19-1175; Rev 0; 12/96 High-Accuracy Step-Down Controller with Synchronous Rectifier for CPU Power MAX798† The high-performance, step-down DC-DC converter provides main CPU power in battery-powered systems. This buck controller achieves 96% efficiency by using synchronous rectification and Maxim’s proprietary Idle Mode™ control scheme to extend battery life at full-load (up to 10A) and no-load outputs. The MAX798’s high accuracy meets the demanding requirements of the latest-generation CPUs. Excellent dynamic response corrects output transients caused by the latest dynamic-clock CPUs within five 300kHz clock cycles. Unique bootstrap circuitry drives inexpensive N-channel MOSFETs, reducing system cost and eliminating the crowbar switching currents found in some PMOS/NMOS switch designs. The MAX798 has a logic-controlled and synchronizable fixed-frequency pulse-width-modulating (PWM) operating mode, which reduces noise and RF interference in sensitive mobile-communications and pen-entry applications. The SKIP override input allows automatic switchover to idle-mode operation (for high-efficiency pulse skipping) at light loads, or forces fixed-frequency mode for lowest noise at all loads. ____________________________Features ♦ 96% Efficiency ♦ 4.5V to 30V Input Range ♦ 1.6V to 6V Adjustable Precision Output ♦ ♦ ♦ ♦ ♦ ±0.4% Max Total Load-Regulation Error 0.06%/V Max Line-Regulation Error 5V Linear-Regulator Output Precision 2.505V Reference Output Automatic Bootstrap Circuit ♦ 150kHz/300kHz Fixed-Frequency PWM Operation ♦ Programmable Soft-Start ♦ 1.2mA Typical Quiescent Current (VIN = 12V, VOUT = 2.5V) ♦ 1µA Typical Shutdown Current ______________Ordering Information PART MAX798ESE TEMP. RANGE PIN-PACKAGE -40°C to +85°C 16 Narrow SO ________________________Applications Notebook and Subnotebook Computers PDAs and Mobile Communicators __________Typical Operating Circuit INPUT 4.5V TO 30V __________________Pin Configuration 5V, 25mA 4.7µF TOP VIEW V+ VL BST SS 1 16 DH SKIP 2 15 LX REF 3 14 BST GND 4 MAX798 RSENSE VOUT 0.1µF MAX798 LX 13 DL ON DL SHDN SYNC 5 12 PGND SHDN 6 11 VL PGND FB 7 10 V+ CSH 9 CSL CSH 8 N DH OFF CSL SO GND N FB Idle Mode is a trademark of Maxim Integrated Products. †U.S. and foreign patents pending. ________________________________________________________________ Maxim Integrated Products 1 For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800 MAX798 _______________General Description MAX798 High-Accuracy Step-Down Controller with Synchronous Rectifier for CPU Power ABSOLUTE MAXIMUM RATINGS V+ to GND .................................................................-0.3V, +36V GND to PGND........................................................................±2V VL to GND ...................................................................-0.3V, +7V BST to GND ...............................................................-0.3V, +36V DH to LX ........................................................-0.3V, (BST + 0.3V) LX to BST.....................................................................-7V, +0.3V SHDN to GND............................................................-0.3V, +36V SYNC, SS, REF, SKIP, DL to GND ...................-0.3V, (VL + 0.3V) CSH, CSL to GND .......................................................-0.3V, +7V VL Short Circuit to GND..............................................Momentary REF Short Circuit to GND ...........................................Continuous VL Output Current ...............................................................50mA Continuous Power Dissipation (TA = +70°C) SO (derate 8.70mW/°C above +70°C) ........................696mW Operating Temperature Range MAX798ESE....................................................-40°C to +85°C Storage Temperature Range .............................-65°C to +160°C Lead Temperature (soldering, 10sec) .............................+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 (V+ = +15V, GND = PGND = 0V, IVL = IREF = 0A, TA = 0°C to +85°C, unless otherwise noted.) PARAMETER CONDITIONS MIN TYP MAX UNITS 4.5 30 V 1.6 6 V STEP-DOWN CONTROLLER Input Supply Range Nominal Adjustable Output Voltage Range External resistor divider Feedback Voltage CSH - CSL = 40mV, SKIP = VL Load Regulation 0mV < (CSH - CSL) < 80mV, SKIP = VL Line Regulation 6V < V+ < 30V 1.576 1.6 0.04 1.624 V ±0.4 % 0.05 %/V CSH - CSL, positive 80 100 120 CSH - CSL, negative -40 -100 -160 SS Source Current 2.0 4.0 6.5 SS Fault Sink Current 2.0 Current-Limit Voltage mV µA mA INTERNAL REGULATOR AND REFERENCE SHDN = 2V, 0mA <IVL < 25mA, 5.5V < V+ < 30V 4.75 5.25 V VL Fault Lockout Voltage Rising edge, hysteresis = 15mV 3.8 4.0 V VL/CSL Switchover Voltage Rising edge, hysteresis = 25mV 4.2 4.7 V Reference Output Voltage No external load (Note 1) 2.537 V Reference Fault Lockout Voltage Falling edge Reference Load Regulation 0µA < IREF < 100µA CSL Shutdown Leakage Current SHDN = 0V, CSL = 6V, V+ = 0V or 30V, VL = 0V V+ Shutdown Current V+ Off-State Leakage Current Dropout Power Consumption Quiescent Power Consumption VL Output Voltage 2 2.463 2.505 1.8 2.3 V 20 mV 0.1 1 µA SHDN = 0V, V+ = 30V, CSL = 0V or 6V 1 5 µA FB = CSH = CSL = 6V, VL switched over to CSL 1 5 µA V+ = 4V, CSL = 0V (Note 2) 6.6 10.5 mW CSH = CSL = 6V 6.4 8.5 mW _______________________________________________________________________________________ High-Accuracy Step-Down Controller with Synchronous Rectifier for CPU Power (V+ = +15V, GND = PGND = 0V, IVL = IREF = 0A, TA = 0°C to +85°C, unless otherwise noted.) PARAMETER Oscillator Frequency CONDITIONS SYNC = REF SYNC = 0V or 5V MIN 270 125 SYNC High Pulse Width 200 SYNC Low Pulse Width 200 SYNC Rise/Fall Time 195 89 91 SYNC = 0V or 5V 93 96 SHDN, SKIP Input Low Voltage Input Current UNITS kHz ns SYNC = REF SYNC Input High Voltage MAX 330 175 ns Guaranteed by design Oscillator Sync Range Maximum Duty Cycle TYP 300 150 200 ns 340 kHz % VL - 0.5 V 2.0 SYNC 0.8 SHDN, SKIP 0.5 SHDN, 0V or 30V 2 SYNC, SKIP 1 CSH, CSL, CSH = CSL = 4V, device not shut down V µA µA 50 FB, FB = 1.6V ±100 nA DL Sink/Source Current DL forced to 2V 1 A DH Sink/Source Current DH forced to 2V, BST - LX = 4.5V 1 DL On-Resistance High or low 7 Ω DH On-Resistance High or low, BST - LX = 4.5V 7 Ω A _______________________________________________________________________________________ 3 MAX798 ELECTRICAL CHARACTERISTICS (continued) MAX798 High-Accuracy Step-Down Controller with Synchronous Rectifier for CPU Power ELECTRICAL CHARACTERISTICS (V+ = +15V, GND = PGND = 0V, IVL = IREF = 0A, TA = -40°C to +85°C, unless otherwise noted.) (Note 3) PARAMETER CONDITIONS MIN TYP MAX UNITS 5.0 30 V 1.6 6.0 V 1.6 1.640 V ±1 % 0.04 0.06 %/ V STEP-DOWN CONTROLLER Input Supply Range Nominal Adjustable Output Voltage Range External resistor divider Feedback Voltage CSH - CSL = 40mV, SKIP = VL Load Regulation 0mV < (CSH - CSL) < 80mV, SKIP = VL Line Regulation 6V < V+ < 30V Current-Limit Voltage 1.560 CSH - CSL, positive 70 CSH - CSL, negative -40 130 -100 -160 mV INTERNAL REGULATOR AND REFERENCE VL Output Voltage SHDN = 2V, 0mA < IVL < 25mA, 5.5V < V+ < 30V 4.7 5.3 V VL Fault Lockout Voltage Rising edge, hysteresis = 15mV 3.75 4.05 V VL/CSL Switchover Voltage Rising edge, hysteresis = 25mV 4.15 4.75 V Reference Output Voltage No external load (Note 1) 2.438 Reference Load Regulation 0µA < IREF < 100µA V+ Shutdown Current SHDN = 0V, V+ = 30V, CSL = 0V or 6V V+ Off-State Leakage Current FB = CSH = CSL = 6V, VL switched over to CSL Quiescent Power Consumption CSH = CSL = 6V 2.505 2.562 V 30 mV 1 10 µA 1 10 µA 6.4 9.1 mW OSCILLATOR AND INPUTS/OUTPUTS Oscillator Frequency SYNC = REF 250 300 350 SYNC = 0V or 5V 110 150 190 SYNC High Pulse Width 250 SYNC Low Pulse Width 250 Oscillator Sync Range 210 Maximum Duty Cycle kHz ns ns 320 SYNC = REF 88 91 SYNC = 0V or 5V 92 96 kHz % DL On-Resistance High or low 7 Ω DH On-Resistance High or low, BST - LX = 4.5V 7 Ω Note 1: Since the reference uses VL as its supply, V+ line-regulation error is insignificant. Note 2: At very low input voltages, quiescent supply current can increase due to excess PNP base current in the VL linear regulator. This occurs only if V+ falls below the preset VL regulation point (5V nominal). The typical maximum quiescent current in dropout will not exceed 16mA. Note 3: All -40°C to +85°C specifications above are guaranteed by design. 4 _______________________________________________________________________________________ High-Accuracy Step-Down Controller with Synchronous Rectifier for CPU Power PIN NAME FUNCTION 1 SS 2 SKIP Disables pulse-skipping mode when high. Connect to GND for normal use. Don’t leave SKIP unconnected. With SKIP grounded, the device will automatically change from pulse-skipping operation to full PWM operation when the load current exceeds approximately 30% of maximum. 3 REF Reference Voltage Output. Bypass to GND with 0.33µF minimum. 4 GND Low-Noise Analog Ground and Feedback Reference Point 5 SYNC Oscillator Synchronization and Frequency Select. Tie to GND or VL for 150kHz operation; tie to REF for 300kHz operation. A high-to-low transition begins a new cycle. Drive SYNC with 0V to 5V logic levels (see the Electrical Characteristics table for VIH and VIL specifications). SYNC capture range is 195kHz to 340kHz. 6 SHDN Shutdown Control Input, active low. Logic threshold is set at approximately 1V (VTH of an internal N-channel MOSFET). Tie SHDN to V+ for automatic start-up. 7 FB 8 CSH Current-Sense Input, high side. Current-limit level is 100mV referred to CSL. 9 CSL Current-Sense Input, low side 10 V+ Battery Voltage Input (4.5V to 30V). Bypass V+ to PGND close to the IC with a 0.1µF capacitor. Connects to a linear regulator that powers VL. 11 VL 5V Internal Linear-Regulator Output. VL is also the supply voltage rail for the chip. VL is switched to the output voltage via CSL (VCSL > 4.5V) for automatic bootstrapping. Bypass to GND with 4.7µF. VL can supply up to 5mA for external loads. 12 PGND 13 DL Low-Side Gate-Drive Output. Normally drives the synchronous-rectifier MOSFET. Swings 0V to VL. 14 BST Boost Capacitor Connection for high-side gate drive (0.1µF) 15 LX Switching Node (inductor) Connection. Can swing 2V below ground without hazard. 16 DH High-Side Gate-Drive Output. Normally drives the main buck switch. DH is a floating driver output that swings from LX to BST, riding on the LX switching-node voltage. Soft-Start Timing Capacitor Connection. Ramp time to full current limit is approximately 1ms/nF. Feedback Input. Regulates at FB = 1.6V. Connect FB to a resistor divider to set the output voltage. Power Ground _______________________________________________________________________________________ 5 MAX798 ______________________________________________________________Pin Description MAX798 High-Accuracy Step-Down Controller with Synchronous Rectifier for CPU Power VIN, 4.5V TO 30V R3 10Ω 2.2µF 0.33µF 4.7µF SKIP V+ C1 VL BST REF DH ON Q1 L1 12µH 0.1µF OFF C2 SHDN LX 15mΩ VOUT, +2.5V @ 4.2A C3 47pF MAX798 C4 Q2 DL PGND R4 1M C5 470pF R1 CSH 0.01µF SS CSL R2 FB SYNC Q1, Q2 = C1, C2 = C4 = L1 = R1 = R2 = f= GND SILICONIX Si4410DY or IRF7413 10µF/30V SANYO OS-CON (30SA10) 470µF/4V SPRAGUE 594D SERIES (594D477X0004R2T) SUMIDA CDRH127 120 6.49kΩ, 1% 11.5kΩ, 1% 150kHz Figure 1. Standard Application Circuit _______________Detailed Description The MAX798 is a BiCMOS, switch-mode power-supply controller designed primarily for buck-topology regulators in battery-powered applications where high accuracy, high efficiency, and low quiescent supply current are critical. The MAX798 also works well in other topologies such as boost, inverting, and CUK due to the flexibility of its floating high-speed gate driver. Light-load efficiency is enhanced by automatic idlemode operation—a variable-frequency pulse-skipping 6 mode that reduces losses due to MOSFET gate charge. The step-down power-switching circuit consists of two N-channel MOSFETs, a rectifier, and an LC output filter. The output voltage is the average of the AC voltage at the switching node, which is adjusted and regulated by changing the duty cycle of the MOSFET switches. The gate-drive signal to the N-channel high-side MOSFET must exceed the battery voltage and is provided by a flying capacitor boost circuit that uses a 100nF capacitor connected between BST and LX. _______________________________________________________________________________________ High-Accuracy Step-Down Controller with Synchronous Rectifier for CPU Power PWM Controller Blocks: • Multi-Input PWM Comparator • Current-Sense Circuit • PWM Logic Block • Gate-Driver Outputs Bias Generator Blocks: • +5V Linear Regulator • Automatic Bootstrap Switchover Circuit • +2.505V Reference MAX798 The MAX798 contains the following seven major circuit blocks, which are shown in Figure 2. BATTERY VOLTAGE V+ TO CSL +5V LINEAR REGULATOR 4.5V OUT VL +5V AT 5mA SHDN BST DH PWM LOGIC SKIP LX MAIN OUTPUT DL +2.505V REF +2.505V AT 100µA PGND PWM COMPARATOR REF CSH 1X 1.6V CSL GND ON/OFF FB SHDN SYNC SS MAX798 Figure 2. Functional Diagram _______________________________________________________________________________________ 7 MAX798 High-Accuracy Step-Down Controller with Synchronous Rectifier for CPU Power These internal IC blocks aren’t powered directly from the battery. Instead, a +5V linear regulator steps down the battery voltage to supply both the IC internal rail (VL pin) as well as the gate drivers. The synchronousswitch gate driver is directly powered from +5V VL, while the high-side-switch gate driver is indirectly powered from VL via an external diode-capacitor boost circuit. An automatic bootstrap circuit turns off the +5V linear regulator and powers the IC from its output voltage if the output is above 4.5V. PWM Controller Block The heart of the current-mode PWM controller is a multiinput open-loop comparator that sums three signals: output voltage error signal with respect to the reference voltage, current-sense signal, and slope compensation ramp (Figure 3). The PWM controller is a direct summing type, lacking a traditional error amplifier and the phase shift associated with it. This direct-summing configuration approaches the ideal of cycle-by-cycle control over the output voltage. For more comprehensive information on the MAX798 internal blocks, please refer to the MAX796/MAX797/ MAX799 data sheet. __________Applications Information Powering VL from a +5V Supply If the circuit’s output voltage is greater than the VL/CSL switchover voltage, the IC automatically bootstraps and runs off its output. Running from the high-efficiency output rather than the VL linear regulator is particularly desirable if the input voltage is high. If the output is not greater than the VL/CSL switchover threshold, efficiency can be improved by powering VL from another efficient system supply (Figure 4). VL can be connected directly to a +5V supply, provided its maximum excursions are within a 4.5V to 6V range and the main output voltage’s maximum is less than 4.2V. If the circuit’s output voltage is greater than 4.2V, the IC can activate its internal bootstrap switch and connect the circuit output to VL. 8 Adjusting the Output Voltage The output voltage is set by an external resistor divider between the output voltage and GND, with the midpoint connected to FB (Figure 5). The output voltage can be adjusted from 1.6V to 6V, according to the formula in Figure 5. Recommended R2 values range from 5kΩ to 100kΩ. For best noise immunity, place R1 and R2 close to FB. For a 1.6V output, connect the output voltage directly to FB. Remote sensing of the output voltage is easily achieved by connecting the top of R1 and, if desired, the bottom of R2, to the remote sense point. Bypassing and Compensation Components The MAX798 is designed to deliver a more accurate output voltage than the MAX797. A major source of the MAX797’s output error is the decrease in output voltage with increasing load. This error is greatly reduced in the MAX798 by increasing the gain of the voltage-sense signal relative to the current-sense signal. As a result of this increased gain, the MAX798 is slightly more noise sensitive than the MAX797, and requires some small compensation components as well. On the other hand, output capacitor ESR requirements can be greatly relaxed compared to the MAX797, with the limiting factor being the maximum total output voltage ripple that the application can tolerate. To control noise problems, place the bypass capacitors on REF, VL, and V+ as close as possible to the IC, and use a 10Ω series resistor (R3, Figure 1) on V+ to form a small lowpass filter. Feed-forward components (R4, C3, and C5) are chosen for stable switching at 150kHz with the components shown. For 300kHz switching and a 4.7µH inductor, use R4 = 470kΩ and C5 = 220pF. Keep the components that connect to FB (R4, C5, R1, R2) close to the IC’s FB pin. Design Procedure With the exception of the items previously mentioned, follow the design procedure for the MAX797. The MAX796/MAX797/MAX799 data sheet contains all necessary information on component values, component selection, layout, and additional applications. _______________________________________________________________________________________ High-Accuracy Step-Down Controller with Synchronous Rectifier for CPU Power MAX798 CSH 1X CSL 1.6V FROM FEEDBACK DIVIDER MAIN PWM COMPARATOR BST R LEVEL SHIFT Q S DH LX SLOPE COMP OSC 30mV SKIP VL 4µA CURRENT LIMIT 24R SS 2.5V SHDN SHOOTTHROUGH CONTROL N MAX798 1R SYNCHRONOUS RECTIFIER CONTROL R –100mV S VL Q LEVEL SHIFT DL PGND Figure 3. PWM Controller Detailed Block Diagram _______________________________________________________________________________________ 9 MAX798 High-Accuracy Step-Down Controller with Synchronous Rectifier for CPU Power 5V ±10% V+ VBATT DH REMOTE SENSE LINES MAIN OUTPUT 0.1µF V+ MAX798 VL DL CSH MAX798 R1 CSL DH FB VOUT DL GND R2 (VOUT (MAX) < 4.2V) R1 VOUT = 1.6V 1 + ––– R2 ( Figure 4. Powering VL from a Separate +5V Supply 10 ) Figure 5. Adjusting the Output Voltage ______________________________________________________________________________________ High-Accuracy Step-Down Controller with Synchronous Rectifier for CPU Power MAX798 ___________________Chip Information TRANSISTOR COUNT: 1008 ________________________________________________________Package Information DIM D 0°-8° A 0.101mm 0.004in. e B A1 E C H L Narrow SO SMALL-OUTLINE PACKAGE (0.150 in.) A A1 B C E e H L INCHES MAX MIN 0.069 0.053 0.010 0.004 0.019 0.014 0.010 0.007 0.157 0.150 0.050 0.244 0.228 0.050 0.016 DIM PINS D D D 8 14 16 MILLIMETERS MIN MAX 1.35 1.75 0.10 0.25 0.35 0.49 0.19 0.25 3.80 4.00 1.27 5.80 6.20 0.40 1.27 INCHES MILLIMETERS MIN MAX MIN MAX 0.189 0.197 4.80 5.00 0.337 0.344 8.55 8.75 0.386 0.394 9.80 10.00 21-0041A ______________________________________________________________________________________ 11 High-Accuracy Step-Down Controller with Synchronous Rectifier for CPU Power MAX798 NOTES 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. 12 __________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600 © 1996 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.