19-1776; Rev 1; 10/01 High-Voltage, Step-Down DC-DC Controller in µMAX ________________________Applications Automotive Electronics ____________________________Features ♦ High-Voltage Operation (up to 36V IN) ♦ Efficiency >90% ♦ Output Power Capability Exceeds 50W ♦ 10-Pin µMax Package ♦ Low-Dropout Voltage ♦ 100% (max) Duty Cycle ♦ 90µA Quiescent Current ♦ 4µA Shutdown Current ♦ Up to 330kHz Switching Frequency ♦ Output Voltage 5V or 3.3V (MAX1744) Adjustable 1.25V to 18V (MAX1745) ♦ Current-Limited Control Scheme Telecom Systems Ordering Information Wall-Cube-Powered Devices Industrial Control Systems PART Firewire/IEEE1394 TEMP RANGE PIN-PACKAGE MAX1744EUB -40°C to +85°C 10 µMAX MAX1744AUB -40°C to +125°C 10 µMAX MAX1745EUB -40°C to +85°C 10 µMAX MAX1745AUB -40°C to +125°C 10 µMAX Typical Operating Circuit Pin Configuration TOP VIEW IN 4.5V TO 36V IN ON 5V OFF SHDN 3.3V 3/5 VH EXT GND 1 P MAX1744 VL CS REF GND OUT OUT 3.3V OR 5V 10 IN VL 2 REF 3 3/5 (FB) 4 7 SHDN OUT 5 6 CS MAX1744 MAX1745 9 EXT 8 VH µMAX ( ) ARE FOR MAX1745 ONLY. ________________________________________________________________ Maxim Integrated Products For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com. 1 MAX1744/MAX1745 General Description The MAX1744/MAX1745 are step-down DC-DC controllers capable of handling up to 36V inputs. These parts use a proprietary current-limited control scheme for excellent light- and full-load efficiency, while their 330kHz (max) switching frequency permits small external components for space-critical applications. Operation to 100% duty cycle permits the lowest possible dropout voltage. The MAX1744 contains an internal feedback network that provides a pin-selectable output voltage of either 3.3V or 5V. The MAX1745 uses an external feedback network to generate an adjustable output voltage between 1.25V and 18V. The MAX1744/MAX1745 are available in a space-saving 10-pin µMAX package. MAX1744/MAX1745 High-Voltage, Step-Down DC-DC Controller in µMAX ABSOLUTE MAXIMUM RATINGS IN, EXT, SHDN to GND...........................................-0.3V to +38V VH to GND..............................................................-0.3V to +34V VH, EXT to IN............................................................-7V to +0.3V CS, OUT to GND ....................................................-0.3V to +20V FB, 3/5, REF to GND .....................................-0.3V to (VL + 0.3V) VL to GND...................................................................-0.3V to 6V Continuous Power Dissipation (TA = +70°C) 10-Pin µMAX (derate 5.6mW/°C above 70°C) .............444mW Operating Temperature Range MAX174_EUB ..................................................-40°C to +85°C MAX174_AUB ................................................-40°C to +125°C Junction Temperature ......................................................+150°C Storage Temperature Range .............................-65°C to +150°C Lead Temperature (soldering, 10s) ................................+300°C Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. ELECTRICAL CHARACTERISTICS (VIN = SHDN = 5.5V to 36V, 3/5 = GND, ILOAD = 0, TA = 0°C to +85°C, unless otherwise noted. Typical values at VIN = SHDN = 36V, TA = +25°C.) PARAMETER CONDITIONS Input Voltage Range MIN MAX UNITS 36 V Supply Current into IN SHDN = VIN = 5.5V to 36V 90 140 µA Shutdown Supply Current SHDN = GND 4 12 µA Output Voltage (MAX1744) 3/5 = VL 4.85 5.00 5.15 3/5 = GND 3.20 3.30 3.40 28 44 µA 1.25 1.28 V 50 nA V OUT Input Current (MAX1744) 3/5 = VL, VOUT = 5V FB Threshold Voltage (MAX1745) Falling edge, hysteresis = 8mV 1.22 VH Output Voltage with Respect to IN VIN = 5.5V to 36V, IVH = 100µA to 20mA -6.0 -5.3 -4.3 V VL Output Voltage VIN = 5.5V to 36V, IVL = 100µA to 2mA 4.5 5.0 5.5 V V FB Input Current (MAX1745) -50 VL Undervoltage Lockout CS Threshold Voltage CS Input Current SHDN, 3/5 Logic-High Threshold 2.0 3.0 4.1 VCS = VOUT = 2.5V to 18V 85 100 115 VCS = VOUT = GND 80 110 150 0 15 25 VCS = VOUT = 2.5V to 18V VCS = VOUT = GND -25 VIN = 4.5V to 36V 2.4 0 mV µA V SHDN, 3/5 Logic-Low Threshold VIN = 4.5V to 36V 0.4 V 3/5 Input Current SHDN = GND ±1 µA 3/5 = GND ±1 SHDN = 36V 12 SHDN Input Current µA 8 20 Ω Minimum EXT Off-Time 1.5 2.0 2.5 µs Minimum EXT On-Time 0.7 1.0 1.5 µs EXT Resistance 2 TYP 4.5 Output Line Regulation Figure 1, 5.5V < VIN < 36V, ILOAD = 1A 5 mV/V Output Load Regulation Figure 1, VIN = 12V, 30mA < ILOAD < 2A 15 mV/A Reference Voltage IREF = 0 1.22 1.25 1.28 V REF Load Regulation 0 ≤ IREF ≤ 100µA 4 10 mV REF Line Regulation VIN = 4.5V to 36V, IREF = 0 30 60 µV/V _______________________________________________________________________________________ High-Voltage, Step-Down DC-DC Controller in µMAX (VIN = SHDN = 5.5V to 36V, 3/5 = GND, ILOAD = 0, TA = -40°C to +85°C, unless otherwise noted.) (Note 1) PARAMETER CONDITIONS Input Voltage Range MIN UNITS 36 V Supply Current into IN SHDN = VIN = 5.5V to 36V 140 µA Shutdown Supply Current SHDN = GND 12 µA Output Voltage (MAX1744) 4.5 MAX 3/5 = VL 4.85 5.15 3/5 = GND 3.20 3.40 OUT Input Current (MAX1744) 3/5 = VL, VOUT = 5V FB Threshold Voltage (MAX1745) Falling edge, hysteresis = 8mV FB Input Current (MAX1745) VH Output Voltage with Respect to IN VIN = 5.5V to 36V, IVH = 100µA to 20mA VL Output Voltage VIN = 5.5V to 36V, IVL = 100µA to 2mA µA 1.22 1.28 V -50 50 nA -6.0V -4.3V V 4.5 5.5 V 2.0 4.1 V VCS = VOUT = 2.5V to 18V 85 115 VCS = VOUT = GND 80 150 VL Undervoltage Lockout CS Threshold Voltage 44 V VCS = VOUT = 2.5V to 18V mV 0 25 VCS = VOUT = GND -25 0 SHDN, 3/5 Logic-High Threshold VIN = 4.5V to 36V 2.4 SHDN, 3/5 Logic-Low Threshold VIN = 4.5V to 36V 0.4 V 3/5 Input Current SHDN = GND ±1 µA 3/5 = GND ±1 SHDN = 36V 12 CS Input Current SHDN Input Current µA 20 Ω µs 0.7 1.5 µs 1.22 1.28 V 1.5 Minimum EXT On-Time V 2.5 EXT Resistance Minimum EXT Off-Time µA Reference Voltage IREF = 0 REF Load Regulation 0 ≤ IREF ≤ 100µA 10 mV REF Line Regulation VIN = 4.5V to 36V, IREF = 0 60 µV/V _______________________________________________________________________________________ 3 MAX1744/MAX1745 ELECTRICAL CHARACTERISTICS MAX1744/MAX1745 High-Voltage, Step-Down DC-DC Controller in µMAX ELECTRICAL CHARACTERISTICS (VIN = SHDN = 5.5V to 36V, 3/5 = GND, ILOAD = 0, TA = -40°C to +125°C, unless otherwise noted.) (Note 1) PARAMETER CONDITIONS Input Voltage Range MIN 36 V SHDN = VIN = 5.5V to 36V 140 µA Shutdown Supply Current SHDN = GND 15 µA 3/5 = VL 4.85 5.15 3/5 = GND 3.20 3.40 OUT Input Current (MAX1744) 3/5 = VL, VOUT = 5V FB Threshold Voltage (MAX1745) Falling edge, hysteresis = 8mV FB Input Current (MAX1745) VH Output Voltage with Respect to IN VIN = 5.5V to 36V, IVH = 100µA to 20mA VL Output Voltage VIN = 5.5V to 36V, IVL = 100µA to 2mA CS Threshold Voltage CS Input Current SHDN, 3/5 Logic-High Threshold 44 V µA 1.22 1.28 V -50 50 nA -6.0V -4.3V V 4.5 5.5 V 1.6 4.1 V VCS = VOUT = 2.5V to 18V 85 115 VCS = VOUT = GND 80 150 VL Undervoltage Lockout 0 25 VCS = VOUT = GND VCS = VOUT = 2.5V to 18V -25 0 VIN = 4.5V to 36V 2.4 mV µA V SHDN, 3/5 Logic-Low Threshold VIN = 4.5V to 36V 0.4 V 3/5 Input Current SHDN = GND ±1 µA 3/5 = GND ±1 SHDN = 36V 12 SHDN Input Current Minimum EXT Off-Time 20 Ω µs 0.7 1.5 µs 1.22 1.28 V 1.5 Minimum EXT On-Time µA 2.5 EXT Resistance Reference Voltage IREF = 0 REF Load Regulation 0 ≤ IREF ≤ 100µA 10 mV REF Line Regulation VIN = 4.5V to 36V, IREF = 0 80 µV/V Note 1: Specifications to -40°C are guaranteed by design, not production tested. 4 UNITS Supply Current into IN Output Voltage (MAX1744) 4.5 MAX _______________________________________________________________________________________ High-Voltage, Step-Down DC-DC Controller in µMAX EFFICIENCY vs. LOAD CURRENT (VOUT = +3.3V) D 40 A: VIN = +5.5V B: VIN = +12.0V C: VIN = +24.0V D: VIN = +36.0V 20 40 A: VIN = +7.2V B: VIN = +12.0V C: VIN = +24.0V D: VIN = +36.0V 20 0 0.001 0.01 0.1 1 10 0.0001 0.001 0.01 0.1 1 100 95 90 85 80 0 10 10 20 30 40 LOAD CURRENT (A) LOAD CURRENT (A) INPUT VOLTAGE (V) IN PIN QUIESCENT CURRENT vs. INPUT VOLTAGE (3.5V TO 5.5V) SWITCHING FREQUENCY vs. INPUT VOLTAGE IN PIN QUIESCENT CURRENT vs. TEMPERATURE 3 2 1 100 80 60 40 94 VOUT = 3.3V 4.5 92 91 90 89 88 86 85 0 3.5 93 87 VOUT = 3.3V IOUT = 2.0A 20 0 MAX1744/5toc06 120 95 QUIESCENT CURRENT (µA) 4 MAX1744/5toc05 5 140 SWITCHING FREQUENCY (kHz) MAX1744/5toc04 6 5.5 0 10 20 30 -50 40 -25 0 25 50 75 100 INPUT VOLTAGE (V) INPUT VOLTAGE (V) TEMPERATURE (°C) EXT RISE AND FALL TIMES vs. CAPACITANCE EXT RISE AND FALL TIMES vs. TEMPERATURE CURRENT-SENSE TRIP LEVEL vs. TEMPERATURE 60 tFALL 40 tRISE 35 30 25 tFALL 20 15 10 tRISE 125 MAX1744/5toc09 40 115 CURRENT-SENSE TRIP LEVEL (mV) 80 VIN = +5V CL = 1000pF 45 tRISE AND tFALL (ns) 100 MAX1744/5toc08 VIN = +5V 20 50 MAX1744/5toc07 120 tRISE AND tFALL (ns) 105 0 0.0001 QUIESCENT CURRENT (mA) D C 60 110 MAX1744/5toc03 A 80 EFFICIENCY (%) C B QUIESCENT CURRENT (µA) A 80 MAX1744/5toc02 B EFFICIENCY (%) 100 MAX1744/5toc01 100 60 IN PIN QUIESCENT CURRENT vs. INPUT VOLTAGE (5.5V TO 36V) EFFICIENCY vs. LOAD CURRENT (VOUT = +5.0V) 110 105 100 95 90 5 0 85 0 0 1000 2000 3000 CAPACITANCE (pF) 4000 5000 -50 -25 0 25 50 75 TEMPERATURE (°C) 100 125 -50 -25 0 25 50 75 100 125 TEMPERATURE (°C) _______________________________________________________________________________________ 5 MAX1744/MAX1745 Typical Operating Characteristics (Circuit of Figure 1, TA = +25°C, unless otherwise specified.) Typical Operating Characteristics (continued) (Circuit of Figure 1, TA = +25°C, unless otherwise specified.) MAX1744 ENTERING/EXITING SHUTDOWN REFERENCE OUTPUT VOLTAGE CHANGE vs. TEMPERATURE MAX1744/5toc11 MAX1744/5toc10 5 REFERENCE OUTPUT VOLTAGE CHANGE (%) 4 3 2 RL = 3.3Ω VOUT 2V/div 1 0 -1 SHUTDOWN PULSE 5V/div -2 -3 -4 -5 -50 -25 0 25 50 75 100 2ms/div 125 TEMPERATURE (°C) LOAD-TRANSIENT RESPONSE LINE-TRANSIENT RESPONSE A MAX1744/5toc13 MAX1744/5toc12 MAX1744/MAX1745 High-Voltage, Step-Down DC-DC Controller in µMAX A B B 50µs/div VIN = 7.2V, VOUT = 3.3V, LOAD CURRENT = 0.1A TO 2A A: VOUT, 50mV/div, 3.3V AC-COUPLED B: LOAD CURRENT, 1A/div 6 4ms/div VOUT = 5V, LOAD CURRENT = 1A A: VOUT, 100mV/div, AC-COUPLED B: VIN, 6V TO 12V, 5V/div _______________________________________________________________________________________ High-Voltage, Step-Down DC-DC Controller in µMAX PIN NAME PIN FUNCTION MAX1744 MAX1745 1 GND GND 2 VL VL 5V Linear Regulator Output. VL provides power to the internal circuitry and can supply up to 1mA to an external load. Bypass VL to GND with 4.7µF or greater capacitor. 3 REF REF 1.25V Reference Output. REF can supply up to 100µA to an external load. Bypass REF to GND with a 0.1µF or greater ceramic capacitor. 4 3/5 — 3.3V or 5V Selection. Connect 3/5 to GND to set the output voltage to 3.3V. Connect 3/5 to VL to set the output voltage to 5V. 4 — FB Feedback Input for Adjustable Output Operation. Connect to an external voltage-divider between the output and FB to set the output voltage. The regulation voltage threshold is 1.25V. 5 OUT OUT 6 CS CS 7 SHDN SHDN 8 VH VH High-Side Linear Regulator Output. VH provides a regulated output voltage that is 5V below IN. The external P-channel MOSFET gate is driven between IN and VH. Bypass VH to IN with a 4.7µF or greater capacitor (see the Capacitor Selection section). 9 EXT EXT Gate Drive for External P-Channel MOSFET. EXT swings between IN and VH. 10 IN IN Ground Sense Input for Fixed 5V or 3.3V Output Operation (MAX1744) and Negative Current-Sense Input (MAX1744/5). OUT is connected to an internal voltage-divider (MAX1744). OUT does not supply current. Current-Sense Input. Connect the current-sense resistor between CS and OUT. External MOSFET is turned off when the voltage across the resistor is equal to or greater than the current limit trip level (100mV). Active-Low Shutdown Input. Connect SHDN to IN for normal operation. Drive SHDN to low to shut the part off. In shutdown mode, the reference, output, external MOSFET, and internal regulators are turned off. Positive Supply Input. Bypass IN to GND with a 0.47µF or greater ceramic capacitor. Detailed Description The MAX1744/MAX1745 are high-voltage step-down DC-DC converter controllers. These devices offer high efficiency over a wide range of input/output voltages and currents, making them optimal for use in applications such as telecom, automotive, and industrial control. Using an external P-channel MOSFET and current-sense resistor allows design flexibility and improved efficiency. The MAX1744/MAX1745 automatically switch from PWM operation at medium and heavy loads to pulse-skipping operation at light loads to improve light-load efficiency. The low 90µA quiescent current further optimizes these parts for applications where low input current is critical. Operation to 100% duty cycle allows the lowest possible dropout voltage, which allows a wider input voltage variation. The small size, high switching frequency, and low parts count minimize the required circuit board area and component cost. Figure 1 shows the MAX1744 typical application circuit. Operating Modes When delivering low output currents, the MAX1744/ MAX1745 operate in discontinuous-conduction mode. Current through the inductor starts at zero, rises as high as the current limit, then ramps down to zero during each cycle (Figure 3). The switch waveform exhibits ringing, which occurs at the resonant frequency of the inductor and stray capacitance, due to residual energy trapped in the core when the commutation diode (D1 in Figure 1) turns off. When delivering medium-to-high output currents, the MAX1744/MAX1745 operate in PWM continuous-conduction mode (Figure 4). In this mode, current always flows through the inductor and never ramps to zero. The control circuit adjusts the switch duty cycle to maintain regulation without exceeding the peak switching current set by the current-sense resistor. _______________________________________________________________________________________ 7 MAX1744/MAX1745 Pin Description MAX1744/MAX1745 High-Voltage, Step-Down DC-DC Controller in µMAX INPUT 4.5V TO 36V C2 4.7µF LOW ESR C3 4.7µF 0.47µF VH IN ON 5V OFF SHDN 3.3V 3/5 EXT MAX1744 C5 4.7µF VL C4 0.1µF P M1 FAIRCHILD NDS9407 L1 22µH CS OUT D1 NIHON EC2IQ506 REF GND RSENSE 40mΩ OUT 3.3V OR 5V 2A C1 220µF Figure 1. Typical Application Circuit 100% Duty Cycle and Dropout The MAX1744/MAX1745 operate with a duty cycle up to 100%. This feature extends the input voltage range by turning the MOSFET on continuously when the supply voltage approaches the output voltage. This services the load when conventional switching regulators with less than 100% duty cycle would fail. Dropout voltage is defined as the difference between the input and output voltages when the input is low enough for the output to drop out of regulation. Dropout depends on the MOSFET drain-to-source on-resistance, current-sense resistor, and inductor series resistance, and is proportional to the load current: Dropout voltage= [ IOUT x RDS(ON) + RSENSE + RINDUCTOR ] Regulation Control Scheme The MAX1744/MAX1755 have a unique operating scheme that allows PWM operation at medium and high current, with automatic switching to pulse-skipping mode at lower currents to improve light-load efficiency. Figure 2 shows the simplified block diagram. Under medium- and heavy-load operation, the inductor current is continuous and the part operates in PWM mode. In this mode, the switching frequency is set by either the 1µs minimum on-time or the 2µs minimum offtime, depending on the duty cycle. The duty cycle is approximately the output voltage divided by the input voltage. If the duty cycle is less than 33%, the minimum on-time controls the frequency; and the frequency is 8 approximately f ≈ 1MHz ✕ D, where D is the duty cycle. If the duty cycle is greater than 33%, the off-time sets the frequency; and the frequency is approximately f ≈ 500kHz ✕ (1 - D). In both cases, the voltage is regulated by the error comparator. For low duty cycles (<33%), the MOSFET is turned on for the minimum on-time, causing fixed-ontime operation. During the MOSFET on-time, the output voltage rises. Once the MOSFET is turned off, the voltage drops to the regulation threshold (set by the internal voltage-divider for the MAX1745 and by the external voltage-divider for the MAX1744), at which time another cycle is initiated. For high duty cycles (>33%), the MOSFET remains off for the minimum off-time, causing fixed-off-time operation. In this case, the MOSFET remains on until the output voltage rises to the regulation threshold. Then the MOSFET turns off for the minimum off-time, initiating another cycle. By switching between fixed-on-time and fixed-off-time operation, the MAX1744/MAX1745 can operate at high input-output ratios, yet still operate up to 100% duty cycle for low dropout. Note that when transitioning from fixed-on-time to fixed-off-time operation, the output voltage drops slightly due to the output ripple voltage. In fixed-on-time operation, the minimum output voltage is regulated, but in fixed-off-time operation, the maximum output voltage is regulated. Thus, as the input voltage drops below approximately three times the output voltage, a decrease in line regulation can be expected. The drop in voltage is approximately VDROP ≈ VRIPPLE / 2. _______________________________________________________________________________________ High-Voltage, Step-Down DC-DC Controller in µMAX MAX1744/MAX1745 EXT IN REF VH SHDN VH LINEAR REGULATOR VL VL LINEAR REGULATOR Q 1.25 REFERENCE TRIG MINIMUM ON-TIME ONE SHOT OUT ERROR COMPARATOR Q (FB) TRIG MINIMUM OFF-TIME ONE SHOT 3/5 Q S R ( ) MAX1745 ONLY - - - MAX1744 ONLY SHDN - + CS 100mV Figure 2. Simplified Functional Diagram At light output loads, the inductor current is discontinuous, causing the MAX1744/MAX1745 to operate at lower frequencies, reducing the MOSFET gate drive and switching losses. In discontinuous mode, under most circumstances, the on-time will be the fixed minimum on-time of 1µs. If the inductor value is small, or the current-sense resistor large, the current limit will be tripped before the minimum on-time, terminating the on-time and thus setting the fixed on-time. If the inductance is too large, or the output capacitance high and equivalent series resistance (ESR) low, then the MOSFET remains on longer than the minimum ontime, until the output capacitor charges beyond the error comparator’s (VOUT / 1.25V) ✕ 8mV hysteresis, causing the part to operate in hysteretic mode. Operating in hysteretic mode results in lower frequency operation. The transition to hysteretic mode occurs at the critical output capacitor ESR: where IRIPPLE is the inductor ripple current, and can be determined by: IRIPPLE = (VIN - VOUT) ✕ tON(MIN) / L where tON(MIN) is the minimum on-time (1µs) for minimum on-time-control, or: IRIPPLE = (VOUT) ✕ tOFF(MIN) / L where tOFF(MIN) is the minimum off-time (2µs) for minimum off-time-control. ESRCRITICAL = (VOUT / 1.25V) ✕ 8mV / IRIPPLE _______________________________________________________________________________________ 9 MAX1744/MAX1745 High-Voltage, Step-Down DC-DC Controller in µMAX A A B B C C 10µs/div 10µs/div CIRCUIT OF FIGURE 1, VIN = 18V, VOUT = 3.3V, ILOAD = 100mA A: MOSFET DRAIN, 10V/div B: OUT, 50mV/div, 3.3V DC OFFSET C: INDUCTOR CURRENT, 1A/div CIRCUIT OF FIGURE 1, VIN = 18V, VOUT = 3.3V, ILOAD = 1.5A A: MOSFET DRAIN, 10V/div B: OUT, 50mV/div, 3.3V DC OFFSET C: INDUCTOR CURRENT, 1A/div Figure 3. Discontinuous-Conduction Mode, Light-Load-Current Waveform Figure 4. Continuous-Conduction Mode, Heavy-Load-Current Waveform VL Linear Regulator Shutdown Mode The MAX1744/MAX1745 contain a 5V low-side linear regulator (VL) that powers the internal circuit and can supply up to 1mA to an external load. This allows the MAX1744/MAX1745 to operate up to 36V input, while maintaining low quiescent current and high switching frequency. When the input voltage goes below 5.5V, this regulator goes into dropout and the IN pin quiescent current will rise. See the Typical Operating Characteristics. Bypass VL with a 4.7µF or greater capacitor. When SHDN is low, the device enters shutdown mode. In this mode, the internal circuitry is turned off. EXT is pulled to IN, turning off the external MOSFET. The shutdown supply current drops to less than 10µA. SHDN is a logic-level input. Connect SHDN to IN for normal operation. VH Linear Regulator The MAX1744/MAX1745 contain a high-side linear regulator (VH) that regulates its output to 5V below IN (the positive supply input voltage). This regulator limits the external P-channel MOSFET gate swing (EXT), allowing high input voltage operation without exceeding the MOSFET gate-source breakdown. Bypass VH with a 4.7µF or greater capacitor between IN and VH. Quiescent Current The devices’ typical quiescent current is 90µA. However, actual applications draw additional current to supply MOSFET switching currents, OUT pin current, external feedback resistors (if used), and both the diode and capacitor leakage currents. For example, in the circuit of Figure 1, with IN at 30V and VOUT at 5V, typical no-load supply current for the entire circuit is 100µA. 10 Reference The 1.25V reference is suitable for driving small external loads. It has a guaranteed 10mV maximum load regulation while sourcing load currents up to 100µA. The reference is turned off during shutdown. Bypass the reference with 0.1µF for normal operation. Place the bypass capacitor within 0.2in (5mm) of REF, with a direct trace to GND. Design Information Setting the Output Voltage The MAX1744’s output voltage can be selected to 3.3V or 5V under logic control by using the 3/5 pin. Connect the 3/5 pin to GND to ensure a 3.3V output, or connect the 3/5 pin to VL to ensure a 5V output. The MAX1745’s output voltage is set using two resistors, R2 and R3 (Figure 5), which form a voltage-divider between the output and FB. R2 is given by: V R2= R3 x OUT − 1 VREF ______________________________________________________________________________________ High-Voltage, Step-Down DC-DC Controller in µMAX FROM OUTPUT R2 TO FB Current-Sense-Resistor Selection The current-sense comparator limits the peak switching current to VCS/RSENSE, where RSENSE is the value of the current-sense resistor and VCS is the current-sense threshold. VCS is typically 100mV. Minimizing the peak switching current will increase efficiency and reduce the size and cost of external components. However, since available output current is a function of the peak switching current, the peak current limit must not be set too low. Set the peak current limit to 1.3 times the maximum load current by setting the current-sense resistor to: RCS = VCS(MIN) 1.3 x IOUT(MAX) Inductor Selection The essential parameters for inductor selection are inductance and current rating. The MAX1744/MAX1745 operate with a wide range of inductance values. In many applications, values between 4.7µH and 100µH take best advantage of the controller’s high switching frequency. Calculate the minimum inductance value as follows: L(MIN) = (VIN - VOUT ) x 1µs VCS(MIN) RCS where 1µs is the minimum on-time. Inductor values between 2 and 10 times L(MIN) are recommended. With high inductor values, the MAX1744/MAX1745 begin continuous-conduction operation at a lower fraction of the full load (see the Detailed Description section). The inductor’s saturation and heating current ratings must be greater than the peak switching current to prevent overheating and core saturation. Saturation occurs when the inductor’s magnetic flux density reaches the maximum level the core can support, and inductance starts to fall. The heating current rating is the maximum DC current the inductor can sustain without overheating. For optimum efficiency, the inductor windings’ resistance should be less than the current-sense resistance. If necessary, use a toroid, pot-core, or shielded-core inductor to minimize radiated noise. Table 1 lists inductor types and suppliers for various applications. R3 Figure 5. Adjustable-Output Operation Using the MAX1745 External Switching Transistor The MAX1744/MAX1745 drive a P-channel enhancement-mode MOSFET. The EXT output swings from VH to IN. Be sure that the MOSFET’s on-resistance is specified for 5V gate drive or less. Table 1 recommends MOSFET suppliers. Four important parameters for selecting a P-channel MOSFET are drain-to-source breakdown voltage, current rating, total gate charge (Qg), and RDS(ON). The drain-to-source breakdown voltage rating should be at least a few volts higher than VIN. Choose a MOSFET with a maximum continuous drain-current rating higher than the peak current limit: ID(MAX) ≥ ILIM(MAX) = VCS(MAX) RSENSE The Qg specification should be 80nC or less to ensure fast drain voltage rise and fall times, and reduce power losses during transition through the linear region. Qg specifies all of the capacitances associated with charging the MOSFET gate. EXT pin rise and fall times vary with different capacitive loads, as shown in the Typical Operating Characteristics. RDS(ON) should be as low as practical to reduce power losses while the MOSFET is on. It should be equal to or less than the current-sense resistor. ______________________________________________________________________________________ 11 MAX1744/MAX1745 where VREF = 1.25V. Since the input bias current at FB has a maximum value of 50nA, large values (10kΩ to 200kΩ) can be used for R3 with no significant accuracy loss. For 1% error, the current through R2 should be at least 100 times FB’s input bias current. MAX1744/MAX1745 High-Voltage, Step-Down DC-DC Controller in µMAX Capacitor Selection Table 1. Component Suppliers COMPANY COUNTRY PHONE FAX 803-946-0690 AVX USA 803-626-3123 or 800-282-4975 Coilcraft USA 847-639-6400 847-639-1469 Coiltronics USA 516-241-7876 516-241-9339 Dale/Vishay USA 402-564-3131 402-563-6418 Kemet USA 408-986-0424 408-986-1442 International Rectifier USA 310-322-3331 310-322-3332 IRC USA 512-992-7900 512-992-3377 Motorola USA 602-303-5454 602-994-6430 Nichicon USA Japan 847-843-7500 81-7-5231-8461 847-843-2798 81-7-5256-4158 Nihon USA Japan 805-867-2555 81-3-3494-7411 805-867-2698 81-3-3494-7414 Sanyo USA Japan 619-661-6835 81-7-2070-6306 619-661-1055 81-7-2070-1174 Choose filter capacitors to service input and output peak currents with acceptable voltage ripple. ESR in the output capacitor is a major contributor to output ripple, so low-ESR capacitors are recommended. LowESR tantalum, polymer, or ceramic capacitors are best. Low-ESR aluminum electrolytic capacitors are tolerable, but standard aluminum electrolytic capacitors are not recommended. Voltage ripple is the sum of contributions from ESR and the capacitor value: VRIPPLE ≈ VRIPPLE,ESR + VRIPPLE,C For tantalum capacitors, the ripple is determined by the ESR, but for ceramic capacitors, the ripple is mostly due to the capacitance. Voltage ripple as a consequence of ESR is approximated by: VRIPPLE,ESR ≈ (RESR )∆ Ip−p The ripple due to the capacitance is approximately: VRIPPLE,C ≈ 408-988-8000 Siliconix USA 408-970-3950 or 800-554-5565 Sprague USA 603-224-1961 603-224-1430 Sumida USA Japan 847-956-0666 81-3-3607-5111 847-956-0702 81-3-3607-5144 USA 714-255-9500 714-255-9400 United Chemi-Con LI2PEAK 2CVO Estimate input and output capacitor values for given voltage ripple as follows: CIN = 1 LI2 2 ∆L VRIPPLE,CINVIN COUT = 1 LI2 2 ∆L VIN VRIPPLE,COUT VOUT VIN − VOUT Diode Selection The MAX1744/MAX1745’s high switching frequency demands a high-speed rectifier. Schottky diodes, such as the 1N5817–1N5822 family or surface-mount equivalents, are recommended. Ultra-high-speed rectifiers with reverse recovery times around 50ns or faster should be used for high output voltages, where the increased forward drop causes less efficiency degradation. Make sure that the diode’s peak current rating exceeds the peak current limit set by RSENSE, and that its breakdown voltage exceeds VIN. Schottky diodes are preferred for heavy loads due to their low forward voltage, especially in low-voltage applications. For high-temperature applications, some Schottky diodes may be inadequate due to their high leakage currents. In such cases, ultra-high-speed rectifiers are recommended, although a Schottky diode with a higher reverse voltage rating can often provide acceptable performance. 12 where I∆L is the change in inductor current. These equations are suitable for initial capacitor selection; final values should be set by testing a prototype or evaluation kit. When using tantalum capacitors, use good soldering practices to prevent excessive heat from damaging the devices and increasing their ESR. Also, ensure that the tantalum capacitors’ surge-current ratings exceed the startup inrush and peak switching currents. Pursuing output ripple lower than the error comparator’s hysteresis (0.6% of the output voltage) is not practical, since the MAX1744/MAX1745 will switch at slower frequencies, increasing inductor ripple current threshold. Choose an output capacitor with a working voltage rating higher than the output voltage. The input filter capacitor reduces peak currents drawn from the power source and reduces noise and voltage ______________________________________________________________________________________ High-Voltage, Step-Down DC-DC Controller in µMAX degrade performance. The current-sense resistor must be placed within 0.2 inches (5mm) of the controller IC, directly between OUT and CS. Place voltage feedback resistors (MAX1745) next to the FB pin (no more than 0.2in) rather than near the output. Place the 0.47µF input bypass capacitor within 0.2in (5mm) of IN. Refer to the MAX1744 Evaluation Kit manual for a twolayer PC board example. Chip Information TRANSISTOR COUNT: 645 Layout Considerations High-frequency switching regulators are sensitive to PC board layout. Poor layout introduces switching noise into the current and voltage feedback signals and may ______________________________________________________________________________________ 13 MAX1744/MAX1745 ripple at IN, caused by the circuit’s switching action. Use a low-ESR capacitor. Two smaller-value low-ESR capacitors can be connected in parallel if necessary. Choose input capacitors with working voltage ratings higher than the maximum input voltage. Place a surface-mount ceramic capacitor very close to IN and GND. This capacitor bypasses the MAX1744/ MAX1745, minimizing the effects of spikes and ringing on the power source (IN). Bypass REF with 0.1µF. This capacitor should be placed within 0.2 inches (5mm) of the IC, next to REF, with a direct trace to GND. MAX1744/MAX1745 High-Voltage, Step-Down DC-DC Controller in µMAX 10LUMAX.EPS Package Information Note: MAX1744/MAX1745 do not feature exposed pads 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. 14 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 © 2001 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.