re EV AVAILABLE MIAAILM 1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter General Description The MAX1678 is a high-efficiency, low-voltage, syn- chronous-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 37yA quiescent supply current. The device includes a 19, 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 ring- ingall in an ultra-small, 1.1 mm-high pMAX 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 2uA shut- down mode. Applications Pagers Remote Controls Pointing Devices Personal Medical Monitors Single-Cell Battery-Powered Devices Typical Operating Circuit Features 0.87V Guaranteed Start-Up Up to 90% Efficiency Built-In Synchronous Rectifier (no external diode) Ultra-Small pMAX Package, 1.1mm High 37pHA Quiescent Current (85pA from 1.5V battery) 2HA Logic-Controlled Shutdown Power-Fail Detector Dual Mode 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 -t+tfethUC TOC OHrmhUhU OHmhUCU Hh OH Ordering Information PART TEMP. RANGE PIN-PACKAGE MAX1678EUA -40C to +85C 8 uMAX Note: To order these devices shipped in tape-and-reel, add a -T to the part number. Pin Configuration INPUT 0.87V TO Vout OUTPUT LX OUT 3.3V i . + MAXIM TP Tv MAX1678 = BATT ON __ "Lore SON LOW- BATTERY } PFI PFO |- LOW-BATTERY DETECTOR INPUT GND FB DETECTOR OUTPUT TOP VIEW Batt [4] 8] OUT [2] maxim FF] pei [2 7] Lx vro[s| MAE FS ox SHON [4 | [5 | FB uMAX Dual Mode is a trademark of Maxim Integrated Products. MAXIM 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. SZLIOLXVWMAX1678 1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter ABSOLUTE MAXIMUM RATINGS BATT, OUT,LX, SHDN to GND uo. eeeeeteeeeeeereeeee -0.3V to +6.0V Operating Temperature Range ........-ceseceeseeeee -40C to +85C OUT, LX Current... eeeeeeeeeees Junction Temperature ...... eee eeeeceeeeeeeeeeeeeeeeeeeeneeeeeeeeeeeeeeees +150C FB, PFI, PFO to GND Storage Temperature Range ..........:cceeeeeeereeeee -65C to +165C Reverse Battery Current (TA = +25C) (Note 1)... 220mA Lead Temperature (Soldering, 10SCC) ........:ccsesseserereees +300C Continuous Power Dissipation (Ta = +70C) MAX (derate 4.1mMW/C above +70C) uo ee eee 330mW Note 1: The reverse battery current is measured from the Typical Operating Circuit's input terminal to GND when the battery is con- nected 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 = VSHDN = 1.3V, ILOAD = 0, FB = GND, Ta = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX | UNITS Voltage. Operating Input VBATT(MIN) 07 Vv Voltage. Operating Input VBATT(MAX) 55 Vv Start-Up Voltage (Note 2) RL = 3kQ, Ta = +25C 0.87 Vv Start-Up Voltage Tempco -2 mvV/C Output Voltage (Fixed Mode) VOUT VeB < 0.1V 3.16 3.3 3.44 Vv (Adpstable Mo Se External feedback 2.0 55 Vv FB Set Voltage VFB External feedback 1.19 1.23 1.26 Vv N-Channel On-Resistance VOUT = 3.3V 1 1.5 Q P-Channel On-Resistance VOUT = 3.3V 1.5 2.2 Q P-Channel Catch Diode Voltage IDIODE = 100mA, P-channel switch off 0.8 Vv Maximum Peak LX Current ILX(MAX) 550 mA On-Time Constant K 0.9V < VBATT < 3.3V (ton = K/ VBATT) 5.60 8 11.2 V-us Quiescent Current into OUT IQ,OUT VouT = 3.5V 37 65 pA Quiescent Current into BATT lQ,BATT 4 8 LA Shutdown Current into OUT ISHDN,OUT | VOUT = 3.5V 0.1 1 pA Shutdown Current into BATT ISHDN,BATT. | VBATT = 1V 2 3.5 pA Efficiency n ILOAD = 20mA, VBATT = 2.5V (Figure 7) 90 % FB Input Current VeB = 1.3V 0.1 10 nA PFI Trip Voltage VIL, PFI Falling PFI hysteresis 2% 590 614 632 mV PFI Input Current VPFI = 650mV 0.1 10 nA PFO Low Output Voltage VoL VpeFl = 0, VoUT = 3.3V, ISINK = 1mA 0.04 0.4 Vv PFO Leakage Current VPFI = 650mV, VpFO = 6V 0.01 1 pA SHDN Input Low Voltage VIL 0.2xVBaTT| V SHDN Input High Voltage ViH 0.8 x VBATT V SHDN Input Current SHDN = GND or BATT 0.1 10 nA 2 MAXUM1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter ELECTRICAL CHARACTERISTICS (VBATT = VSHDN = 1.3V, ILoaD = 0, FB = GND, Ta = -40C to +85C, unless otherwise noted.) (Note 3) PARAMETER SYMBOL CONDITIONS MIN MAX UNITS Voltage. Operating Input VBATT(MAX) 55 V Output Voltage (Fixed Mode) VOUT Vee < 0.1V 3.12 3.48 Vv (Adjustable Me bo External feedback 2.0 5.5 V FB Set Voltage VFB External feedback 1.417 1.28 Vv N-Channel On-Resistance VOUT = 3.3V 1.5 Q P-Channel On-Resistance VOUT = 3.3V 2.2 Q On-Time Constant K 0.9V < VBATT < 3.3V (ton = K/ VBATT) 5.60 11.2 V-us Quiescent Current into OUT IQ,OUT VouT = 3.5V 65 pA Quiescent Current into BATT lQ,BATT 8 pA Shutdown Current into OUT ISHDN,OUT | VOUT = 3.5V 1 pA Shutdown Current into BATT ISHDN,BATT | VBATT= 1V 3.5 pA FB Input Current VFB = 1.3V 10 nA PFI Trip Voltage VIL,PFI Falling PFI hysteresis 2% 580 642 mV PFI Input Current VpPFI = 650mV 10 nA PFO Low Output Voltage VOL VPFI = 0, VOUT = 3.3V, ISINK = 1mA 0.4 Vv PFO Leakage Current VpFL = 650mV, VPFO = 6V 1 pA SHDN Input Low Voltage VIL 0.2 x VBATT V SHDN Input High Voltage VIH 0.8 x VBATT V SHDN Input Current SHDN = GND or BATT 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. MAXIM SZLIOLXVWMAX1678 1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter (Circuit of Figure 7 (Fixed Mode, 3.3V) or Figure 8 (Adjustable Mode), Ta = +25C, unless otherwise noted.) EFFICIENCY vs. LOAD CURRENT (Vout = 2.4V, L1 = 22uH) 100 5 100 90 Vin =2.0V 5 90 80 Vin = 1.5V 80 _ 70 _ 70 % % = = 60 3 3 am 50 & 50 5 Vin=1.2V S Ee 40 Vin =0.85V IN fF 40 30 30 20 L1=22uH 20 40 SUMIDA CD43-220 40 Ri = 200k0, R2 = 200k 0 0 0.01 04 1 10 100 200 LOAD CURRENT (mA) EFFICIENCY vs. LOAD CURRENT (Vout = 3.3V, Lt = 22uH) 100 5 100 90 Vin =2.5V Vins 1.5V 2 90 80 Vin=2.0V 80 _ 70 70 x se 2 8 Vine 1.2V 2, 80 a a g 5 Vin = 0.85V So * 40 fF 40 30 30 20 L1 =22uH 20 40 SUMIDA CD43-220 10 FB = GND 0 0 0.01 0.1 1 10 100 200 LOAD CURRENT (mA) EFFICIENCY vs. LOAD CURRENT (Vout = 5.0V, Lt = 22uH) 100 s 100 Vin=4 g 90 on 5 90 Vin =3.0V 80 80 _ 70 _ 70 = 6 = 6 3 3 aH 50 H 50 ie fe H# 40 fF 40 30 30 20 Lt =22uH 20 40 SUMIDA CD43-220 40 Ri = 619kQ2, R2 = 200k 0 0 0.01 0.1 1 10 100 200 LOAD CURRENT (mA) 0.01 0.01 EFFICIENCY vs. LOAD CURRENT (Vout = 2.4V, Lt = SUMIDA 47yH) 0.1 2.0V Vin =1.5V Vin=1 Vin = 0.85V Li =47yH SUMIDA CD43-470 MAX1678-02 Ri =200kQ, R2 = 200kQ 1 10 LOAD CURRENT (mA) 100 200 EFFICIENCY vs. LOAD CURRENT (VouT = 3.3V, L1 = SUMIDA 47H) =2.0V Vin=2.5V 0.1 Vin=1.2V Vin = 0.85V Vin=1.5V L1 =47uH MAX1678-05 SUMIDA CD43-470 FB = GND 1 10 LOAD CURRENT (mA) 100 200 EFFICIENCY vs. LOAD CURRENT (Vout = 5.0V, Lt = SUMIDA 47uH) Vin = Vin=4.5V 0.1 Vin=1.2V Vin = 0.85V L1=47H SUMIDA CD43-470 Ri = 619kQ, R2 = 200kQ 1 10 LOAD CURRENT (mA) MAX1678-08 100 200 EFACIENCY (2%) EFFICIENCY (2%) EFFICIENCY (%) 100 90 80 70 60 50 40 30 20 100 90 80 70 60 50 40 30 20 100 80 70 60 50 40 30 20 0.01 0.01 Typical Operating Characteristics EFFICIENCY vs. LOAD CURRENT (VouT 0.1 = 2.4V, L1 = TDK 47H) Vin =2.0V Vin =,1.5V MAX1678-03 Vin=1.2V Vin = 0.85V L1=47H TDK NLC453232T-470K Ri = 200kQ, R2 = 200kQ 1 10 100. 200 LOAD CURRENT (mA) EFFICIENCY vs. LOAD CURRENT (Vout = 3.3V, L1 = TDK 47uH) Vin =2.5V =2.0V MAX1678-08 Vin =1.5V 0.1 EFFICI Vin=1.2V Vin = 0.85V Li =47yH TDK NLC453232T-470K FB =GND 1 10 100 200 LOAD CURRENT (mA) ENCY vs. LOAD CURRENT (Vout = 5.0V, L1 = TDK 47H) Vin=4 MAX1678-09 BV Vin =3.0V 0.1 Vin=2.0V Vin=1 | Vin = 0.85V Li =47yH TDK NLC453232-470K Ri =619kQ, R2 = 200kQ 1 10 100. 200 LOAD CURRENT (mA) MAXIMHigh-Efficiency, Step-Up DC-DC Converter Typical Operating Characteristics (continued) 1-Cell to 2-Cell, Low-Noise, (Circuit of Figure 7 (Fixed Mode, 3.3V) or Figure 8 (Adjustable Mode), Ta = +25C, unless otherwise noted.) 90 85 80 Bb 70 EFFICIENCY (2%) 65 60 55 50 SHUTDOWN BATTERY CURRENT (1A) oO 140 120 MAXIMUM LOAD CURRENT (mA) 8 & 8 8 B Q MAXIM EFFICIENCY WITH DIFFERENT INDUCTORS Veart = 1.2V VouT=3.3V 74 [] ILoap = 20mA MAX1678-10 DS1608C-473 CD43-470 LQH4N470K 22nH DT 6080-223 224A CD43-220 47pH NLC453239T-470K 22H NLC4532327-220K 47H LOH3C470K tH 7p 47pH Jd a COILCRAFT SUMIDA MURATA SHUTDOWN BATTERY CURRENT vs. INPUT VOLTAGE LZ MAX1678-13 T T 3.3V FIXED MODE | Lt =47H SUMIDA CD43-470 LZ VA LZ YY 0 1 2 3 4 5 6 INPUT VOLTAGE (V) MAXIMUM LOAD CURRENT vs. INPUT VOLTAGE (L1 = 22uH) Li = 22uH SUMIDA CD43-220 I | | Vout =2.4V MAX1678-18 Vout = 3.3V Vout = 5.0V 005 1015 20 25 30 35 40 45 50 INPUT VOLTAGE (V) NO-LOAD BATTERY CURRENT (,1A) ON-TIME CONSTANT (V-1s) MAXIMUM LOAD CURRENT (mA) 1000 100 9.0 8.8 86 8.4 8.2 8.0 78 76 140 120 100 80 40 20 40 -20 0 20 40 60 80 NO-LOAD BATTERY CURRENT vs. INPUT VOLTAGE MAX1678-11 Vout =5.0V Ri =3MQ, R2=1MQ Vout = 3.0V FB = GND Vout =2.4V Ri = 1MQ, R2 = iMQ Li =47yH SUMIDA CD43-470 0 05 1015 20 25 30 35 4.0 45 50 INPUT VOLTAGE (V} ON-TIME CONSTANT (K) vs. TEMPERATURE MAX1678-14 Veatr = 1.3V 100 TEMPERATURE (C) MAXIMUM LOAD CURRENT vs. INPUT VOLTAGE (L1 = SUMIDA 47H) MAX1678-17 Li =47yH SUMIDA 0 05 1015 20 25 30 35 40 45 50 INPUT VOLTAGE (V) QUIESCENT CURRENT (1A) START-UP INPUT VOLTAGE (V) MAXIMUM LOAD CURRENT (mA) 0.9 0.8 0.7 0.6 140 120 100 80 60 40 20 -40 -20 BATT AND OUT QUIESCENT CURRENT vs. TEMPERATURE MaX1678-12 SZLIOLXVW Vpatt = 1.3V Vout = 3.6V FB =GND 0 20 40 60 80 100 TEMPERATURE (C) MINIMUM START-UP INPUT VOLTAGE vs. LOAD CURRENT L1=47u4H | SUMIDA CD43-470 4 3 3.3V FIXED MODE YL | LA an WITHOUT DIODE \ | / aK | | WITH EXTERNAL SCHOTTKY DIODE (FIGURE 3) 0 5 0 1 20 2 30 3 LOAD CURRENT (mA) MAXIMUM LOAD CURRENT vs. INPUT VOLTAGE (L1 = TDK 47H) MAX1678-18 L1=47H TDK NLC453232T-470K 0 05 1015 20 25 30 35 40 45 50 INPUT VOLTAGE (V}MAX1678 1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter Typical Operating Characteristics (continued) (Circuit of Figure 7 (Fixed Mode, 3.3V) or Figure 8 (Adjustable Mode), Ta = +25C, unless otherwise noted.) SWITCHING WAVEFORM LOAD- TRANSIENT RESPONSE A B ct i 5us/div 100ps/div Vout = 3.3V, Vpatt = 1.2V, lLoap = 10mA, Cour = 10,1F, Vout = 3.3V, Veatr = 1.2V, Cour = 10uF, L1 = SUMIDA CD43-470 L1 = SUMIDA CD43-470, A:LX, 2V/div B: Vout, 50mV/div AC COUPLED A: Vout, 50mV/div, AC COUPLED _B: INDUCTOR CURRENT, C: INDUCTOR CURRENT, 100mA/div C: LOAD, 2mA to 12mA 100mA/div LINE- TRANSIENT RESPONSE POWER-UP RESPONSE ii i A B B i c 200j19/div 100ps/div Vout = 3.3V, Veatr = 1.2V, ILoap = 10mA, Cour = 10uF, Vout =3.3V, Veatt = 1.2V, ILoap = 10mA, Cour = 10,1F, L1 = SUMIDA CD43-470 Li = SUMIDA CD43-470 A: Vour, 50mV/div, AC COUPLED ~B: Vin, 1Wdiv, 1.2V to 2.2V A: Vout, 1V/div B: INDUCTOR CURRENT, 100mA/div C: SHDN, 5V/div . . . Pin Description PIN NAME FUNCTION 1 BATT 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 614mvV. 4 SHDN Active-Low Shutdown. Connect SHDN to BATT for normal operation. 5 EB Dual-Mode Feedback Input. Connect FB to GND for fixed-output operation (3.3V). Connect FB to a feed- back-resistor network for adjustable output voltage operation (2V to 5.5V). FB regulates to 1.23V. GND Ground 7 LX 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 8 OUT the filter capacitor close to OUT. 6 MAXIMA1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter BACKUP torr TIMER ZERO-CROSSING DETECTION BATT >__@eJ OUT DAMPING ton = K/Veatt ose} SWITCH ---- ] vn DAMP TON TOFF = PDRV p in CONTROL LOGIC NORV L, PFI - LX + @ PFO MAXIM LC | MAX1678 + FB + N s REF START-UP OSCILLATOR oF GND _ | OUT 1.7V SHDN START-UP COMPARATOR Figure 1. Functional Diagram Detailed Description The MAX1678 consists of an internal 192, 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 pow- ered 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: 1) Primary battery is good and load is active: |n this state the load draws tens of milliamperes and the MAX1678 typically offers 80% to 90% efficiency. MAXIM 2) Primary battery is good and load is sleeping: |n 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 sup- plied by the backup battery, and the MAX1678 draws 0.1p/A current from the OUT pin. 4) Primary and backup battery dead: The DC-DC con- verter can restart from this condition. SZLIOLXVWMAX1678 1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter Operating Principle The MAX1678 employs a proprietary constant-peak- current control scheme that combines the ultra-low qui- escent current of traditional pulse-skipping PFM converters with high-load efficiency. When the error comparator detects that the output volt- age 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 volt- age 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: K VBaTT ton = where K is typically 8V-ps. The peak inductor current (assuming a lossless circuit) can be calculated from the following equation: K IPEAK = 7 The P-channel MOSFET (synchronous rectifier) turns on when the N-channel MOSFET turns off. The circuit oper- ates 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 capaci- tance 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 DC- DC 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 Vx ff Vout 4 BATT Var Pereeed-------- | OEAD TING (ONTIMB t IPEAK te . I I I I , (NTIME ! I o tt (DEAD TIME $$$ >< t OR DEAD TIME Figure 2. Switching Waveforms Vout Cour TL OUT Vin = MAXLAA MAX1678 PORV | | Pp LU TIMING Lx CIRCUIT NDRV | 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 mini- mum start-up voltage is a function of load current (see Typical Operating Characteristics). |n normal operation, when the voltage at the OUT pin exceeds 1.7V, the DC- DC converter is powered from the OUT pin (boot- strapped) 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 synchro- nous rectifier (from LX to OUT) as shown in Figure 3 (see Typical Operating Characteristics). MAXIMA1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter Shutdown Mode Pulling the SHDN pin low places the MAX1678 in shut- down mode (ISHDN = 2HA 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. Reverse-Battery Protection The MAX1678 can sustain/survive battery reversal up to the package power-dissipation limit. An internal 5Q resistor in series with a diode limits reverse current to less than 220mA, preventing damage. Prolonged oper- ation above 220mA reverse-battery current can degrade the devices performance. 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: Vv. R3 = rex ye - } PFI where VTH is the desired threshold of the power-fail detector, and VpFi is the 614mvV threshold of the power- fail comparator. Since PFI leakage is 10nA max, select feedback resistor R4 in the 100kQ to 1MQ range. 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, effec- tively depleting the inductors remaining energy. When the energy in the inductor is insufficient to supply cur- rent 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. MAXIM rN MAXIMA OUT MAX1678 L PDRV + |. TIMING CIRCUIT DAMP NDRV | Figure 4. Simplified Diagram of Damping Switch 1Vidiv VeatT = 2.5V Vour =3.3V L1 =47uH 2us/div Figure 5. LX Ringing Without Damping Switch (example only) T | vf Mele ja WI a 1Vidiv Veart =1 BV Vout =3.3V Lt =47uH 2us/div Figure 6. LX Ringing With Damping Switch SZLIOLXVWMAX1678 1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter Applications Information Output Voltage Selection 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. Since FB leakage is 10nA max, select feedback resistor R2 in the 100kQ to 1MQ range. R1 is given by: V, R1 = rex | ut - } VREF where VREF = 1.23V. Maximum Output Current and Inductor Selection The MAX1678 is designed to work well with a 47pH inductor in most low-power applications. 47pH is a suf- ficiently low value to allow the use of a small surface- mount coil, but large enough to maintain low ripple. The Typical Operating Characteristics section shows perfor- mance curves with several 47H and 22uH coils. Low VBATT Vout 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-ys factor that governs the inductor charge time. Nominally, M = 0.9 and K = 8V-us. M should be further reduced by 0.1 for each ohm of inductor resistance. The inductors saturation-current rating must exceed the worst-case peak current limit set by the MAX1678s timing algorithm: 1 K xX L lourmax) = Mx > x KMAx PEAK = where KMAX = 11.2V-ps. It is usually acceptable to exceed most coil saturation-current ratings by 20% with no ill effects; however, the maximum recommended | PEAK for the MAX1678 internal switches is 550mA, so inductor values below 22uH are not recommended. For optimum efficiency, inductor series resistance should be less than 150mV/IPEAK. Table 1 lists suggested inductors and sup- pliers. Table 1. Suggested Inductors and Suppliers inductance values supply higher output current but PIN INDUCTOR PHONE also increase ripple and reduce efficiency. Note that DS1608C-223 values below 22uH are not recommended due to Coilcratt DS1608C-473 (847) 639-6400 MAX1678 switch limitations. Higher inductor values : : LQH4N470k, reduce peak inductor current (and consequent ripple Murata (814) 237-1431 : : . LQH3C470K and noise) and improve efficiency, but also limit output Current. Sumida Sota (847) 956-0666 The relationship between current and inductor value is NLO4532327. 220K roximately: : , 5 app ately TDK NLC453232T-470K (847) 390-4373 ut Ney a INPUT 0.87V TOV u 087V TO Vour ATuH, 200mA or Ct 10uF IX PFI QUT 3.3VOUT ~ MAXIAA a MAX1678 2 LL 10uF PFO = aa SHON ~ GND FB i _ BATT LX Vour=2V TOS.5V PFI QUT MA AXLM MAX1678 PFO FB SHDN Figure 7. 3.3V Standard Application Circuit 10 Figure 8. Adjustable Output Circuit MAXIMA1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter Capacitor Selection Choose input and output capacitors to service input and output peak currents with acceptable voltage rip- ple. Capacitor ESR is a major contributor to output rip- ple (usually more than 60%). A 10uUF, 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 (10uF to 47pF). The input filter capacitor (CIN) also reduces peak cur- rents 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 perfor- mance. Low-ESR aluminum electrolytic capacitors are tolerable, and standard aluminum electrolytic capaci- tors should be avoided. Capacitance and ESR variation over temperature need to be taken into consideration for best performance in applications with wide operat- ing temperature ranges. Table 2 lists suggested capac- itors and suppliers. Minimizing Noise and Voltage Ripple EMI and output voltage ripple can be minimized by fol- lowing 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. 3) Choose the largest inductor value that satisfies the load requirement, to minimize peak switching cur- rent and the resulting ripple and noise. 4) Use low-ESR input and output filter capacitors. 5) Follow sound circuit-board layout and grounding rules (see the PC Board Layout and Grounding sec- tion). 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 feed- back paths and voltage gradients in the ground plane. Both of these factors can result in instability or regula- tion 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 componentssuch as the MAX1678, inductor, input filter capacitor, and output filter capaci- toras close together as possible. Keep their traces short, direct, and wide (250 mil or 1.25mm), and place their ground pins close together in a star-ground con- figuration. 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 compo- nent-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. Table 2. Recommended Surface-Mount Capacitor Manufacturers Ur) DESCRIPTION MANUFACTURER PHONE 595D-series tantalum Sprague 603-224-1961 4.7 to 47 TAJ, TPS-series tantalum AVX 803-946-0690 TDK 847-390-4373 4.7 to 10 X7R ceramic AVX 803-946-0690 4.7 to 22 X7R ceramic Taiyo Yuden 408-573-4150 MAXIM 11 SZLIOLXVWMAX1678 1-Cell to 2-Cell, Low-Noise, High-Efficiency, Step-Up DC-DC Converter TRANSISTOR COUNT: 840 Chip Information Package Information Lh te Gop NOTES: 1. D&E DO NOT INCLUDE MOLD FLASH. 2. MOLD FLASH OR PROTRUSIONS NOT TO EXCEED .15mm.006">. 3. CONTROLLING DIMENSION: INCHES 8LUMAXD.EPS INCHES MILLIMETERS MIN | MAx | MIN | MAX A [0.036 |0.044 |0.91 111 Al |0.004 |0.008 [0.10 0.20 B {0.010 [0.014 {0.25 0.36 C_ (0.005 [0.007 |0.13 0.18 D |0116 [O1e0 [2.95 3.05 e 0.0256 0.65 E [0116 [0120 [2.95 3.05 H_ [0188 |0198 |4.78 5.03 L_ {0.016 |0026 {0.41 0.66 iy 0 6 0 6 PROPRIETARY INFORMATION MIA AILSVI TITLE: 8LD_ uMAX PACKAGE OUTLINE DwG. APPROVAL ENT CONTROL NO REV 21-0036 rl 12 MAXIMA