Adjustable-Output, Switch-Mode Current Sources with Synchronous Rectifier

Transcription

1 9-245; Rev 0; 7/97 EALUATION KIT AAILABLE Current Sources with Synchronous Rectifier General Description The MAX640/MAX64 CMOS, adjustable-output, switch-mode current sources operate from a +5.5 to +26 input, and are ideal for microprocessor-controlled battery chargers. Charging current, maximum output voltage, and pulse-trickle charge are programmed with external resistors. Programming the off-time modifies the switching frequency, suppressing undesirable harmonics in noise-sensitive circuits. The MAX640 s highside current sensing allows the load to connect directly to ground, eliminating ground-potential errors. The MAX64 incorporates a low-side current sense. The MAX640/MAX64 step-down pulse-width-modulation (PWM) controllers use an external P-channel MOSFET switch and an optional, external N-channel MOSFET synchronous rectifier for increased efficiency. An internal low-dropout linear regulator provides power for the internal reference and circuitry as well as the gate drive for the N-channel synchronous rectifier. The MAX640/MAX64 are available in space-saving, 6-pin narrow QSOP packages. Applications Battery-Powered Equipment Laptop, Notebook, and Palmtop Computers Handy Terminals Portable Consumer Products Cordless Phones Cellular Phones PCS Phones Backup Battery Charger Pin Configuration TOP IEW LDOL TOFF D D0 CC MAX640 MAX64 6 IN 5 LDOH 4 PDR 3 NDR 2 PGND Features 95% Efficiency +5.5 to +26 Input Supply Range 2 to 24 Adjustable-Output oltage Range 00% Maximum Duty Cycle (Low Dropout) Up to 500kHz PWM Operation Optional Synchronous Rectifier 6-Pin QSOP Package Current-Sense Accuracy: 2% (MAX64) 5.3% (MAX640) Ordering Information PART MAX640C/D MAX640EEE MAX64C/D TEMP. RANGE 0 C to +70 C -40 C to +85 C 0 C to +70 C PIN-PACKAGE Dice* 6 QSOP Dice* MAX64EEE -40 C to +85 C 6 QSOP *Dice are specified at, DC parameters only. Typical Operating Circuit IN = +5.5 TO +26 R TOFF D0 D TOFF CC IN GND MAX640 LDOH PDR NDR PGND 0 LDOL OUT P MAX640/MAX GND QSOP Maxim Integrated Products For free samples & the latest literature: or phone For small orders, phone ext

2 MAX640/MAX64 ABSOLUTE MAXIMUM RATINGS IN to GND to +28 LDOH to IN to -6 LDOL to GND to +6 PDR to GND... ( LDOH - 0.3) to ( IN + 0.3) NDR to GND to ( LDOL + 0.3) TOFF,,,, CC to GND to ( LDOL + 0.3) D0, D to GND to +6, to GND to +28 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 ( IN = +2, OUT = 6, Circuit of Figure 2, T A = 0 C to +85 C, unless otherwise noted. Typical values are at.) PARAMETER Input oltage Range SYMBOL IN PGND to GND...±0.3 Continuous Power Dissipation (T A = +70 C) QSOP (derate 8.30mW/ C above +70 C) mW Operating Temperature Range MAX64_EEE C to +85 C Storage Temperature Range C to +50 C Lead Temperature (soldering, 0sec) C CONDITIONS MIN TYP MAX UNITS Linear-Regulator Output oltage, IN Referenced LDOH IN = 5.5 to 26, I LOAD = 0 to 20mA IN - IN - IN Linear-Regulator Output oltage, Ground Referenced LDOL IN = 5.5 to 26, I LOAD = 0 to 20mA Full-Scale Current-Sense Threshold Quarter-Scale Current-Sense Threshold Current-Sense Line Regulation Output Current Compliance Quiescent IN Supply Current Output Current in Off Mode LDOL Undervoltage Lockout Reference oltage Reference Load Regulation Input Current MAX640 MAX64 MAX640 MAX64 IN = OUT to 26 OUT = 2 to 24 D0 or D = high D0 = D = low (off mode) D0 = D = low I = 0 to 50µA FET Drive Output Resistance PFET and NFET drive 2 Ω Off-Time Range 0 µs Off-Time Accuracy R TOFF = 62kΩ µs Pulse-Trickle Mode Duty-Cycle Period MAX640 MAX64 D0 = low, D = high, R TOFF = 00kΩ ms m m %/ %/ ma µa µa m µa Pulse-Trickle Mode Duty Cycle (Note ) D0 = low, D = high, R TOFF = 00kΩ 2.5 % Note : This ratio is generated by a :8 clock divider and is not an error source for current calculations. 2

3 ELECTRICAL CHARACTERISTICS (continued) ( IN = +2, OUT = 6, Circuit of Figure 2, T A = 0 C to +85 C, unless otherwise noted. Typical values are at.) PARAMETER ELECTRICAL CHARACTERISTICS ( IN = +2, OUT = 6, Circuit of Figure 2, to +85 C, unless otherwise noted.) PARAMETER Input oltage Range Linear-Regulator Output oltage, IN Referenced SYMBOL SYMBOL IN LDOH IN = 5.5 to 26, I LOAD = 0 to 20mA CONDITIONS CONDITIONS MIN TYP MAX PWM Maximum Duty Cycle 00 % Input Low oltage IL D0, D 0.8 Input High oltage IH D0, D 2.4 Input Leakage Current I IN D0, D ± µa MIN TYP MAX IN - IN UNITS UNITS MAX640/MAX64 Linear-Regulator Output oltage, Ground Referenced LDOL IN = 5.5 to 26, I LOAD = 0 to 20mA Full-Scale Current-Sense Threshold MAX640 MAX m Quarter-Scale Current-Sense Threshold MAX640 MAX m Output Current Compliance OUT = 2 to 24 (MAX640) 0.4 %/ Quiescent IN Supply Current D0 or D = high 4 ma Output Current in Off Mode D0 = D = low µa LDOL Undervoltage Lockout Reference oltage Reference Load Regulation I = 0 to 50µA 0 m Input Current µa FET Drive Output Resistance Off-Time Range 2 Ω.5 8 µs Off-Time Accuracy R TOFF = 62kΩ µs Pulse-Trickle Mode Duty-Cycle Period D0 = low, D = high, R TOFF = 50kΩ ms PWM Maximum Duty Cycle 00 % Input Low oltage IL D0, D 0.8 Input High oltage IH D0, D 2.4 Input Leakage Current I IN D0, D ± µa 3

4 MAX640/MAX64 Typical Operating Characteristics (Circuit of Figure 2,, unless otherwise noted.) EFFICIENCY (%) EFFICIENCY vs. OUTPUT OLTAGE IN = 2 IN = OUTPUT OLTAGE () IN = 26 MAX640/4-TOC0 OUTPUT CURRENT (A) MAX640 OUTPUT CURRENT vs. INPUT OLTAGE ( OUT = 4) INPUT OLTAGE () MAX640/4 TOC02 OUTPUT CURRENT (A) MAX640 OUTPUT CURRENT vs. OUTPUT OLTAGE OUTPUT OLTAGE () MAX640/4-TOC03 OUTPUT CURRENT (A) MAX64 OUTPUT CURRENT vs. INPUT OLTAGE ( OUT = 4) MAX640/4 TOC04 OUTPUT CURRENT (A) MAX64 OUTPUT CURRENT vs. OUTPUT OLTAGE MAX640/4-TOC05 QUIESCENT CURRENT (ma) QUIESCENT CURRENT vs. INPUT OLTAGE (NO-LOAD) MAX640/4-TOC INPUT OLTAGE () OUT () INPUT OLTAGE () OFF-MODE SUPPLY CURRENT (ma) OFF-MODE SUPPLY CURRENT (NO-LOAD) INPUT OLTAGE () MAX640/4-TOC07 SWITCHING FREQUENCY (khz) 0, SWITCHING FREQUENCY vs. R TOFF OUT = +6 OUT = T OFF (kω) 4 MAX640/4 TOC 08 A B LOAD = 3 LINE-TRANSIENT RESPONSE A: OUTPUT CURRENT, D = D0 = A/div B: INPUT OLTAGE, 0/div 2ms/div MAX640/4 TOC 09 0A 0

5 Typical Operating Characteristics (continued) (Circuit of Figure 2,, unless otherwise noted.) A B CURRENT-MODE CHANGE RESPONSE TIME 2ms/div IN = 2, =, R LOAD = 4Ω, NO OUTPUT CAPACITOR A: OUTPUT CURRENT, D0 = D = 0 A/div B: LOAD OLTAGE, AC coupled, 500m/div MAX640/4 TOC 0 0A 0 A B IN = 2, R LOAD = 4Ω A: D0 = D = 2/div EXITING OFF MODE 20µs/div B: OUTPUT CURRENT, 0.5A/div MAX640/4 TOC MAX640/MAX64 Pin Description PIN NAME FUNCTION LDOL 2 TOFF Internal, Ground-Referenced Low-Dropout Linear Regulator Output. Bypass with a 0.µF capacitor in parallel with a 4.7µF capacitor to GND. Off-Time Select Input. A resistor (R TOFF ) connected from this pin to GND programs the off-time for the hysteretic PWM step-down converter. This resistor also sets the period in duty-cycle mode. See Duty-Cycle Mode and Programming the Off-Time. 3, 4 D, D0 Digital Inputs. Select mode of operation (Table ). 5 CC Constant-Current Loop Compensation Input. Bypass with a 0.0µF capacitor to GND. 6 Reference oltage Output ( = 2). Bypass with a 0.µF capacitor to GND. 7 8 Current Select Input. Program the desired current level by applying a voltage at between 0 and, (I = / 3.3R SENSE ). See Figure 3. Maximum Output oltage Termination Input. When exceeds the reference voltage, the comparator resets the internal PWM latch, shutting off the external P-channel FET. 9 GND Ground 0 Negative Current-Sense Comparator Input Positive Current-Sense Comparator Input 2 PGND High-Current Ground Return for the output drivers 3 NDR Gate Drive for an optional N-channel FET synchronous rectifier 4 PDR Gate Drive for the P-channel FET 5 LDOH Internal, Input-Referenced Low-Dropout Linear Regulator Output. Bypass with a 0.33µF capacitor to IN. 6 IN Power-Supply Input. Input of the internal, low-dropout linear regulators. 5

6 MAX640/MAX64 A IN LDOL LDOH REG A2 PDR Gm MODE CONTROL B A MUX SEL NDR PGND MAX640 MAX64 D0, D CC TOFF Figure. MAX640/MAX64 Functional Diagram 6

7 47µF 0.33µF IN LDOH /2 IR7309 CC GND P 47µH R3 R4 OUT 4.7µF 0.µF LDOL PDR 4.7µF 0.µF MAX64 D0 D R TOFF TOFF /2 IR7309 NDR N PGND BATT R 0.µF 00mΩ R2 47µF 0.33µF IN LDOH /2 IR7309 LDOL PDR 0.µF MAX640 D0 D R TOFF TOFF /2 IR7309 NDR N PGND R R2 R3 P 47µH 00mΩ OUT BATT MAX640/MAX64 0.0µF CC GND R4 0.0µF Figure 2a. Standard Application Circuit Detailed Description The MAX640/MAX64 switch-mode current sources utilize a hysteretic, current-mode, step-down pulsewidth-modulation (PWM) topology with constant offtime. Internal comparators control the switching mechanism. These comparators monitor the current through a sense resistor (R SENSE ) and the voltage at. When inductor current reaches the current limit [( - ) / R SENSE ], the P-channel FET turns off and the N-channel FET synchronous rectifier turns on. Inductor energy is delivered to the load as the current ramps down. This ramp rate depends on R TOFF and inductor values. When off-time expires, the P-channel FET turns back on and the N-channel FET turns off. Two digital inputs, D0 and D, select between four possible current levels (Table ). In pulse-trickle mode, the Figure 2b. Standard Application Circuit part operates for 2.5% of the period set by R TOFF, resulting in a lower current for pulse-trickle charging. Figure is the MAX640/MAX64 functional diagram. Figure 2 shows the standard application circuits. Charge Mode: Programming the Output Currents The sense resistor, R SENSE, sets two charging current levels. Choose between these two levels by holding D0 high, and toggling D either high or low (Table ). The fast-charge current level equals CS / R SENSE where CS is the full-scale current-sense voltage of 50m. Alternatively, calculate this current by / (3.3R SENSE ). The top-off current equals / (3.3R SENSE ). A resistor-divider from to GND programs the voltage at (Figure 3). 7

8 MAX640/MAX64 The voltage at is given by: R = R2 ( / - ); 0kΩ < R2 < 300kΩ where = 2 and is proportional to the desired output current level. Table. Selecting Output Current Levels D DO MODE OUTPUT CURRENT (A) 0 0 OFF 0 0 Top-Off / (3.3R SENSE ) 0 Pulse-Trickle / (3.3R SENSE ) 2.5% duty cycle Fast Charge / (3.3R SENSE ) R R2 MAX640 MAX64 MAX64 Figure 3. Adjusting the Output Current Level L MAX640 R SENSE BATT R3 R4 Figure 4a. Setting the Maximum Output oltage Level BATT R SENSE Figure 4b. Setting the Maximum Output oltage Level The MAX640/MAX64 are specified for between 0 and. For >, output current increases linearly (with reduced accuracy) until it clamps at 4. Pulse-Trickle Mode: Selecting the Pulse-Trickle Current Pulling D0 low and D high selects pulse-trickle mode. This current equals / (3.3R SENSE ) and remains on for 2.5% of the period set by R TOFF. Pulse-trickle current maintains full charge across the battery and can slowly charge a cold battery before fast charging commences. L R3 R4-7 PERIOD = 3.2 x 0 x R TOFF (sec) Off Mode: Turning Off the Output Current Pulling D0 and D low turns off the P-channel FET and hence the output current flow. This mode also controls end of charge and protects the battery against excessive temperatures. Setting the Maximum Output oltage Level The maximum output voltage should be programmed to a level higher than the output/battery voltage (I LOAD x R LOAD ). An external resistor-divider between the output and ground (Figure 4) sets the voltage at. Once the voltage at exceeds the reference, the internal comparator turns off the P-channel FET, terminating current flow. Select R4 in the 0kΩ to 500kΩ range. R3 is given by: R3 = R4 ( OUT / ) - 8

9 where = 2 and OUT is the desired output voltage. Programming the Off-Time When programming the off-time, consider such factors as maximum inductor current ripple, maximum output voltage, inductor value, and inductor current rating. The output current ripple is less than the inductor current ripple and depends heavily on the output capacitor s size. Perform the following steps to program the off-time: ) Select the maximum output current ripple. I R (A) 2) Select the maximum output voltage. OUT (MAX)() 3) Calculate the inductor value range as follows: L MIN = ( OUTMAX x µs) / I R L MAX = ( OUTMAX x 0µs) / I R 4) Select an inductor value in this range. 5) Calculate t OFF as follows: t = L x I R OFF OUTMAX 6) Program t OFF by selecting R TOFF from: R TOFF = (29.3 x 0 9 ) x t OFF 7) Calculate the switching frequency by: fs = / (t ON + t OFF ) where t ON = (I R x L) / ( IN - OUT ) and I R = ( OUT x t OFF ) / L. L is the inductor value, IN is the input voltage, OUT is the output voltage, and I R is the output peak-to-peak current ripple. Note that R TOFF sets both the off-time and the pulsetrickle charge period. Reference The on-chip reference is laser trimmed for a precise 2 at. can source no more than 50µA. Bypass with a 0.µF capacitor to ground. Constant-Current Loop: AC Loop Compensation The constant-current loop s output is brought out at CC. To reduce noise due to variations in switching currents, bypass CC with a nf to 00nF capacitor to ground. A large capacitor value maintains a constant average output current but slows the loop response to changes in switching current. A small capacitor value speeds up the loop response to changes in switching current, generating increased ripple at the output. Select C CC to optimize the ripple vs. loop response. Synchronous Rectification Synchronous rectification reduces conduction losses in the rectifier by shunting the Schottky diode with a lowresistance MOSFET switch. In turn, efficiency increases by about 3% to 5% at heavy loads. To prevent crossconduction or shoot-through, the synchronous rectifier turns on shortly after the P-channel power MOSFET Table 2. Component Manufacturers COMPONENT Inductor MOSFETs Sense Resistor Capacitors Rectifier Sumida Coilcraft Coiltronics International Rectifier Siliconix MANUFACTURER turns off. The synchronous rectifier remains off for 90% of the off-time. In low-cost designs, the synchronous rectifier FET may be replaced by a Schottky diode. Component Selection External Switching Transistors The MAX640/MAX64 drive an enhancement-mode P-channel MOSFET and a synchronous-rectifier N- channel MOSFET (Table 2). When selecting a P-channel FET, some important parameters to consider are on-resistance (r DS(ON) ), maximum drain-to-source voltage ( DS max), maximum gate-to-source voltage ( GS max), and minimum threshold voltage ( TH min). In high-current applications, MOSFET package power dissipation often becomes a dominant design factor. I2R power losses are the greatest heat contributor for both high-side and low-side MOSFETs. Switching losses affect the upper MOSFET only (P-channel), since the Schottky rectifier or the N-FET body diode clamps the switching node before the synchronous rectifier turns on. Rectifier Diode If an N-channel MOSFET synchronous rectifier is not used, a Schottky rectifier is needed. The MAX640/ 9 Dale IRC AX Sprague Motorola Nihon CDRH25 series D0336P series UP2 series IRF7309 S4539DY WSL-200 series LR200-0 series TPS series 595D series MBAR5340t3 IN587-IN5822 NSQ03A04 MAX640/MAX64

10 MAX640/MAX64 I/0 I/0 CH0 CH D0 D MAX640 PDR NDR PGND R3 N DC IN P LOW-SIDE IS SHORTED R SENSE T BATT GND R4 Figure 5. Microcontroller Battery Charger MAX64 s high switching frequency demands a highspeed rectifier (Table 2). Schottky diodes such as the N587 N5822 are recommended. Make sure the Schottky diode s average current rating exceeds the peak current limit and that its breakdown voltage exceeds the output voltage ( OUT ). For high-temperature applications, Schottky diodes may be inadequate due to their high leakage current; high-speed silicon diodes such as the MUR05 or ECFS can be used instead. At heavy loads and high temperatures, the benefits of a Schottky diode s low forward voltage may outweigh the disadvantage of high leakage current. If the application uses an N-channel MOSFET synchronous rectifier, a parallel Schottky diode is usually unnecessary except with very high charge current (> 3 amps). Best efficiency is achieved with both an N-channel MOSFET and a Schottky diode. Inductor alue Refer to the section Programming the Off-Time to select the proper inductor value. There is a trade-off between inductor value, off-time, output current ripple, and switching frequency. Applications Information All-Purpose Microcontroller Battery Charger: NiCd, NiMH In applications where a microcontroller is available, the MAX640/MAX64 can be used as a low-cost battery charger (Figure 5). The controller takes over fast charge, pulse-trickle charge, charge termination, and other smart functions. By monitoring the output voltage at OUT, the controller initiates fast charge (set D0 and D high), terminates fast charge and initiates top-off (set D0 high and D low), enters trickle charge (set D0 low and D high), or shuts off and terminates current flow (set D0 and D low). Layout and Grounding Due to high current levels and fast switching waveforms, proper PC board layout is essential. High-current ground paths should be connected in a star 0

11 configuration to PGND. These traces should be wide to reduce resistance and as short as possible to reduce stray inductance. All low-current ground paths should be connected to GND. Place the input bypass capacitor as close as possible to the IN pin. See MAX640 E kit for layout example. Chip Information TRANSISTOR COUNT: 233 Package Information QSOP.EPS MAX640/MAX64