19-2547; Rev 0; 7/02 Smallest 1A, 1.4MHz Step-Down Regulators Features The MAX1973/MAX1974 are constant-frequency 1.4MHz pulse-width-modulated (PWM) current-mode step-down regulators. The output voltage can be set as low as 0.75V using an external voltage-divider, or it can be set to preset outputs of 1V, 1.5V (MAX1974), 1.8V, or 2.5V (MAX1973) without requiring external resistors. The MAX1973 also includes a voltage-margining feature that offsets the output voltage up or down 4% to facilitate board-level production testing. ♦ Tiny Circuit Footprint of 0.19in2 A fixed 1.4MHz operating frequency ensures operation outside the DSL frequency band, provides fast transient response, and allows the use of small external components. Only 4.7µF input and output ceramic capacitors are needed for 1A applications. Forced PWM operation ensures a constant switching frequency over all load conditions. Output voltage accuracy is ±1% over load, line, and temperature operating ranges. The MAX1973 features voltage margining; the MAX1974 provides a POK output to indicate when the output has reached 90% of its nominal regulation voltage. Both devices are available in small 10-pin µMAX packages. ♦ Built-In ±4% Logic-Controlled Voltage Margining (MAX1973) ♦ Ultra-Low Circuit Height of 1.8mm ♦ 4.7µF Ceramic Input and Output Capacitors ♦ 2.6V to 5.5V Input Voltage ♦ 1A Output Current ♦ 1% Accurate ♦ Preset 1V, 1.5V, 1.8V, 2.5V, or 0.75V to VIN Adjustable Output ♦ Fixed-Frequency PWM Current-Mode Operation ♦ 1.4MHz Switching Frequency, Operate Outside DSL Band ♦ 100% Duty-Cycle Dropout Capability ♦ Small External Components Applications Network Equipment Ordering Information TEMP RANGE PIN-PACKAGE Cellular Base Stations MAX1973EUB PART -40°C to +85°C 10 µMAX DSL and Wireless Modems/Routers MAX1974EUB -40°C to +85°C 10 µMAX Optical Modules Central-Office DSL and Telecom Pin Configurations DSP/ASIC Core and IO supplies TOP VIEW Selector Guide appears at end of data sheet. Typical Operating Circuit OUTPUT 1.25V TO VIN 1A INPUT 2.6V TO 5.5V IN LX FBSEL 1 COMP 2 FB 3 SS GND 10 CTL1 9 IN 8 LX 4 7 PGND 5 6 CTL2 MAX1973 µMAX COMP FB FBSEL 1 FBSEL MAX1973 SS CTL1 CTL2 GND PGND MAX1974 OUTPUT DOWN TO 0.75V VOLTAGE MARGINING ON/OFF 10 ON COMP 2 FB 3 SS 4 7 PGND GND 5 6 POK MAX1974 9 IN 8 LX µMAX ________________________________________________________________ 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 MAX1973/MAX1974 General Description MAX1973/MAX1974 Smallest 1A, 1.4MHz Step-Down Regulators ABSOLUTE MAXIMUM RATINGS IN, POK, CTL1, CTL2, FBSEL, ON to GND ..............-0.3V to +6V COMP, FB, SS to GND ................................-0.3V to (VIN + 0.3V) PGND to GND .......................................................-0.3V to +0.3V LX Current (Note 1) ...............................................-2.4A to +2.4A Continuous Power Dissipation (TA = +70°C) 10-Pin µMAX (derate 5.6mW/°C above +70°C) .......... 444mW Operating Temperature Range ...........................-40°C to +85°C Storage Temperature Range .............................-65°C to +150°C Lead Temperature (soldering, 10s) .................................+300°C Note 1: LX has internal clamp diodes to IN and PGND. Applications that forward bias these diodes should take care not to exceed the IC package power dissipation limit. 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 = VCTL_ = 3.3V, FB = OUT, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER CONDITIONS MIN TYP MAX UNITS 5.5 V 2.35 2.5 V 4.0 7.5 mA 3 5 mA 0.1 10 µA IN IN Voltage Range IN Undervoltage Lockout Threshold 2.6 Rising and falling, hysteresis is 25mV (typ) Supply Current Switching with no load Supply Current in Dropout VOUT set for 3.6V Shutdown Supply Current VIN = 5.5V 2.2 FB Output Voltage Range MAX1973 1.25 VIN MAX1974 0.75 VIN MAX1973 FB Regulation Voltage MAX1974 FBSEL not connected 1.2375 1.25 1.2625 FBSEL = GND 1.7820 1.8 1.8180 FBSEL = IN 2.4750 2.5 2.5250 FBSEL not connected 0.7425 0.75 0.7575 FBSEL = GND 0.99 1.00 1.01 FBSEL = IN 1.485 1.500 1.515 V V FB Regulation Voltage Positive Voltage Margining MAX1973, CTL1 = GND, CTL2 = IN +3 +4 +5 % FB Regulation Voltage Negative Voltage Margining MAX1973, CTL1 = IN, CTL2 = GND -3 -4 -5 % 10 30 70 kΩ -0.1 0.01 +0.1 µA FB Input Resistance to GND in Preset Output Modes FB Input Bias Current FBSEL not connected SS (REFERENCE OUTPUT) SS Voltage 2 MAX1974 0.75 MAX1973 1.25 _______________________________________________________________________________________ V Smallest 1A, 1.4MHz Step-Down Regulators MAX1973/MAX1974 ELECTRICAL CHARACTERISTICS (continued) (VIN = VCTL_ = 3.3V, FB = OUT, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) MIN TYP MAX UNITS SS Source Current PARAMETER CONDITIONS -25 -20 -15 µA SS Sink Current 10 20 35 µA SS to GND Resistance in Shutdown 5 40 100 Ω FBSEL Low Input Threshold 0.3 V High Input Threshold Input Bias Current FBSEL = GND or IN, VIN = 5.5V VIN 0.3 V -20 10 +20 µA Transconductance from FB to COMP 40 60 80 µS COMP to GND Resistance in Shutdown 5 40 100 Ω COMP Clamp Voltage Low 0.6 0.9 1.2 V Clamp Voltage High 1.35 1.75 2.15 V LX On-Resistance High VIN = 3.3V 0.23 0.46 Ω On-Resistance Low VIN = 3.3V 0.16 0.32 Ω Current-Sense Transresistance 0.275 0.335 0.425 V/A Positive Current-Limit Threshold 1.1 1.6 1.75 A Negative Current-Limit Threshold -1.2 -0.8 -0.4 A VLX = VIN = 5.5V LX Shutdown Leakage Current LX = GND, VIN = 5.5V Switching Frequency 20 -20 1.2 1.4 µA 1.6 MHz 1.6 V +1 µA mV CTL1, CTL2 (MAX1973), ON (MAX1974) Logic-Low Input Threshold 0.6 V Logic-High Input Threshold Logic Input Current -1 POK (MAX1974 only) Output Low Voltage Output Valid Threshold for POK POK sinking 1mA Percentage of nominal regulation voltage 10 100 Rising 90 92.5 95 Falling 88 90 92 % _______________________________________________________________________________________ 3 MAX1973/MAX1974 Smallest 1A, 1.4MHz Step-Down Regulators ELECTRICAL CHARACTERISTICS (continued) (VIN = VCTL_ = 3.3V, FB = OUT, TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER CONDITIONS MIN TYP MAX UNITS THERMAL SHUTDOWN Thermal-Shutdown Threshold +170 °C Thermal-Shutdown Hysteresis 20 °C ELECTRICAL CHARACTERISTICS (VIN = VFB = VCTL_ = 3.3V, TA = -40°C to +85°C, unless otherwise noted.) (Note 2) PARAMETER CONDITIONS MIN TYP MAX UNITS 2.6 5.5 V 2.2 2.5 V 7.5 mA IN IN Voltage Range IN Undervoltage Lockout Threshold Rising and falling, hysteresis is 25mV (typ) Supply Current Switching with no load Supply Current in Dropout VOUT set for 3.6V 5 mA Shutdown Supply Current VIN = 5.5V 10 µA FB Output Voltage Range MAX1973 1.25 VIN MAX1974 0.75 VIN FBSEL not connected 1.2375 1.2625 MAX1973 FB Regulation Voltage MAX1974 FBSEL = GND 1.7820 1.8180 FBSEL = IN 2.4750 2.5250 FBSEL not connected 0.7425 0.7575 FBSEL = GND 0.99 1.01 FBSEL = IN 1.485 1.515 V V FB Regulation Voltage Positive Voltage Margining MAX1973, CTL1 = GND, CTL2 = IN 3 5 % FB Regulation Voltage Negative Voltage Margining MAX1973, CTL1 = IN, CTL2 = GND -3 -5 % 10 70 kΩ -0.15 +0.15 µA SS Source Current -25 -15 µA SS Sink Current 10 35 µA FB Input Resistance to GND in Preset Output Modes FB Input Bias Current FBSEL not connected SS (REFERENCE OUTPUT) 4 _______________________________________________________________________________________ Smallest 1A, 1.4MHz Step-Down Regulators MAX1973/MAX1974 ELECTRICAL CHARACTERISTICS (VIN = VCTL_ = 3.3V, FB = OUT, TA = -40°C to +85°C, unless otherwise noted.) (Note 2) PARAMETER CONDITIONS SS to GND Resistance in Shutdown MIN TYP MAX UNITS 5 40 100 Ω FBSEL Low Input Threshold 0.3 V VIN 0.4 V -20 +20 µA Transconductance from FB to COMP 40 80 µS COMP to GND Resistance in Shutdown 5 100 Ω Clamp Voltage Low 0.6 1.2 V Clamp Voltage High 1.3 2.2 V 0.46 Ω High Input Threshold Input Bias Current FBSEL = GND or IN, VIN = 5.5V COMP LX On-Resistance High VIN = 3.3V On-Resistance Low VIN = 3.3V 0.32 Ω Current-Sense Transresistance 0.275 0.425 V/A Positive Current-Limit Threshold 1.10 1.85 A -1.20 -0.35 A Negative Current-Limit Threshold VLX = VIN = 5.5V LX Shutdown Leakage Current LX = GND, VIN = 5.5V Switching Frequency 20 -20 1.2 1.6 µA MHz CTL1, CTL2 (MAX1973), ON (MAX1974) Logic-Low Input Threshold 0.6 Logic-High Input Threshold Logic Input Current -1 V 1.6 V 1 µA 100 mV POK (MAX1974 only) Output Low Voltage POK sinking 1mA Output Valid Threshold for POK Percentage of nominal regulation voltage Rising 90 95 Falling 88 92 % Note 2: Specifications to -40°C are guaranteed by design and not production tested. _______________________________________________________________________________________ 5 Typical Operating Characteristics (Circuits of Figure 2, 3, and 4; TA = +25°C, unless otherwise noted.) EFFICIENCY vs. LOAD CURRENT VOUT = 1.8V 40 70 60 50 40 30 30 20 20 10 0 VIN = 5V 0 0.01 0.1 1 0.1 NO-LOAD CURRENT vs. INPUT VOLTAGE FB VOLTAGE vs. LOAD CURRENT 4 1.250 1.249 1.248 1.247 MAX1973 VIN = 5V R1 = 22kΩ R2 = 13kΩ 1.246 2 1.245 VOUT = 2.5V 0 1 2 3 4 0.2 0.4 0.6 LOAD CURRENT (A) 0.2 0.4 0.6 0.8 1.0 1.0 0.5 VOUT = 1.8V 0 -0.5 2.5V -1.0 1V -1.5 -2.0 -2.5 -3.0 VIN = 5V 0.01 0.1 1 SWITCHING FREQUENCY vs. SUPPLY VOLTAGE MAX1973/74 toc07 1.40 SWITCHING FREQUENCY (MHz) TA = +85°C 1.35 1.30 TA = +25°C 1.25 TA = -40°C 1.20 1.15 1.10 2 3 10 LOAD CURRENT (A) LOAD CURRENT (A) INPUT VOLTAGE (V) 4 5 6 SUPPLY VOLTAGE (V) 6 0.8 -4.0 0 5 0.10 -3.5 1.244 0 VOUT = 3.3V 0 MAX1973/74 toc05 1.251 FB VOLTAGE (V) 6 0.15 CHANGE IN OUTPUT VOLTAGE vs. LOAD CURRENT 1.252 MAX1973/74 toc04 8 0.20 1 LOAD CURRENT (A) 10 VOUT = 2.5V 0.25 0 0.01 LOAD CURRENT (A) 12 0.30 0.05 10 VIN = 3.3V MAX1973/74 toc03 VOUT = 2.5V MAX1973/74 toc06 60 50 80 DROPOUT VOLTAGE (V) VOUT = 1V 70 VOUT = 3.3V 90 EFFICIENCY (%) EFFICIENCY (%) 80 DROPOUT VOLTAGE vs. LOAD CURRENT 0.35 CHANGE IN OUTPUT VOLTAGE (mV) MAX1973/74 toc01 VOUT = 2.5V 90 100 MAX1973/74 toc02 EFFICIENCY vs. LOAD CURRENT 100 NO-LOAD CURRENT (mA) MAX1973/MAX1974 Smallest 1A, 1.4MHz Step-Down Regulators _______________________________________________________________________________________ 100 1000 Smallest 1A, 1.4MHz Step-Down Regulators MAX1974 STARTUP WAVEFORMS MAX1973 HIGH-CURRENT SWITCHING WAVEFORMS MAX1973/74 toc08 MAX1973/74 toc09 10V/div 0 2V/div 0 ON POK 20mV/div VOUT VLX 5V/div 1V/div VOUT 0 0 200mA/div 500mA/div IL IIN 0 0 VIN = 5V, 100kΩ PULLUP RESISTOR POK TO VOUT 500ns/div VIN = 5V, VOUT = 2.5V, IOUT = 800mA MAX1973 LOW-CURRENT SWITCHING WAVEFORMS MAX1973 LOAD TRANSIENT MAX1973/74 toc11 MAX1973/74 toc10 VOUT 20mV/div 20mV/div VOUT 5V/div VLX 0 500mA/div IL 500mA/div IL 0 0 500ns/div VIN = 5V, VOUT = 2.5V, IOUT = 10mA 500ns/div VIN = 5V, VOUT = 2.5V, IOUT = 400mA TO 800mA MAX1973 LOAD TRANSIENT MAX1973 LINE TRANSIENT MAX1973/74 toc13 MAX1973/74 toc12 200mV/div VOUT IL 500mA/div 50mV/div VOUT VIN 2V/div 0 0 20µs/div VIN = 5V, VOUT = 2.5V, IOUT = 600mA TO 800mA 200µs/div VIN = 3.3V TO 5V TO 3.3V, IOUT = 800mA _______________________________________________________________________________________ 7 MAX1973/MAX1974 Typical Operating Characteristics (continued) (Circuits of Figure 2, 3, and 4; TA = +25°C, unless otherwise noted.) MAX1973/MAX1974 Smallest 1A, 1.4MHz Step-Down Regulators Typical Operating Characteristics (continued) (Circuits of Figure 2, 3, and 4; TA = +25°C, unless otherwise noted.) MAX1974 POK AND INPUT VOLTAGE MAX1973 VOLTAGE MARGIN STEP CHANGE RESPONSE MAX1973/74 toc15 MAX1973/74 toc14 VOUT 1V/div VPOK 2V/div VCTL1 0 10V/div 0 10V/div 0 VCTL2 IIN 500mA/div 0 VOUT 1V/div VIN 0 2V/div 0 1ms/div VIN = 5V, VOUT = 2.5V, IOUT = 800mA, -4% TO +4% TO -4% 0 20ms/div MAX1974 WITH 100kΩ PULLUP RESISTOR FROM POK TO IN, ILOAD = 100mA Pin Description PIN FUNCTION MAX1974 1 FBSEL FBSEL Feedback-Mode Selector. Connect FBSEL to GND to set the output voltage to 1.8V (MAX1973) or 1V (MAX1974). Connect FBSEL to IN to set the output voltage to 2.5V (MAX1973) or 1.5V (MAX1974). Leave FBSEL unconnected to set the output voltage using a resistor-divider at FB. 2 COMP COMP Compensation. Connect a series RC network to GND. COMP is internally pulled to GND when the device is in shutdown or in undervoltage lockout (see the Compensation Components section). 3 FB FB Feedback Input. Connect to the output if a preset voltage is used, or to a resistordivider from the output to GND for an adjustable output voltage. SS SS Soft-Start Pin and Reference Output. Bypass to GND with at least 0.01µF. Connect 0.1µF to GND for a soft-start ramp time of 6.25ms for the MAX1973, or 3.75ms for the MAX1974. SS is internally pulled to GND when the device is shut down or in undervoltage lockout. 4 5 GND GND CTL2 — — POK 7 PGND PGND 8 LX LX Inductor Connection. Connect an inductor from LX to the output. 9 IN IN Input Supply Voltage. Input voltage range is 2.6V to 5.5V. Connect a 4.7µF capacitor from IN to PGND. CTL1 — Control Input 1. Controls Enable/Disable and voltage margining (see Table 1). — ON Enable Input. Connect to IN or drive high for normal operation. Drive low to put device in shutdown. 6 10 8 NAME MAX1973 Ground Control Input 2. Controls enable/disable and voltage margining (see Table 1). Power-OK Output. Open-drain output goes low when output is below 90% of nominal output. POK is also low when the device is shut down or in undervoltage lockout. Power Ground _______________________________________________________________________________________ Smallest 1A, 1.4MHz Step-Down Regulators The MAX1973/MAX1974 are 1.4MHz fixed-frequency PWM current-mode step-down DC/DC converters. A high 1.4MHz switching frequency allows use of small inductors and small capacitors for filtering and decoupling. An internal synchronous rectifier improves efficiency and eliminates the need for an external Schottky freewheeling diode. On-chip current sensing uses the on-resistance of the internal MOSFETs, eliminating current-sensing resistors and improving efficiency. The input voltage range is 2.6V to 5.5V. The output voltage is selectable to one of two presets, or adjustable by using a resistor-divider. The output voltage of the MAX1973 is preset to 1.8V or 2.5V by connecting FBSEL to GND or IN, respectively. The MAX1974 is preset to 1.0V or 1.5V by connecting FBSEL to GND or IN, respectively. In adjustable mode (see the Output Voltage Selection section), the output voltage is programmable down to 0.75V on the MAX1974, and down to 1.25V on the MAX1973. PWM Control Scheme The MAX1973/MAX1974 use a fixed-frequency PWM current-mode control scheme. The heart of the PWM current-mode controller is an open-loop comparator that compares the integrated voltage feedback signal against the sum of the amplified current-sense signal and the slope compensation ramp (see Figure 1). At each rising edge of the internal clock, the internal highside MOSFET turns on until the PWM comparator trips. During this on-time, current ramps up through the inductor, sourcing current to the output and storing energy in a magnetic field. The current-mode feedback system regulates the peak inductor current as a function of the output voltage error signal. Because the average inductor current is nearly the same as the peak inductor current (assuming that the inductor value is relatively high to minimize ripple current), the circuit acts as a switch-mode transconductance amplifier. It pushes the output LC filter pole, normally found in a voltage-mode PWM, to a higher frequency. To preserve inner loop stability and eliminate inductor staircasing, an internal slope-compensation ramp is summed into the main PWM comparator. During the second half of the switching cycle (off-time), the internal high-side MOSFET turns off and the internal low-side N-channel MOSFET turns on. The inductor releases the stored energy as its current ramps down while still providing current to the output. The output capacitor stores charge when the inductor current exceeds the load current and discharges when the inductor current is lower, smoothing the voltage across the load. Under overload conditions, when the inductor current exceeds the current limit, the high-side MOSFET is not turned on at the rising edge of the clock, and the low-side MOSFET remains on to let the inductor current ramp down. 100% Duty-Cycle Operation The MAX1973/MAX1974 can operate at 100% duty cycle. In this state, the high-side P-channel MOSFET is turned on (not switching). The dropout voltage in 100% duty-cycle operation is the output current multiplied by the sum of the on-resistance of the P-channel MOSFET (RDS(ON)P) and the inductor resistance (RL). VDROPOUT = IOUT ✕ ( RDS(ON)P + RL ) Current Sense and Current Limit The current-sense circuit amplifies the current-sense voltage generated by the high-side MOSFET’s on-resistance and the inductor current (RDS(ON) ✕ INDUCTOR). This amplified current-sense signal and the internal slope compensation signal are summed together at the PWM comparator’s inverting input. The PWM comparator turns off the internal high-side MOSFET when this sum exceeds the integrated feedback voltage. The internal high-side MOSFET has a current limit of 1.6A (typ). If the current flowing out of LX exceeds this maximum, the high-side MOSFET turns off and the synchronous rectifier MOSFET turns on. This lowers the duty cycle and causes the output voltage to droop until the current limit is no longer exceeded. There is also a synchronous rectifier current limit of -0.85A, to protect the device from current flowing into LX. If this negative current limit is exceeded, the synchronous rectifier turns off, and the inductor current continues to flow through the high-side MOSFET body diode back to the input until the beginning of the next cycle, or until the inductor current drops to zero. Soft-Start To reduce the supply inrush current, soft-start circuitry ramps up the output voltage during startup by charging the SS capacitor with a 20µA current source. When SS reaches its nominal value, the output is in full regulation. The soft-start time (tSS) is determined from: V t SS = SS × CSS ISS where VSS is the soft-start (reference) voltage (1.25V for the MAX1973; 0.75V for the MAX1974), I SS is 20µA, and CSS is the value of the capacitor connected to SS. Soft-start occurs when power is first applied and when the device exits shutdown. The part also goes through _______________________________________________________________________________________ 9 MAX1973/MAX1974 Detailed Description MAX1973/MAX1974 Smallest 1A, 1.4MHz Step-Down Regulators MAX1974 ONLY VOLTAGE CLAMP POK COMP OUT IN PMOS CURRENT SENSE FEEDBACK SELECT FB CTL1 VOLTAGE MARGINING MAX1973 ONLY LX ERROR AMP PWM LOGIC BLOCK CTL2 PWM COMPARATOR ON SOFT-START Σ NMOS CURRENT LIMIT MAX1974 ONLY PGND SS REFERENCE GND 1.4MHz OSCILLATOR SLOPE COMP REFERENCE READY MAX1973 MAX1974 CHIP ENABLE BIAS IN UNDERVOLTAGE LOCK OUT Figure 1. Functional Diagram soft-start when coming out of undervoltage lockout (UVLO) or thermal-overload protection. Undervoltage Lockout (UVLO) If VIN drops below 2.35V (typ), the MAX1973/MAX1974 assume that the supply voltage is too low to provide a valid output voltage, and the UVLO circuit inhibits switching. Once VIN rises above 2.4V, UVLO is disabled and the soft-start sequence begins. 10 Thermal-Overload Protection Thermal-overload protection limits total power dissipation and protects the IC from damage in case of an overload or short-circuit condition. When the IC junction temperature (TJ) exceeds +170°C, the device shuts down. The part turns on again after the junction temperature cools by 20°C. This results in a pulsed output during continuous thermal-overload conditions. ______________________________________________________________________________________ Smallest 1A, 1.4MHz Step-Down Regulators A shutdown feature is included on both the MAX1973 and the MAX1974. Shutdown turns off the IC and reduces the supply current about 0.1µA. For the MAX1974, drive ON high for normal operation, or low for shutdown. For the MAX1973, drive both CTL1 and CTL2 high for normal operation, or drive both low for shutdown. For a simple enable/shutdown function with no voltage margining on the MAX1973, connect CTL1 to CTL2 and drive as one input. Power-OK Output (POK) A power-OK output (POK) is provided on the MAX1974. This is an open-drain output indicating when the output voltage is in regulation. If the output voltage falls below 90% of its nominal value, POK goes low. POK remains low until the output voltage rises to 92.5% of its nominal value. At that point, POK goes high impedance. To use POK as a logic output, connect a 10kΩ to 100kΩ pullup resistor from POK to the power supply of the logic receiving the POK signal. POK continues to function in shutdown or UVLO. Note that a minimum voltage of 1V at IN is required to ensure that POK provides a valid output. When VIN drops to zero, POK is high impedance. See the Typical Operating Characteristics. Applications Information Output Voltage Selection The output voltage can be set to one of two preset values, or can be set by an external resistor-divider. For preset output voltages, connect FB to the output as shown in Figures 2 and 3. Connect FBSEL to GND or IN to select the desired preset output voltage (see Table 2). To set the output voltage to a value other than the preset values, FBSEL is not connected, and FB is connected to a voltage-divider as shown in Figures 4 and 5. Select a value for R2 in the 1kΩ to 22kΩ range, and then calculate the value of R1 from the following equation: V R1 = R2 × OUT − 1 VFB Table 1. CTL_ Input Functions (MAX1973) CTL1 CTL2 GND GND GND IN IN GND IN IN FUNCTION Shutdown Positive voltage margining, regulation voltage increased 4% from normal operation Negative voltage margining, regulation voltage lowered 4% from normal operation Normal operation Table 2. Preset Output Voltages FBSEL OUTPUT VOLTAGE MAX1973 MAX1974 GND 1.8V 1V IN 2.5V 1.5V Not Connected Adjustable down to 1.25V Adjustable down to 0.75V For the MAX1973, VFB = 1.25V, allowing its output to be set down to 1.25V. For the MAX1974, VFB = 0.75V, allowing its output to be set down to 0.75V The MAX1973/MAX1974 PWM circuitry is capable of a stable minimum duty cycle of 17%. This limits the minimum output voltage that can be generated to 0.17 ✕ VIN. Instability may result for VIN/VOUT ratios below 0.17. Inductor Selection A 2.2µH to 4.7µH inductor with a saturation current of at least 1.25A is recommended for full-load (1mA) applications. For lower load currents, the inductor current rating can be reduced. For most applications, use an inductor with a current rating 1.25 times the maximum required output current. For best efficiency, the inductor’s DC resistance should be as small as possible. See Table 3 for recommended inductors and manufacturers. For most designs, the inductor value (L INIT) can be derived from the following equation: LINIT = VOUT (VIN − VOUT ) VIN × LIR × IOUT(MAX) × fSW ______________________________________________________________________________________ 11 MAX1973/MAX1974 Voltage Margining and Shutdown A voltage-margining feature is provided on the MAX1973 to shift the output voltage up or down by 4%. Voltage margining is useful for the automatic testing of systems at high and low supply conditions to find potential failures. See Table 1 for the MAX1973 voltage margining and shutdown truth table. MAX1973/MAX1974 Smallest 1A, 1.4MHz Step-Down Regulators 3.3µH VIN = 2.6V TO 5.5V 4.7µF IN LX COMP FB 1.8V 36kΩ 4.7µF VIN = 2.6V TO 5.5V 4.7µF LX COMP FB RC CC 470pF MAX1973 3.3µH IN MAX1973 CTL1 FBSEL SS CTL2 SS CTL2 GND PGND GND PGND FBSEL 1.25V TO VIN R1 4.7µF CTL1 R2 0.1µF 0.1µF Figure 2. MAX1973 with 1.8V Preset Output VIN = 2.6V TO 5.5V 4.7µF Figure 4. MAX1973 with Adjustable Output Voltage Set by R1 and R2 3.3µH IN LX COMP FB 1.5V 43kΩ 330pF IN 4.7µF VIN = 2.6V TO 5.5V 4.7µF IN FB 0.75V TO VIN R1 4.7µF IN MAX1974 IN ON R2 POK POK GND COMP RC FBSEL 100kΩ SS LX CC ON FBSEL MAX1974 0.1µF 3.3µH IN SS POK 100kΩ 0.1µF PGND GND PGND POK Figure 3. MAX1974 with Preset 1.5V Output Figure 5. MAX1974 with Adjustable Output Voltage Set by R1 and R2 where fSW is the switching frequency (1.4✕106 Hz), and LIR is the inductor ripple current as a percentage of the maximum load current. Keep LIR between 20% and 40% for best compromise of cost, size, and performance. The peak inductor current is approximately: capacitor must meet the ripple current requirement (IRMS) imposed by the switching currents defined by the following equation: LIR IL(PEAK) = 1 + × IOUT(MAX) 2 Input Capacitor A 4.7µF ceramic input capacitor is recommended for most applications because of its low equivalent series resistance (ESR), equivalent series inductance (ESL), and cost. To ensure stability over a wide temperature range, an X5R or X7R dielectric is recommended. The input capacitor reduces peak currents drawn from the power source and reduces noise and voltage ripple on the input caused by the circuit’s switching. The input 12 I IRMS = OUT VOUT (VIN − VOUT ) VIN Choose a capacitor that exhibits less than 10°C temperature rise at the maximum operating RMS current for optimum long-term reliability. Output Capacitor A 4.7µF ceramic output capacitor is recommended for most applications because of its low ESR, ESL, and lower cost. To ensure stability over a wide temperature range, an X5R or X7R dielectric is recommended. Key selection parameters for a ceramic output capacitor are capacitance, ESR, and voltage rating. These affect the overall stability, output ripple voltage, and transient ______________________________________________________________________________________ Smallest 1A, 1.4MHz Step-Down Regulators PART INDUCTANCE (µH) ESR (mΩ) SATURATION CURRENT (A) DIMENSIONS L ✕ W ✕ H (mm) Coilcraft LPO1704-32M 3.3 160 1.3 5.5 ✕ 6.6 ✕ 1 Sumida CDRD3D16-R3 3.3 85 1.1 4 ✕ 4 ✕ 1.8 Toko A682AY-3R3M 3.3 134 0.97 4.4 ✕ 4.4 ✕ 3.1 MANUFACTURER response of the DC-DC converter. With ceramic capacitors, the voltage ripple from ESL is negligible. Output ripple is generated by variations in the charge stored in the output capacitance, and the voltage drop across the capacitor ESR. VRIPPLE = VRIPPLE(C) + VRIPPLE(ESR) The output voltage ripple due to the output capacitance is: IP−P VRIPPLE(C) = 8 × COUT × fSW The output voltage ripple due to capacitor ESR is: VRIPPLE(ESR) = IP−P × ESR IP-P is the peak-to-peak inductor current: V −V V IP−P = IN OUT × OUT fSW × L VIN These equations are suitable for initial capacitor selection, but final values should be set by testing a prototype or evaluation circuit. As a rule, a smaller ripple current results in less output voltage ripple. Because the inductor ripple current is inversely proportional to inductor value, output voltage ripple decreases with larger inductance. Load transient response depends on the selected output capacitor. During a load transient, the output voltage instantly changes by ESR ✕ ∆ILOAD. Before the controller can respond, the output deviates further, depending on the inductor and output capacitor values. After a short time (see the Typical Operating Characteristics), the controller responds by regulating the output voltage back to its nominal state. The controller response time depends on the closed-loop bandwidth. With a higher bandwidth the response time is faster. However, to maintain stable operation, the bandwidth should not be set above fSW/10. Compensation Components An internal transconductance error amplifier compensates the control loop. Connect a series resistor and capacitor between COMP and GND to form a pole-zero pair. The external inductor, output capacitor, compensation resistor, and compensation capacitor determine the loop bandwidth and stability. The inductor and output capacitor are chosen based on performance, size, and cost. Additionally, the compensation resistor and capacitor are selected to optimize the control loop. Table 4 and Table 5 list typical component values. The rest of this section is a more detailed discussion on calculating compensation components. The controller uses a current-mode control scheme that regulates the output voltage by forcing the required current through the external inductor. The voltage across the internal high-side MOSFET’s on-resistance is used to sense inductor current. Current-mode control eliminates the double pole caused by the inductor and output capacitor found in other control schemes. Simple Type 1 compensation with a single resistor (RC) and capacitor (CC) is all that is needed to provide a stable and high-bandwidth loop. Use the formula below to calculate the value of C C, then use the nearest standard value: CC = 1 1 VFB × × gm × 0.5 × IOUT(MAX) RCS 2π × fC where VFB is 1.25V for the MAX1973 and 0.75V for the MAX1974, the current-sense transresistance (RCS) is 0.26Ω (typ), and the transconductance from FB to COMP (g m ) is 50µS (typ). For best stability and response performance, the closed-loop unity-gain frequency (fC) should be approximately 140kHz (onetenth the switching frequency). Use the following equation to calculate RC: RC = COUT CC × VOUT 0.5 × IOUT(MAX) Below is a numerical example of calculating compensation values for a circuit using the MAX1973 with 2.5V output and maximum output current of 1A: MAX1973 ______________________________________________________________________________________ 13 MAX1973/MAX1974 Table 3. Recommended Inductors MAX1973/MAX1974 Smallest 1A, 1.4MHz Step-Down Regulators VOUT = 2.5V IOUT(MAX) = 1A COUT = 4.7µF PC Board Layout A properly designed PC board layout is important in any switching regulator. The switching power stage requires particular attention. Follow these guidelines for good PC board layout: 1) Place decoupling capacitors as close to IC pins as possible. Keep the power ground plane (connected to PGND) and signal ground plane (connected to GND) separate. Connect the two ground planes with a single connection from PGND to GND. 2) Input and output capacitors are connected to the power ground plane; all other capacitors are connected to the signal ground plane. 3) Keep the high-current paths as short and wide as possible. VFB = 1.25V RCS = 0.26Ω gm = 50µS fC = 140kHz CC = = VFB 0.5 × IOUT(MAX) 1.25 × 1 1 × gm × RCS 1 2π × fC 1 × × 50 × 10−6 × = 547pF 0.5 × 1 0.26 2π × 140000 4) If possible, connect IN, LX, and PGND separately to a large land area to help cool the IC to further improve efficiency and long-term reliability. 5) Ensure all feedback connections are short and direct. Place feedback resistors (if used) as close to the IC as possible. 6) Route high-speed switching nodes (LX) away from sensitive analog areas (FB, COMP, SS). Select the nearest standard value: CC = 560pF RC = COUT CC 4.7 × 10−6 560 × 10 −12 × × VOUT 0.5 × IOUT(MAX) 2.5 0.5 × 1 = = 41.9kΩ Select the nearest standard value: RC = 43kΩ Table 4. Recommended Components for the MAX1973 VOUT (V) CIN (µF) COUT (µF) CC (pF) RC (kΩ) 2.5 4.7 4.7 560 43 1.8 4.7 4.7 560 30 Table 5. Recommended Components for the MAX1974 VOUT (V) CIN (µF) COUT (µF) CC (pF) RC (kΩ) 1.5 4.7 4.7 330 43 1.0 4.7 4.7 330 27 Selector Guide PART FEATURES OUTPUT PRESET MAX1973EUB Voltage Margining 1.8V or 2.5V MAX1974EUB Power-OK Output 1V or 1.5V 14 Chip Information TRANSISTOR COUNT: 1998 PROCESS: BiCMOS ______________________________________________________________________________________ Smallest 1A, 1.4MHz Step-Down Regulators 10LUMAX.EPS e 4X S 10 INCHES 10 H ÿ 0.50±0.1 0.6±0.1 1 1 0.6±0.1 BOTTOM VIEW TOP VIEW D2 MILLIMETERS MAX DIM MIN 0.043 A 0.006 0.002 A1 A2 0.030 0.037 0.116 0.120 D1 0.114 0.118 D2 0.116 E1 0.120 E2 0.114 0.118 H 0.187 0.199 L 0.0157 0.0275 L1 0.037 REF b 0.007 0.0106 e 0.0197 BSC c 0.0035 0.0078 0.0196 REF S α 0∞ 6∞ MAX MIN 1.10 0.05 0.15 0.75 0.95 3.05 2.95 3.00 2.89 3.05 2.95 2.89 3.00 4.75 5.05 0.40 0.70 0.940 REF 0.177 0.270 0.500 BSC 0.090 0.200 0.498 REF 0∞ 6∞ E2 GAGE PLANE A2 c A b D1 A1 α E1 L L1 FRONT VIEW SIDE VIEW PROPRIETARY INFORMATION TITLE: PACKAGE OUTLINE, 10L uMAX/uSOP APPROVAL DOCUMENT CONTROL NO. 21-0061 REV. I 1 1 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. Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 15 © 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products. MAX1973/MAX1974 Package Information (The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information, go to www.maxim-ic.com/packages.)