19-1381; Rev 0; 7/98 KIT ATION EVALU E L B AVAILA 1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter Features 0.87V Guaranteed Start-Up The device includes a 1, N-channel MOSFET power switch, a synchronous rectifier that acts as the catch diode, a reference, pulse-frequency-modulation (PFM) control circuitry, and circuitry to reduce inductor ringing--all in an ultra-small, 1.1mm-high MAX package. The output voltage is preset to 3.3V or can be adjusted from +2V to +5.5V using only two resistors. Efficiencies up to 90% are achieved for loads up to 50mA. The device also features an independent undervoltage comparator (PFI/PFO) and a logic-controlled 2A shutdown mode. 37A Quiescent Current (85A from 1.5V battery) Up to 90% Efficiency Built-In Synchronous Rectifier (no external diode) Ultra-Small MAX Package, 1.1mm High 2A Logic-Controlled Shutdown Power-Fail Detector Dual ModeTM Output: Fixed 3.3V Adjustable 2V to 5.5V 45mA Output Current at 3.3V for 1-Cell Input 90mA Output Current at 3.3V for 2-Cell Input Inductor-Damping Switch Suppresses EMI Ordering Information Applications PART MAX1678EUA Pagers Remote Controls TEMP. RANGE -40C to +85C PIN-PACKAGE 8 MAX Note: To order these devices shipped in tape-and-reel, add a -T to the part number. Pointing Devices Personal Medical Monitors Single-Cell Battery-Powered Devices Pin Configuration Typical Operating Circuit INPUT 0.87V TO VOUT OUT LX OUTPUT 3.3V TOP VIEW MAX1678 BATT ON OFF SHDN LOW-BATTERY DETECTOR INPUT PFI GND PFO FB LOW-BATTERY DETECTOR OUTPUT BATT 1 8 OUT PFI 2 7 LX PFO 3 6 GND SHDN 4 5 FB MAX1678 MAX Dual Mode is a trademark of Maxim Integrated Products. ________________________________________________________________ Maxim Integrated Products 1 For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800. For small orders, phone 408-737-7600 ext. 3468. MAX1678 General Description The MAX1678 is a high-efficiency, low-voltage, synchronous-rectified, step-up DC-DC converter intended for use in devices powered by 1 to 3-cell alkaline, NiMH, or NiCd batteries or a 1-cell lithium battery. It guarantees a 0.87V start-up voltage and features a low 37A quiescent supply current. MAX1678 1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter ABSOLUTE MAXIMUM RATINGS BATT, OUT,LX, SHDN to GND ..............................-0.3V to +6.0V OUT, LX Current.......................................................................1A FB, PFI, PFO to GND ................................-0.3V to (VOUT + 0.3V) Reverse Battery Current (TA = +25C) (Note 1) ...............220mA Continuous Power Dissipation (TA = +70C) MAX (derate 4.1mW/C above +70C) .......................330mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +165C Lead Temperature (soldering, 10sec) .............................+300C Note 1: The reverse battery current is measured from the Typical Operating Circuit's input terminal to GND when the battery is connected backward. A reverse current of 220mA will not exceed package dissipation limits but, if left for an extended time (more than 10 minutes), may degrade performance. 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 = V SHDN = 1.3V, ILOAD = 0, FB = GND, TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER SYMBOL Minimum Operating Input Voltage VBATT(MIN) Maximum Operating Input Voltage VBATT(MAX) Start-Up Voltage (Note 2) CONDITIONS MIN TYP 0.7 RL = 3k, TA = +25C 0.87 VFB < 0.1V 3.16 Output Voltage Range (Adjustable Mode) External feedback 2.0 FB Set Voltage VFB External feedback 1.19 V V -2 VOUT UNITS V 5.5 Start-Up Voltage Tempco Output Voltage (Fixed Mode) MAX mV/C 3.3 1.23 3.44 V 5.5 V 1.26 V N-Channel On-Resistance VOUT = 3.3V 1 1.5 P-Channel On-Resistance VOUT = 3.3V 1.5 2.2 P-Channel Catch Diode Voltage IDIODE = 100mA, P-channel switch off 0.8 V 550 mA Maximum Peak LX Current On-Time Constant ILX(MAX) K Quiescent Current into OUT IQ,OUT Quiescent Current into BATT IQ,BATT 0.9V < VBATT < 3.3V (tON = K / VBATT) 5.60 VOUT = 3.5V 8 11.2 V-s 37 65 A 4 8 A Shutdown Current into OUT ISHDN,OUT VOUT = 3.5V 0.1 1 A Shutdown Current into BATT ISHDN,BATT VBATT = 1V 2 3.5 A ILOAD = 20mA, VBATT = 2.5V (Figure 7) 90 Efficiency FB Input Current PFI Trip Voltage VFB = 1.3V VIL,PFI PFI Input Current PFO Low Output Voltage VOL PFO Leakage Current SHDN Input Low Voltage VIL SHDN Input High Voltage VIH SHDN Input Current 2 % 0.1 10 nA 614 632 mV VPFI = 650mV 0.1 10 nA VPFI = 0, VOUT = 3.3V, ISINK = 1mA 0.04 0.4 V VPFI = 650mV, VPFO = 6V 0.01 1 A Falling PFI hysteresis 2% 590 0.2 x VBATT 0.8 x VBATT SHDN = GND or BATT V V 0.1 _______________________________________________________________________________________ 10 nA 1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter MAX1678 ELECTRICAL CHARACTERISTICS (VBATT = V SHDN = 1.3V, ILOAD = 0, FB = GND, TA = -40C to +85C, unless otherwise noted.) (Note 3) PARAMETER Maximum Operating Input Voltage Output Voltage (Fixed Mode) SYMBOL MIN VBATT(MAX) VOUT Output Voltage Range (Adjustable Mode) FB Set Voltage CONDITIONS VFB MAX UNITS 5.5 V VFB < 0.1V 3.12 3.48 V External feedback 2.0 5.5 V External feedback 1.17 1.28 V N-Channel On-Resistance VOUT = 3.3V 1.5 P-Channel On-Resistance VOUT = 3.3V 2.2 11.2 V-s 65 A 8 A On-Time Constant K Quiescent Current into OUT IQ,OUT Quiescent Current into BATT IQ,BATT 0.9V < VBATT < 3.3V (tON = K / VBATT) 5.60 VOUT = 3.5V Shutdown Current into OUT ISHDN,OUT VOUT = 3.5V 1 A Shutdown Current into BATT ISHDN,BATT VBATT = 1V 3.5 A VFB = 1.3V 10 nA FB Input Current PFI Trip Voltage VIL,PFI PFI Input Current PFO Low Output Voltage VOL PFO Leakage Current 642 mV VPFI = 650mV 580 10 nA VPFI = 0, VOUT = 3.3V, ISINK = 1mA 0.4 V 1 A VPFI = 650mV, VPFO = 6V SHDN Input Low Voltage VIL SHDN Input High Voltage VIH SHDN Input Current Falling PFI hysteresis 2% 0.2 x VBATT 0.8 x VBATT SHDN = GND or BATT V V 10 nA Note 2: Start-up is guaranteed by correlation to measurements of device parameters (i.e., switch on-resistance, on-time, off-time, and output voltage trip point). Note 3: Specifications to -40C are guaranteed by design and not production tested. _______________________________________________________________________________________ 3 Typical Operating Characteristics (Circuit of Figure 7 (Fixed Mode, 3.3V) or Figure 8 (Adjustable Mode), T A = +25C, unless otherwise noted.) EFFICIENCY vs. LOAD CURRENT (VOUT = 2.4V, L1 = SUMIDA 47H) 50 VIN = 1.2V VIN = 0.85V 10 70 VIN = 1.2V 60 VIN = 0.85V 50 40 L1 = 47H SUMIDA CD43-470 R1 = 200k, R2 = 200k 0.1 1 10 10 0.1 1 10 100 200 0.01 10 100 200 EFFICIENCY vs. LOAD CURRENT (VOUT = 3.3V, L1 = TDK 47H) 100 VIN = 2.0V VIN = 2.5V 90 100 MAX1678-05 MAX1678-04 VIN = 1.5V VIN = 0.85V 40 VIN = 0.85V V = 1.5V IN 60 50 40 30 30 20 L1 = 47H SUMIDA CD43-470 FB = GND 10 0.1 1 10 VIN = 1.2V VIN = 0.85V 40 20 L1 = 47H TDK NLC453232T-470K FB = GND 0 0.01 100 200 60 50 10 0 0 70 30 20 L1 = 22H SUMIDA CD43-220 FB = GND 10 VIN = 2.0V VIN = 1.5V 80 VIN = 1.2V 70 EFFICIENCY (%) EFFICIENCY (%) VIN = 1.2V VIN = 2.5V 90 80 60 0.1 1 10 100 200 0.01 0.1 1 10 100 200 LOAD CURRENT (mA) LOAD CURRENT (mA) LOAD CURRENT (mA) EFFICIENCY vs. LOAD CURRENT (VOUT = 5.0V, L1 = 22H) EFFICIENCY vs. LOAD CURRENT (VOUT = 5.0V, L1 = SUMIDA 47H) EFFICIENCY vs. LOAD CURRENT (VOUT = 5.0V, L1 = TDK 47H) VIN = 3.0V 60 VIN = 1.2V VIN = 0.85V 40 30 EFFICIENCY (%) 80 VIN = 2.0V 70 80 VIN = 1.2V 60 VIN = 0.85V 50 20 L1 = 22H SUMIDA CD43-220 R1 = 619k, R2 = 200k 0 40 1 10 LOAD CURRENT (mA) 100 200 MAX1678-09 70 VIN = 2.0V 60 VIN = 1.2V 50 40 VIN = 0.85V 30 20 L1 = 47H SUMIDA CD43-470 R1 = 619k, R2 = 200k 10 20 L1 = 47H TDK NLC453232-470K R1 = 619k, R2 = 200k 10 0 0 0.1 VIN = 4.5V VIN = 3.0V 90 VIN = 2.0V 70 30 10 100 MAX1678-08 VIN = 3.0V VIN = 4.5V 90 EFFICIENCY (%) VIN = 4.5V 50 100 MAX1678-07 100 0.01 1 EFFICIENCY vs. LOAD CURRENT (VOUT = 3.3V, L1 = SUMIDA 47H) 50 80 0.1 EFFICIENCY vs. LOAD CURRENT (VOUT = 3.3V, L1 = 22H) 70 90 L1 = 47H TDK NLC453232T-470K R1 = 200k, R2 = 200k LOAD CURRENT (mA) VIN = 2.0V 0.01 40 LOAD CURRENT (mA) VIN = 2.5V 80 VIN = 0.85V LOAD CURRENT (mA) 100 90 VIN = 1.2V 50 0 0.01 100 200 60 20 0 0.01 70 30 10 0 VIN = 1.5V 80 20 L1 = 22H SUMIDA CD43-220 R1 = 200k, R2 = 200k VIN = 2.0V 90 30 20 EFFICIENCY (%) 100 EFFICIENCY (%) EFFICIENCY (%) 60 30 4 VIN = 2.0V 80 70 40 VIN = 1.5V MAX1678-06 VIN = 1.5V 80 EFFICIENCY (%) 90 MAX1678-02 VIN = 2.0V 90 100 MAX1678-01 100 EFFICIENCY vs. LOAD CURRENT (VOUT = 2.4V, L1 = TDK 47H) MAX1678-03 EFFICIENCY vs. LOAD CURRENT (VOUT = 2.4V, L1 = 22H) EFFICIENCY (%) MAX1678 1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter 0.01 0.1 1 10 LOAD CURRENT (mA) 100 200 0.01 0.1 1 10 LOAD CURRENT (mA) _______________________________________________________________________________________ 100 200 1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter 100 VOUT = 2.4V R1 = 1M, R2 = 1M L1 = 47H SUMIDA CD43-470 10 4 4 MAX1678-12 20 40 60 80 8.2 8.0 L1 = 47H SUMIDA CD43-470 3.3V FIXED MODE 1.2 1.1 WITHOUT DIODE 1.0 0.9 WITH EXTERNAL SCHOTTKY DIODE (FIGURE 3) 0.8 0.7 0.6 -20 0 20 40 60 80 0 100 5 10 15 20 25 30 MAXIMUM LOAD CURRENT vs. INPUT VOLTAGE (L1 = 22H) MAXIMUM LOAD CURRENT vs. INPUT VOLTAGE (L1 = SUMIDA 47H) MAXIMUM LOAD CURRENT vs. INPUT VOLTAGE (L1 = TDK 47H) VOUT = 3.3V VOUT = 5.0V 100 VOUT = 3.3V 80 60 VOUT = 2.4V VOUT = 5.0V 40 20 L1 = 47H TDK NLC453232T-470K 100 VOUT = 3.3V 80 60 VOUT = 2.4V VOUT = 5.0V 40 20 0 0 120 35 MAX1678-18 L1 = 47H SUMIDA CD43-470 MAXIMUM LOAD CURRENT (mA) VOUT = 2.4V 120 140 MAX1678-17 140 MAX1678-16 L1 = 22H SUMIDA CD43-220 100 1.3 LOAD CURRENT (mA) MAXIMUM LOAD CURRENT (mA) MAXIMUM LOAD CURRENT (mA) 0 TEMPERATURE (C) 60 20 8.4 -40 80 40 -20 MINIMUM START-UP INPUT VOLTAGE vs. LOAD CURRENT 8.6 6 IBATT INPUT VOLTAGE (V) 140 100 5 10 ON-TIME CONSTANT (K) vs. TEMPERATURE 7.6 0 3 15 -40 7.8 2 20 TEMPERATURE (C) 8.8 2 1 25 INPUT VOLTAGE (V) START-UP INPUT VOLTAGE (V) 6 0 30 0 VBATT = 1.3V ON-TIME CONSTANT (V-s) 8 IOUT 5 9.0 MAX1678-13 SHUTDOWN BATTERY CURRENT (A) 3.3V FIXED MODE L1 = 47H SUMIDA CD43-470 VBATT = 1.3V VOUT = 3.6V FB = GND 35 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 TDK 12 120 MAX1678-11 VOUT = 3.0V FB = GND SHUTDOWN BATTERY CURRENT vs. INPUT VOLTAGE 10 QUIESCENT CURRENT (A) 22H NLC453232T-220K VOUT = 5.0V R1 = 3M, R2 = 1M MAX1678-15 MURATA 47H NLC453232T-470K LQH3C470K LQH4N470K COILCRAFT SUMIDA 47H 50 47H 22H 47H 55 CD43-220 60 22H DT1608C-223 65 47H DS1608C-473 70 CD43-470 75 45 40 MAX1678-14 80 1000 NO-LOAD BATTERY CURRENT (A) VBATT = 1.2V VOUT = 3.3V ILOAD = 20mA 85 EFFICIENCY (%) MAX1678-10 90 BATT AND OUT QUIESCENT CURRENT vs. TEMPERATURE NO-LOAD BATTERY CURRENT vs. INPUT VOLTAGE EFFICIENCY WITH DIFFERENT INDUCTORS 0 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 INPUT VOLTAGE (V) INPUT VOLTAGE (V) INPUT VOLTAGE (V) _______________________________________________________________________________________ 5 MAX1678 Typical Operating Characteristics (continued) (Circuit of Figure 7 (Fixed Mode, 3.3V) or Figure 8 (Adjustable Mode), T A = +25C, unless otherwise noted.) Typical Operating Characteristics (continued) (Circuit of Figure 7 (Fixed Mode, 3.3V) or Figure 8 (Adjustable Mode), T A = +25C, unless otherwise noted.) LOAD-TRANSIENT RESPONSE MAX1678-19 MAX1678-20 SWITCHING WAVEFORM A A B B C C 100s/div VOUT = 3.3V, VBATT = 1.2V, COUT = 10F, L1 = SUMIDA CD43-470, A: VOUT, 50mV/div, AC COUPLED B: INDUCTOR CURRENT, C: LOAD, 2mA to 12mA 100mA/div 5s/div VOUT = 3.3V, VBATT = 1.2V, ILOAD = 10mA, COUT = 10F, L1 = SUMIDA CD43-470 A: LX, 2V/div B: VOUT, 50mV/div AC COUPLED C: INDUCTOR CURRENT, 100mA/div POWER-UP RESPONSE MAX1678-22 LINE-TRANSIENT RESPONSE MAX1678-21 MAX1678 1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter A A B B C 200s/div VOUT = 3.3V, VBATT = 1.2V, ILOAD = 10mA, COUT = 10F, L1 = SUMIDA CD43-470 A: VOUT, 50mV/div, AC COUPLED B: VIN, 1V/div, 1.2V to 2.2V 100s/div VOUT = 3.3V, VBATT = 1.2V, ILOAD = 10mA, COUT = 10F, L1 = SUMIDA CD43-470 B: INDUCTOR CURRENT, 100mA/div A: VOUT, 1V/div C: SHDN, 5V/div Pin Description 6 PIN NAME 1 BATT FUNCTION Battery-Power Input 2 PFI Power-Fail Input. When the voltage at PFI is below 614mV, PFO sinks current. 3 PFO Open-Drain Power-Fail Output. PFO sinks current when PFI is below 614mV. 4 SHDN 5 FB 6 GND 7 LX 8 OUT Active-Low Shutdown. Connect SHDN to BATT for normal operation. Dual-Mode Feedback Input. Connect FB to GND for fixed-output operation (3.3V). Connect FB to a feedback-resistor network for adjustable output voltage operation (2V to 5.5V). FB regulates to 1.23V. Ground N-Channel MOSFET Switch Drain and P-Channel Synchronous-Rectifier Drain Power Output and IC Power Input (bootstrapped). OUT is the feedback input for 3.3V operation. Connect the filter capacitor close to OUT. _______________________________________________________________________________________ 1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter MAX1678 BACKUP tOFF TIMER ZERO-CROSSING DETECTION BATT OUT DAMPING SWITCH 0.5REF tON = K/VBATT DAMP TON TOFF PDRV EN CONTROL LOGIC NDRV P PFI LX PFO FB MAX1678 REF RFRDY N START-UP OSCILLATOR REF 1.23V REF 0.5REF GND OUT 1.7V SHDN START-UP COMPARATOR Figure 1. Functional Diagram Detailed Description The MAX1678 consists of an internal 1, N-channel MOSFET power switch, a built-in synchronous rectifier that acts as the catch diode, a reference, PFM control circuitry, and an inductor damping switch (Figure 1). The device is optimized for applications that are powered by 1 to 3-cell alkaline, NiMH, or NiCd batteries, or a 1-cell lithium battery such as pagers, remote controls, and battery-powered instruments. They are designed to meet the specific demands of the operating states characteristic of such systems: 2) Primary battery is good and load is sleeping: In this state the load draws hundreds of microamperes and the DC-DC converter IC draws very low quiescent current. Many applications maintain the load in this state most of the time. 3) Primary battery is dead and DC-DC converter is shut down: In this state the load is sleeping or supplied by the backup battery, and the MAX1678 draws 0.1A current from the OUT pin. 4) Primary and backup battery dead: The DC-DC converter can restart from this condition. 1) Primary battery is good and load is active: In this state the load draws tens of milliamperes and the MAX1678 typically offers 80% to 90% efficiency. _______________________________________________________________________________________ 7 MAX1678 1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter Operating Principle The MAX1678 employs a proprietary constant-peakcurrent control scheme that combines the ultra-low quiescent current of traditional pulse-skipping PFM converters with high-load efficiency. When the error comparator detects that the output voltage is too low, it turns on the internal N-channel MOSFET switch for an internally calculated on-time (Figure 2). During the on-time, current ramps up in the inductor, storing energy in the magnetic field. When the MOSFET turns off during the second half of each cycle, the magnetic field collapses, causing the inductor voltage to force current through the synchronous rectifier, transferring the stored energy to the output filter capacitor and the load. The output filter capacitor stores charge while the current from the inductor is high, then holds up the output voltage until the second half of the next switching cycle, smoothing power flow to the load. The ideal on-time of the N-channel MOSFET changes as a function of input voltage. The on-time is determined as follows: t ON = VBATT (DEAD TIME) VOUT VBATT (ON TIME) IL K VBATT t K VOUT - VBATT IPEAK = K L IPEAK (ON TIME) (DEAD TIME) tON tOFF tON t OR DEAD TIME Figure 2. Switching Waveforms K VBATT where K is typically 8V-s. The peak inductor current (assuming a lossless circuit) can be calculated from the following equation: K IPEAK = L The P-channel MOSFET (synchronous rectifier) turns on when the N-channel MOSFET turns off. The circuit operates at the edge of discontinuous conduction; therefore, the P-channel synchronous rectifier turns off immediately after the inductor current ramps to zero. During the dead time after the P-switch has been turned off, the damping switch connects LX and BATT. This suppresses EMI noise due to LC ringing of the inductor and parasitic capacitance at the LX node (see Damping Switch section). The error comparator starts another cycle when VOUT falls below the regulation threshold. With this control scheme, the MAX1678 maintains high efficiency over a wide range of loads and input/output voltages while minimizing switching noise. Start-Up Operation The MAX1678 contains a low-voltage start-up oscillator (Figure 1). This oscillator pumps up the output voltage to approximately 1.7V, the level at which the main DCDC converter can operate. The 150kHz fixed-frequency oscillator is powered from the BATT input and drives an NPN switch. During start-up, the P-channel synchronous 8 VLX VOUT COUT OUT VIN MAX1678 PDRV TIMING CIRCUIT NDRV L1 P LX N START-UP OSCILLATOR GND Figure 3. External Schottky Diode to Improve Start-Up with Heavy Load rectifier remains off and its body diode (or an external diode, if desired) is used as an output rectifier. The minimum start-up voltage is a function of load current (see Typical Operating Characteristics). In normal operation, when the voltage at the OUT pin exceeds 1.7V, the DCDC converter is powered from the OUT pin (bootstrapped) and the main control circuitry is enabled. Once started, the output can maintain the load as the battery voltage decreases below the start-up voltage. To improve start-up capability with heavy loads, add a Schottky diode in parallel with the P-channel synchronous rectifier (from LX to OUT) as shown in Figure 3 (see Typical Operating Characteristics). _______________________________________________________________________________________ 1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter VOUT VIN OUT MAX1678 PDRV BATT P DAMPING SWITCH TIMING CIRCUIT P DAMP LX Reverse-Battery Protection The MAX1678 can sustain/survive battery reversal up to the package power-dissipation limit. An internal 5 resistor in series with a diode limits reverse current to less than 220mA, preventing damage. Prolonged operation above 220mA reverse-battery current can degrade the device's performance. MAX1678 Shutdown Mode Pulling the SHDN pin low places the MAX1678 in shutdown mode (ISHDN = 2A typical). In shutdown, the internal switching MOSFET turns off, PFO goes high impedance, and the synchronous rectifier turns off to prevent the flow of reverse current from the output back to the input. However, there is still a forward current path through the synchronous-rectifier body diode from the input to the output. Thus, in shutdown, the output remains one diode drop below the battery voltage (VBATT). To disable the shutdown feature, connect SHDN (a logic input) to BATT or OUT. NDRV N GND Figure 4. Simplified Diagram of Damping Switch 1V/div Power-Fail Comparator The MAX1678 has an on-chip comparator for power-fail detection. This comparator can detect a loss of power at the input or output (Figures 7 and 8). If the voltage at the power-fail input (PFI) falls below 614mV, the PFO output sinks current to GND. Hysteresis at PFI is 2%. The power-fail monitor threshold is set by two resistors, R3 and R4, using the following equation: V R3 = R4 x TH - 1 VPFI where VTH is the desired threshold of the power-fail detector, and VPFI is the 614mV threshold of the powerfail comparator. Since PFI leakage is 10nA max, select feedback resistor R4 in the 100k to 1M range. VBATT = 2.5V VOUT = 3.3V L1 = 47H 2s/div Figure 5. LX Ringing Without Damping Switch (example only) 1V/div Damping Switch The MAX1678 is designed with an internal damping switch to minimize ringing at the LX node. The damping switch (Figure 4) connects the LX node to BATT, effectively depleting the inductor's remaining energy. 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 damping switch supplies a path to quickly dissipate this energy, suppressing the ringing at LX. This does not reduce the output ripple, but does reduce EMI. Figures 5 and 6 show the LX node voltage waveform without and with the damping switch. VBATT = 1.8V VOUT = 3.3V L1 = 47H 2s/div Figure 6. LX Ringing With Damping Switch _______________________________________________________________________________________ 9 MAX1678 1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter Applications Information IOUT(MAX) = M x Output Voltage Selection V 1 K x x BATT 2 L VOUT The MAX1678 operates with a fixed 3.3V or adjustable output. To select fixed-voltage operation, connect FB to GND (Figure 7). For an adjustable output between 2V and 5.5V, connect FB to a resistor voltage-divider between OUT and GND (Figure 8). FB regulates to 1.23V. where M is an empirical factor that takes into account losses in the MAX1678 internal switches and in the inductor resistance. K is the V-s factor that governs the inductor charge time. Nominally, M = 0.9 and K = 8V-s. M should be further reduced by 0.1 for each ohm of inductor resistance. Since FB leakage is 10nA max, select feedback resistor R2 in the 100k to 1M range. R1 is given by: The inductor's saturation-current rating must exceed the worst-case peak current limit set by the MAX1678's timing algorithm: KMAX IPEAK = L V R1 = R2 x OUT - 1 VREF where VREF = 1.23V. Maximum Output Current and Inductor Selection The MAX1678 is designed to work well with a 47H inductor in most low-power applications. 47H is a sufficiently low value to allow the use of a small surfacemount coil, but large enough to maintain low ripple. The Typical Operating Characteristics section shows performance curves with several 47H and 22H coils. Low inductance values supply higher output current but also increase ripple and reduce efficiency. Note that values below 22H are not recommended due to MAX1678 switch limitations. Higher inductor values reduce peak inductor current (and consequent ripple and noise) and improve efficiency, but also limit output current. The relationship between current and inductor value is approximately: C1 10F R3 BATT OUT R4 OUT MAX1678 R5 PHONE Coilcraft (847) 639-6400 Murata LQH4N470K, LQH3C470K (814) 237-1431 Sumida CD43-220, CD43-470 (847) 956-0666 TDK NLC453232T-220K, NLC453232T-470K (847) 390-4373 L1 47H R3 BATT LX PFI OUT MAX1678 R4 R1 R5 FB SHDN FB Figure 7. 3.3V Standard Application Circuit VOUT = 2V TO 5.5V OUT PFO SHDN 10 INDUCTOR DS1608C-223, DS1608C-473 3.3VOUT C2 10F PFO GND PIN C1 10F LX PFI Table 1. Suggested Inductors and Suppliers INPUT 0.87V TO VOUT L1 47H, 200mA INPUT 0.87V TO VOUT where K MAX = 11.2V-s. It is usually acceptable to exceed most coil saturation-current ratings by 20% with no ill effects; however, the maximum recommended IPEAK for the MAX1678 internal switches is 550mA, so inductor values below 22H are not recommended. For optimum efficiency, inductor series resistance should be less than 150mV/IPEAK. Table 1 lists suggested inductors and suppliers. R2 GND Figure 8. Adjustable Output Circuit ______________________________________________________________________________________ C2 1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter Minimizing Noise and Voltage Ripple EMI and output voltage ripple can be minimized by following these simple design rules: 1) Place the DC-DC converter and digital circuitry on the opposite corner of the PC board from sensitive RF and analog input stages. 2) Use a closed-core inductor, such as toroid or shielded bobbin, to minimize fringe magnetic fields. 5) Follow sound circuit-board layout and grounding rules (see the PC Board Layout and Grounding section). PC Board Layout and Grounding High switching frequencies and large peak currents make PC board layout an important part of design. Poor design can result in excessive EMI on the feedback paths and voltage gradients in the ground plane. Both of these factors can result in instability or regulation errors. The OUT pin must be bypassed directly to GND, as close to the IC as possible (within 0.2 inches or 5mm). Place power components--such as the MAX1678, inductor, input filter capacitor, and output filter capacitor--as close together as possible. Keep their traces short, direct, and wide (50 mil or 1.25mm), and place their ground pins close together in a star-ground configuration. Keep the extra copper on the board and integrate it into ground as a pseudo-ground plane. On multilayer boards, route the star ground using component-side copper fill, then connect it to the internal ground plane using vias. Place the external voltage-feedback network very close to the FB pin (within 0.2 inches or 5mm). Noisy traces, such as from the LX pin, should be kept away from the voltage-feedback network and separated from it using grounded copper. The MAX1678 evaluation kit manual shows an example PC board layout, which includes a pseudo-ground plane. 3) Choose the largest inductor value that satisfies the load requirement, to minimize peak switching current and the resulting ripple and noise. 4) Use low-ESR input and output filter capacitors. Table 2. Recommended Surface-Mount Capacitor Manufacturers VALUE (F) DESCRIPTION MANUFACTURER PHONE 595D-series tantalum Sprague 603-224-1961 TAJ, TPS-series tantalum AVX 803-946-0690 TDK 847-390-4373 AVX 803-946-0690 Taiyo Yuden 408-573-4150 4.7 to 47 4.7 to 10 4.7 to 22 X7R ceramic X7R ceramic ______________________________________________________________________________________ 11 MAX1678 Capacitor Selection Choose input and output capacitors to service input and output peak currents with acceptable voltage ripple. Capacitor ESR is a major contributor to output ripple (usually more than 60%). A 10F, ceramic output filter capacitor typically provides 50mV output ripple when stepping up from 1.3V to 3.3V at 20mA. Low input to output voltage differences (i.e., 2 cells to 3.3V) require higher capacitor values (10F to 47F). The input filter capacitor (CIN) also reduces peak currents drawn from the battery and improves efficiency. Low-ESR capacitors are recommended. Ceramic capacitors have the lowest ESR, but low-ESR tantalums represent a good balance between cost and performance. Low-ESR aluminum electrolytic capacitors are tolerable, and standard aluminum electrolytic capacitors should be avoided. Capacitance and ESR variation over temperature need to be taken into consideration for best performance in applications with wide operating temperature ranges. Table 2 lists suggested capacitors and suppliers. ___________________Chip Information TRANSISTOR COUNT: 840 Package Information 8LUMAXD.EPS MAX1678 1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter 12 ______________________________________________________________________________________