______________________________________________________________________________________ 7 MAX1774 ELECTRICAL CHARACTERISTICS (continued) Typical Operating Characteristics (Circuit of Figure 1, VIN = +5V, VINC = +3.3V, TA = +25°C, unless otherwise noted.) CORE EFFICIENCY vs. LOAD 90 VIN = +12V 60 VIN = +15V 50 40 VIN = +18V 30 20 70 VIN = +2.7V 60 VIN = +3.3V 50 VIN = +5V 40 30 70 60 50 1 10 100 LOAD (mA) 1000 10,000 0 1 10 100 1000 0.01 0.1 LOAD (mA) 1 10 100 LOAD (mA) VREF ACCURACY vs. TEMPERATURE REFERENCE LOAD REGULATION 1.5 MAX1774-05 0 MAX1774-04 2.0 -0.2 -0.4 1.0 VREF ACCURACY (%) VREF ACCURACY (%) VMAIN = 3.3V 10 0 0 VBBATT = +0.8V 40 20 10 10 VBBATT = +1.0V 30 VCORE = 1.8V 20 VMAIN = 3.3V VBBATT = +2.5V 80 EFFICIENCY (%) EFFICIENCY (%) 70 90 80 VIN = +3.3V VIN = +5V 80 100 MAX1774-02 MAX1774-01 90 BACKUP EFFICIENCY vs. LOAD 100 0.5 0 -0.5 -1.0 -0.6 -0.8 -1.0 -1.2 -1.4 -1.6 -1.5 -2.0 -40 -1.8 -2.0 -20 0 20 60 40 TEMPERATURE (°C) 80 100 0 10 20 30 40 50 60 70 80 IREF (µA) MAIN SWITCHING WAVEFORMS (LIGHT LOAD 100mA) MAX1774-06 MAIN SWITCHING WAVEFORMS (HEAVY LOAD 1A) 5V MAX1774-07 LX 5V/div 4V LX 5V/div 0 0 40mV 20mV 0 20mV VMAIN (AC-COUPLED) 20mV/div 8 VMAIN (AC-COUPLED) 20mV/div 0 -20mV -20mV 1500mA 500mA ILI 500mA/div 0 1000mA IL1 500mA/div 500mA 0 5µs/div MAX1774-03 MAIN EFFICIENCY vs. LOAD 100 EFFICIENCY (%) MAX1774 Dual, High-Efficiency, Step-Down Converter with Backup Battery Switchover 5µs/div _______________________________________________________________________________________ Dual, High-Efficiency, Step-Down Converter with Backup Battery Switchover CORE SWITCHING WAVEFORMS (HEAVY LOAD 500mA) CORE SWITCHING WAVEFORMS (LIGHT LOAD 50mA) MAX1774-09 MAX1774-08 3.3V LX 2V/div 2V LXC 0 0 500mA 0 4V 2V/div 20mV IL2 500mA/div 0 -20mV VCORE (AC-COUPLED) 20mV/div 500mA 0 VCORE (AC-COUPLED) 20mV/div L2 500mA/div 2µs/div 1µs/div CORE LINE-TRANSIENT RESPONSE MAIN LINE-TRANSIENT RESPONSE MAX1774-11 MAX1774-10 4V 12V 10V VINC 2V 2V/div VIN 5V/div 5V 0 0 50mV VCORE (AC-COUPLED) 50mV/div VMAIN (AC-COUPLED) 50mV/div -50mV 100µs/div 1µs/div MAIN LOAD-TRANSIENT RESPONSE MAIN LOAD-TRANSIENT RESPONSE 50mA TO 500mA MAX1774-12 MAX1774-13 1000mA 500mA 0 500mA IMAIN 500mA/div 0 IMAIN 500mA/div 20mV VMAIN (AC-COUPLED) -20mV 20mV/div 10mV 0 0 VMAIN (AC-COUPLED) 10mV/div -10mV 100µs/div 100µs/div _______________________________________________________________________________________ 9 MAX1774 Typical Operating Characteristics (continued) (Circuit of Figure 1, VIN = +5V, VINC = +3.3V, TA = +25°C, unless otherwise noted.) MAX1774 Dual, High-Efficiency, Step-Down Converter with Backup Battery Switchover Typical Operating Characteristics (continued) (Circuit of Figure 1, VIN = +5V, VINC = +3.3V, TA = +25°C, unless otherwise noted.) BACKUP SWITCHOVER RESPONSE TURN-ON RESPONSE MAX1774-15 MAX1774-14 5V VSHDN 5V/div 0 3V VMAIN VCORE 2V VBKUP 5V/div VOUT 1V/div IBBATT 50mA/div 1V 0 400µA INPUT CURRENT 200µA VBIN 10mV/div INPUT CURRENT 200mA/div VMAIN 10mV/div 0 100µs/div 5µs/div Pin Description PIN NAME FUNCTION QSOP QFN 1 30 SHDNM Shutdown for Main Regulator. Low voltage on SHDNM shuts off the main output. For normal operation, connect SHDNM to IN. 2 31 SHDNC Shutdown for Core Regulator. Low voltage on SHDNC shuts off the core output. For normal operation, connect SHDNC to CVL. 3 32 BKUP Open-Drain Backup Input/Output. The device is in backup mode when BKUP is low. BKUP can be externally pulled low to place the device in backup mode. 4 1 MDRV Open-Drain Drive Output. MDRV goes low when the ACI voltage drops below the main voltage plus 220mV and device is not in backup. Connect MDRV to the gate of the main battery P-channel MOSFET to switch the battery to IN when the AC adapter voltage is not present. 5 2 PGNDC 10 Power Ground for the Core Converter. Connect all grounds together close to the IC. 6 3 PGND Power Ground. Ground for NDRV and core output synchronous rectifier. Connect all grounds together close to the IC. 7 4 NDRV N-Channel Drive Output. Drives the main output synchronous-rectifier MOSFET. NDRV swings between CVL and PGND. Low-Side Bypass. CVL is the output of an internal LDO regulator. This is the internal power supply for the device control circuitry as well as the N-channel driver. Bypass CVL with a 1.0µF or greater capacitor to GND. When CS- is above the CVL switchover threshold (2.47V), CVL is powered from the main output. 8 5 CVL 9 6 IN 10 7 PDRV P-Channel Drive Output. Drives the main output high-side MOSFET switch. PDRV swings between IN and CVH. The voltage at CVH is regulated at VIN - 4.2V unless the input voltage is less than 5.5V. 11 8 CVH High-Side Drive Bypass. This is the low-side of the P-channel driver output. Bypass with a 1.0µF capacitor or greater to IN. When the input voltage is less than 5.5V, CVH is switched to PGND. 12 9 LXB Backup Converter Switching Node. Connect an inductor from LXB to the backup battery and a Schottky diode to BIN to complete the backup converter. In backup mode, this step-up converter powers the main output from the backup battery through BIN. Power Supply Input ______________________________________________________________________________________ Dual, High-Efficiency, Step-Down Converter with Backup Battery Switchover Pin Description (continued) NAME FUNCTION QSOP QFN — 10 LXB2 13 11 BIN 14 12 BKOFF Backup Disable Input. Driving BKOFF below +0.5V disables the backup mode. In backup mode, the device enters shutdown when this pin is pulled low. BKOFF can be driven from a digital signal or can be used as a low battery detector to disable the backup converter when the backup battery is low. 15 13 ACI AC Adapter Low-Voltage Detect Input. Connect to adapter DC input. When the voltage at ACI falls below the voltage at INS plus +0.22V, ACO asserts. 16 14 DBI Dead Battery Input. Connect DBI to the main battery through a resistive voltage-divider. When DBI drops below +1.20V, no AC adapter is connected (ACO is low, but main output still available), BKUP asserts. 17 15 LBI Low-Battery Input. Connect LBI to the main battery through a resistive voltage-divider. When the voltage at LBI drops below +1.20V, LBO asserts. 18 16 REF Reference Voltage Output. Bypass REF to GND with a 0.22µF or greater capacitor. — 17, 25, 29 N.C. 19 18 FBM Main Output Feedback. Connect FBM to a resistive voltage-divider to set main output voltage between +1.25V to +5.5V. 20 19 CS+ Main Regulator High-Side Current-Sense Input. Connect the sense resistor between CS+ and CS-. This voltage is used to set the current limit and to turn off the synchronous rectifier when the inductor current approaches zero. 21 20 CS- Main Regulator Low-Side Current-Sense Input. Connect CS- to the main output. 22 21 FBC Core Output Feedback. Connect FBC to a resistive voltage-divider to set core output between +1.0V to +5.0V. 23 22 GND Analog Ground 24 23 INC Core Supply Input 25 24 ACO Low AC Output. Open drain ACO asserts when ACI falls below the main output voltage plus 0.22V. 26 26 LBO Open-Drain Low-Battery Output. LBO asserts when LBI falls below +1.20V. Backup Converter Switching Node. Connect LXB2 to LXB as close to the IC as possible. Backup Battery Input. Connect BIN to the output of the backup boost regulator. Bypass BIN with a 10µF or greater capacitor to GND. When the MAX1774 is in backup mode, BIN powers the main output. No Connection. Not Internally Connected. 27 27 INS Power-Supply Input Voltage Sense Input. Connect INS to the power-supply input voltage. 28 28 LXC Core Converter Switching Node Detailed Description The MAX1774 dual step-down DC-DC converter is designed to power PDA, palmtop, and subnotebook computers. Normally, these devices require two separate power supplies–one for the processor and another higher voltage supply for the peripheral circuitry. The MAX1774 provides an adjustable +1.25V to +5.5V main output designed to power the peripheral circuitry of PDAs and similar devices. The main output delivers up to 2A output current. The lower voltage core converter has an adjustable +1.0V to +5.0V output, providing up to 1.5A output current. Both regulators utilize a proprietary regulation scheme allowing PWM operation at medium to heavy loads, and automatically switch to pulse skipping at light loads for improved efficiency. Under low-battery conditions, the MAX1774 enters backup mode, making use of a low-voltage backup battery and a step-up regulator to power the output. Figure 1 is the MAX1774 typical application circuit. Operating Modes for the Step-Down Converters When delivering low output currents, the MAX1774 operates in discontinuous conduction mode. Current through the inductor starts at zero, rises as high as the minimum current limit (IMIN), then ramps down to zero during _______________________________________________________________________________________ 11 MAX1774 PIN MAX1774 Dual, High-Efficiency, Step-Down Converter with Backup Battery Switchover NOTE: FOR INPUT VOLTAGES TO 28V SEE FIGURE 4 AND FIGURE 5 2.7V TO 5.5V D1 2.7V V IN_AC TO 5.5V MAIN BATTERY NSD03A10 C5 1µF NDS356AP P1 INS IN CVH R4 R1 C6 10µF MDRV P2 DBI PDRV ACI R2 FDS8928A L1 5µH RCS MAIN N1 LBI MAX1774 1MΩ R3 CMAIN 47µF NDRV 1.25V TO 5.5V PGND ON SHDNM CS+ ON SHDNC CS- OFF OFF BIN C1 10µF R10 D2 EP05Q 03L LXB FBM LXB2(QFN ONLY) L3 22µH BACKUP BATTERY 0.9V TO 5.5V R11 BKOFF C2 10µF R5 1MΩ ACO CVL C3 1µF R6 1MΩ C4 0.22µF REF LBO R7 1MΩ GND BKUP INC PGNDC C7 1µF L2 5.4µH CORE LXC R8 1.0V TO CCORE 5.5V 22µF FBC R9 Figure 1. Typical Application Circuit For Low-Input Voltage Applications each cycle (see Typical Operating Characteristics). The switch waveform may exhibit ringing, which occurs at the resonant frequency of the inductor and stray capacitance, due to the residual energy trapped in the core when the rectifier MOSFET turns off. This ringing is normal and does not degrade circuit performance. When delivering medium-to-high output currents, the MAX1774 operates in PWM continuous-conduction mode. In this mode, current always flows through the inductor and never ramps to zero. The control circuit 12 adjusts the switch duty cycle to maintain regulation without exceeding the peak switching current set by the current-sense resistor. 100% Duty Cycle and Dropout The MAX1774 operates with a duty cycle up to 100%, extending the input voltage range by turning the MOSFET on continuously when the supply voltage approaches the output voltage. This services the load when conventional switching regulators with less than ______________________________________________________________________________________ Dual, High-Efficiency, Step-Down Converter with Backup Battery Switchover MAX1774 TOFFMIN VMIN CS+ CS- TONMIN PON VVALLEY S FB PON VIN Q PSW REF R VCLM S VO Q NON NSW R VZERO NONOVERLAP PROTECTION Figure 2. Simplified Control System Block Diagram 100% duty cycle fail. Dropout voltage is defined as the difference between the input and output voltages when the input is low enough for the output to drop out of regulation. Dropout depends on the MOSFET drain-tosource on-resistance, current-sense resistor, and inductor series resistance, and is proportional to the load current: VDROPOUT = IOUT [RDS(ON) + RSENSE + RL] Regulation Control Scheme The MAX1774 has a unique operating scheme that allows PWM operation at medium and high current, automatically switching to pulse-skipping mode at lower currents to improve light-load efficiency. Figure 2 shows a simplified block diagram. Under medium and heavy load operation, the inductor current is continuous and the part operates in PWM mode. In this mode, depending on the duty cycle, either the minimum on-time or the minimum off-time sets the switching frequency. The duty cycle is approximately the output voltage divided by the input voltage. If the duty cycle is less than 50%, the minimum on-time controls the frequency, and the frequency is approximately f ≈ 2.5MHz ✕ D, where D is the duty cycle. If the duty cycle is greater than 50%, the minimum off-time sets the frequency, and the frequency is approximately f ≈ 2.5MHz ✕ (1 - D). In both cases, the error comparator regulates the voltage. For low duty cycles (<50%), the P-channel MOSFET is turned on for the minimum on-time, causing fixed-on-time operation. During the MOSFET on-time, the output voltage rises. Once the MOSFET is turned off, the voltage drops to the regulation threshold, when another cycle is initiated. For high duty cycles (>50%), the MOSFET remains off for the minimum off-time, causing fixed-off-time operation. In this case, the MOSFET remains on until the output voltage rises to the regulation threshold. Then the MOSFET turns off for the minimum off-time, initiating another cycle. By switching between fixed-on-time and fixed-off-time operation, the MAX1774 can operate at high input-output ratios and still operate up to 100% duty cycle for low dropout. When operating from fixed-on-time operation, the minimum output voltage is regulated, but in fixed-off-time operation, the maximum output voltage is regulated. Thus, as the input voltage drops below approximately twice the output voltage, a decrease in line regulation can be expected. The drop in voltage is approximately VDROP ≈ VRIPPLE. At light output loads, the inductor current is discontinuous, causing the MAX1774 to operate at lower frequencies, reducing the MOSFET gate drive and switching losses. In discontinuous mode, under most circumstances, the on-time will be a fixed minimum on-time of 400ns. ______________________________________________________________________________________ 13 MAX1774 Dual, High-Efficiency, Step-Down Converter with Backup Battery Switchover The MAX1774 features four separate current-limit threshold detectors and a watchdog timer for each of its step-down converters. In addition to the more common peak-current detector and zero-crossing detector, each converter also provides a valley-current detector, and a minimum-current detector. The valley-current detector is used to force the inductor current to drop to a lower level after hitting peak current before allowing the Pchannel MOSFET to turn on. This is a safeguard against inductor current significantly overshooting above the peak current when the inductor discharges too slowly when VOUT/L is small. The minimum-current detector ensures that a minimum current is built up in the inductor before turning off the P-channel MOSFET. This helps the inductor to charge the output near dropout when the dl/dt is small (because (VIN - VOUT) / L is small) to avoid multiple pulses and low efficiency. This feature, however, is disabled during dropout and light-load conditions where the inductor current may take too long to reach the minimum current value. A watchdog timer overrides the minimum current after the P-channel MOSFET has been on for longer than about 10µs. Main Step-Down Converter The main step-down converter features adjustable +1.25V to +5.5V output delivering up to 2A from a +2.7V to +28V input (see Setting the Output Voltages ). The use of external MOSFETs and current-sense resistor maximizes design flexibility. The MAX1774 offers a synchronous-rectifier MOSFET driver that improves efficiency by eliminating losses through a diode. The two MOSFET drive outputs, PDRV and NDRV, control these external MOSFETs. The output swing of these outputs is limited to reduce power consumption by limiting the amount of injected gate charge (see Internal Linear Regulators section for details). Current-limit detection for all main converter current limits is sensed through a small-sense resistor at the converters’ output (see Setting the Current Limit section ). Driving the SHDNM pin low puts the main converter in a low-power shutdown mode. The core regulator, low-voltage detectors, and backup converter are still functional when the main converter is in shutdown. When the MAX1774 enters backup mode, the main converter and its current sensor are shut off. Core Step-Down Converter The core step-down converter produces a +1.0V to +5.0V output from a +2.6V to +5.5V input. The low-voltage input allows the use of internal power MOSFETs, taking advantage of their low RDS(ON), improving efficiency and reducing board space. Like the main converter, the core regulator makes use of a synchronousrectifying N-channel MOSFET, improving efficiency and 14 eliminating the need for an external Schottky diode. Current sensing is internal to the device, eliminating the need for an external sense resistor. The maximum and minimum current limits are sensed through the P-channel MOSFET, while the valley current and zero-crossing current are sensed through the N-channel MOSFET. The core output voltage is measured at FBC through a resistive voltage-divider. This divider can be adjusted to set the output voltage level (see Setting the Output Voltages). The core input can be supplied from the main regulator or an external supply that does not exceed +5.5V (see High-Voltage Configuration and Low-Voltage Configuration sections). The core converter can be shut down independent of the main converter by driving SHDNC low. If the main converter output is supplying power to the core and is shut down, SHDNM controls both outputs. In this configuration, the core converter continues to operate when the MAX1774 is in backup mode. Voltage Monitors and Battery Switchover The MAX1774 offers voltage monitors ACI, LBI, DBI, and BKOFF that drive corresponding outputs to indicate low-voltage conditions. The AC adapter low-voltage detect input, ACI, is typically connected to the output of an AC-to-DC converter. When the voltage at ACI drops below the INS sense input plus 0.22V, the low AC output, ACO, is asserted. Figure 3 shows a simplified block diagram. The low and dead battery monitors (LBI and DBI) monitor the voltage at MAIN_BATT through a resistive voltage-divider. When the voltage at LBI falls below +1.20V, the low-battery output flag, LBO, is asserted. When both VIN_AC and MAIN_BATT are present, the MAX1774 chooses one of the two supplies determined by ACI. To facilitate this, the MAX1774 provides an open-drain MOSFET driver output (MDRV). This drives an external P-channel MOSFET used to switch the MAX1774 from the AC input to the battery. MDRV goes low when ACO is low, the main battery is not dead, and the MAX1774 is not in backup mode. The MAX1774 enters backup mode when the voltage at DBI is below +1.20V and VIN_AC is not present to the board. Under these conditions, the BKUP output is asserted (low), and the device utilizes its boost converter and a low-voltage backup battery to supply the main output. The BKUP pin can be driven low externally, forcing the MAX1774 to enter backup mode. If the voltage at BKOFF is less than 0.5V, the backup converter is disabled. BKOFF can be driven from a digital signal, or can be used as a low-battery detector to disable the backup converter when the backup battery is low. ______________________________________________________________________________________ Dual, High-Efficiency, Step-Down Converter with Backup Battery Switchover MAX1774 LBO LBO LBI 1.2V MDRV MDRV DBI DBO 1.2V BKUP BKUP ACI ACO INS NOAC 0.22V BKUP MODE BKOFF 0.5V CS- (MAIN OUT) BIN CS+ MAIN RDY CVL CS- CS+ IN RDY REF REF CVL PDRV MAIN BUCK EN CVH CVH FB SOFT-START NDRV ON PGND SHDNM SHDNC LXB2 (QFN ONLY) ON INC LXB BACKUP BOOST EN CORE BUCK LXC FB PGND FB PGNDC GND MAX1774 FBM FBC Figure 3. Simplified Block Diagram ______________________________________________________________________________________ 15 MAX1774 Dual, High-Efficiency, Step-Down Converter with Backup Battery Switchover Place 1MΩ pullup resistors from the main output to ACO, LBO, and BKUP. Use a 1MΩ pullup resistor from MDRV to IN. When not in backup mode, the backup regulator is isolated from the main output by an internal switch. When the MAX1774 is in backup mode, the main converter is disabled, and the output of the backup regulator is connected to the main output. The core converter is still operable while in backup mode. The backup step-up converter cannot drive the typical main load current. The load at main must be reduced before entering backup mode. If BKUP is de-asserted (goes high), the MAX1774 exits backup mode and resumes operation from the main battery or the AC adapter input. If BKOFF goes low, or the backup battery discharges where it cannot sustain the main output load, the backup converter shuts off. To restart the main converter, apply power to VIN_AC or MAIN_BATT. The backup converter uses an external Schottky diode and internal power NMOS switch. Since this converter shares the same output as the main buck converter, it shares the same feedback network. This automatically sets the backup converter output voltage to that of the main converter. The backup converter generates an output between +1.25V and +5.5V from a +0.9V to +5.5V input, and provides a load current up to 20mA. When the MAX1774 is in backup mode, the main current- sense circuit is turned off to conserve power. When the output is out of regulation, the maximum inductor current limit and zero-current detectors regulate switching. The N-channel MOSFET is turned on until the maximum inductor current limit is reached, and shuts off until the inductor current reaches zero. When the output is within regulation, switching is controlled by the maximum pulse width, LXB, switch current limit, zero crossing, and the feedback voltage. Internal Linear Regulators There are two internal linear regulators in the MAX1774. A high-voltage linear regulator accepts inputs up to +28V, reducing it to +2.8V at CVL to provide power to the MAX1774. If the voltage at CS- is greater than +2.47V, CVL is switched to CS-, allowing it to be driven from the main converter, improving efficiency. CVL supplies the internal bias to the IC and power for the NDRV gate driver. The CVH regulator output provides the low-side voltage for the main regulator’s PDRV output. The voltage at CVH is regulated at 4.2V below VIN to limit the voltage swing on PDRV, reducing gate charge and improving efficiency (Figure 3). 16 Reference The MAX1774 has a trimmed internal +1.25V reference at REF. REF can source no more than 50µA. Bypass REF to GND with a 0.22µF capacitor. Design Procedure Low-Voltage Configuration To improve efficiency and conserve board space, the core regulator operates from low input voltages, taking advantage of internal low-voltage, low-on-resistance MOSFETs. When the input voltage remains below 5.5V, run the core converter directly from the input by connecting INC to IN (Figure 1). This configuration takes advantage of the core’s low-voltage design and improves efficiency. High-Voltage Configuration For input voltages greater than 5.5V, cascade the main and core converters by connecting INC to the main output voltage (Figure 4). In this configuration, the core converter is powered from the main output. Ensure that the main output can simultaneously supply its load and the core input current. Backup Converter Configuration The MAX1774 provides a backup step-up converter to power the device and provide the main output voltage when other power fails. The backup converter operates from a +0.9V to +5.5V battery. For most rechargeable batteries, such as NiCd or NiMH, the simple circuit of Figure 5 can be used to recharge the backup battery. In this circuit, the backup battery is charged through R1 and D10. Consult the battery manufacturer for charging requirements. To prevent the backup battery from overdischarging, connect a resistive voltagedivider from the backup battery to BKOFF. Resistor values can be calculated through the following equation: R12 = R13 ✕ [(VBU / V BKOFF) - 1] where V BKOFF = 0.5V, and VBU is the minimum acceptable backup battery voltage. Choose R13 to be less than 150kΩ. Setting the Output Voltages The main output voltage is set from +1.25V and +5.5V with two external resistors connected as a voltagedivider to FBM (Figure 1). Resistor values can be calculated by the following equation: R10 = R11 ✕ [(VOUTM / VFBM) - 1] where VFBM = +1.25V. Choose R11 to be 40kΩ or less. The core regulator output is adjustable from +1.0V to +5.0V through two external resistors connected as a ______________________________________________________________________________________ Dual, High-Efficiency, Step-Down Converter with Backup Battery Switchover D1 VIN_AC MAIN BATTERY 2.7V TO 20V NSD03A10 C5 1µF NDS356AP P1 1MΩ MAX1774 2.7V TO 28V IN ACI INS CVH R4 R1 C6 10µF MDRV P2 DBI FDS8928A L1 5µH PDRV R2 RCS MAIN N1 LBI CMAIN 47µF NDRV 2.6V TO 5.5V MAX1774 R3 PGND ON SHDNM CS+ ON SHDNC CS- OFF OFF BIN C1 10µF LXB LXB2 (QFN ONLY) FBM L3 22µH BACKUP BATTERY 0.9V TO 5.5V R10 D2 EP05 Q03L R11 BKOFF C2 10µF CVL R5 1MΩ ACO C3 1µF R6 1MΩ C4 0.22µF REF LBO R7 1MΩ GND PGNDC BKUP INC C7 1µF L2 5.4µH CORE LXC R8 1.0V CCORE TO 22µF 5.5V FBC R9 Figure 4. Typical Application Circuit (Cascaded) voltage-divider to FBC (Figure 1). Resistor values can be calculated with the following equation: R8 = R9 ✕ [(VOUTC / VFBC) - 1] where VFBC = +1.0V. Choose R9 to be 30kΩ or less. Setting the Current Limit The main regulator current limit is set externally through a small current-sense resistor, R CS (Figure 1). The value of RCS can be calculated with the following equation: RCS = VCLM / (1.3 ✕ IOUT) where VCLM = 80mV is the current-sense threshold, and IOUT is the current delivered to the output. The core and backup converter current limits are set internally and cannot be modified. Careful layout of the current-sense signal traces is imperative. Place RCS as close to the MAX1774 as possible. The two traces should have matching length and width, be as far as possible from noisy switching sig- ______________________________________________________________________________________ 17 MAX1774 Dual, High-Efficiency, Step-Down Converter with Backup Battery Switchover 2.7V TO 28V D1 VIN_AC MAIN BATTERY 2.7V TO 20V NSD03A10 C5 NDS356AP IN INS CVH R4 P1 C6 10µF R1 MDRV P2 DBI FDS8928A L1 10µH PDRV R2 RCS MAIN N1 LBI ACI R3 CMAIN 47µF NDRV 2.6V TO 5.5V MAX1774 1MΩ PGND R12 ON SHDNM CS+ ON SHDNC CS- OFF OFF BIN D2 EP05 Q03L C1 10µF R10 LXB FBM LXB2 (QFN ONLY) L3 22µH R11 R13 R5 1MΩ BKOFF BACKUP BATTERY ACO C2 10µF 0.9V TO 5.5V R6 1MΩ CVL LBO C3 R7 1MΩ REF C4 0.22µF GND BKUP INC C7 1µF PGNDC L2 CORE LXC R8 CCORE 22µF 1.0V TO 5.5V FBC R9 Figure 5. Typical Application Circuit (with Recharge) nals, and be close together to improve noise rejection. These traces should be used for current-sense signal routing only and should not carry any load current. Refer to the MAX1774 evaluation kit for layout examples. Setting the Voltage Monitor Levels The low battery and dead battery detector trip points can be set by adjusting the resistor values of the 18 divider string (R1, R2, and R3) in Figure 1 according to the following equations: R1 = (R2 + R3) ✕ [(VBD / VTH) - 1] R2 = R3 ✕ [(VBL / VBD) - 1] where VBL is the low battery voltage, VBD is the dead battery voltage, and VTH = +1.20V. Choose R3 to be less than 250kΩ. ______________________________________________________________________________________ Dual, High-Efficiency, Step-Down Converter with Backup Battery Switchover The inductor’s saturation current must be greater than the peak switching current to prevent core saturation. Saturation occurs when the inductor’s magnetic flux density reaches the maximum level the core can support and inductance starts to fall. The inductor heating current rating must be greater than the maximum load current to prevent overheating. For optimum efficiency, the inductor series resistance should be less than the current-sense resistance. Capacitor Selection Choose the output filter capacitors to service input and output ripple current with acceptable voltage ripple. ESR in the output capacitor is a major contributor to output ripple. For the main converter, low-ESR capacitors such as polymer or ceramic capacitors are recommended. For the core converter, choosing a low-ESR tantalum capacitor with enough ESR to generate about 1% ripple voltage across the output is helpful in ensuring stability. Voltage ripple is the sum of contributions from ESR and the capacitor value: VRIPPLE ≈ VRIPPLE,ESR + VRIPPLE,C For tantalum capacitors, the ripple is determined mostly by the ESR. Voltage ripple due to ESR is: VRIPPLE,ESR ≈ (RESR) ✕ IRIPPLE For ceramic capacitors, the ripple is mostly due to the capacitance. The ripple due to the capacitance is approximately: VRIPPLE,C ≈ L IRIPPLE2COUT VOUT where VOUT is the average output voltage. These equations are suitable for initial capacitor selection. Final values should be set by testing a prototype or evaluation kit. When using tantalum capacitors, use good soldering practices to prevent excessive heat from damaging the devices and increasing their ESR. Also, ensure that the tantalum capacitors’ surge-current ratings exceed the startup inrush and peak switching currents. The input filter capacitor reduces peak currents drawn from the power source and reduces noise and voltage ripple at IN, caused by the circuit’s switching. Use a low-ESR capacitor. Two smaller value low-ESR capacitors can be connected in parallel if necessary. Choose input capacitors with working voltage ratings higher than the maximum input voltage. MOSFET Selection The MAX1774 drives an external enhancement-mode Pchannel MOSFET and a synchronous-rectifier N-channel MOSFET. When selecting the MOSFETs, important parameters to consider are on-resistance (RDS(ON)), maximum drain-to-source voltage (V DS(MAX) ), maximum gate-to-source voltage (V GS(MAX) ), and minimum threshold voltage (VTH(MIN)). Chip Information TRANSISTOR COUNT: 4545 PROCESS: BiCMOS Pin Configurations (continued) TOP VIEW SHDNM 1 28 LXC SHDNC 2 27 INS BKUP 3 26 LBO MDRV 4 25 ACO PGNDC 5 PGND 6 24 INC MAX1774 23 GND NDRV 7 22 FBC CVL 8 21 CS- IN 9 20 CS+ PDRV 10 19 FBM CVH 11 18 REF LXB 12 17 LBI BIN 13 16 DBI BKOFF 14 15 ACI 28 QSOP ______________________________________________________________________________________ 19 MAX1774 Inductor Selection The essential parameters for inductor selection are inductance and current rating. The MAX1774 operates with a wide range of inductance values. Calculate the inductance value for either CORE or MAIN, LMIN : L(MIN) = (VIN - VOUT) ✕ (tON(MIN) / lRIPPLE) where tONMIN is typically 400ns, and lRIPPLE is the continuous conduction peak-to-peak lRIPPLE current. In continuous conduction, lRIPPLE should be chosen to be 30% of the maximum load current. With high inductor values, the MAX1774 begins continuous-conduction operation at a lower fraction of full load (see Detailed Description). Dual, High-Efficiency, Step-Down Converter with Backup Battery Switchover QFN 28, 32,44, 48L.EPS MAX1774 Package Information 20 ______________________________________________________________________________________ Dual, High-Efficiency, Step-Down Converter with Backup Battery Switchover QSOP.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. 21 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 © 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products. MAX1774 Package Information (continued)