19-4689; Rev 0; 7/09 KIT ATION EVALU E L B A IL AVA 1MHz, 2A, 2.6V to 5.5V Input, PWM DC-DC Step-Down Regulator with Enable Features The MAX1951A high-efficiency, DC-DC step-down switching regulator delivers up to 2A of output current. The device operates from an input voltage range of 2.6V to 5.5V and provides an adjustable output voltage from 0.8V to VIN, making the MAX1951A ideal for on-board postregulation applications. The MAX1951A total output error is less than ±1.5% over load, line, and temperature. The MAX1951A operates at a fixed frequency of 1MHz with an efficiency of up to 94%. The high operating frequency minimizes the size of external components. Internal soft-start control circuitry reduces inrush current. Short-circuit and thermal-overload protection improve design reliability. The MAX1951A can start up safely with a prebiased or without a preexisting output. This feature simplifies tracking supply designs for core and I/O applications and redundant supply designs. o Compact 0.385in2 Circuit Footprint The MAX1951A is available in an 8-pin SO package and operates over the -40°C to +85°C extended temperature range. o Enable Input Audio Shutdown for Reducing Power Consumption Applications o 10µF Ceramic Input and Output Capacitors, 2µH Inductor for 2A Output o Efficiency Up to 94% o 1.5% Output Accuracy Over Load, Line, and Temperature o Guaranteed 2A Output Current o Operates from 2.6V to 5.5V Supply o Adjustable Output from 0.8V to VIN o Internal Digital Soft-Soft o Short-Circuit and Thermal-Overload Protection o 1MHz Switching Frequency Reduces Component Size o Safe Startup into Prebiased Output Ordering Information ASIC/DSP/µP/FPGA Core and I/O Voltages Set-Top Boxes PART Networking and Telecommunications Servers MAX1951AESA+ TEMP RANGE PIN-PACKAGE -40°C to +85°C 8 SO +Denotes a lead(Pb)-free/RoHS-compliant package. TVs Pin Configuration Typical Operating Circuit OUTPUT 0.8V TO VIN, UP TO 2A INPUT 2.6V TO 5.5V TOP VIEW IN LX + VCC 1 EN 2 GND 3 8 MAX1951A IN 7 LX 6 PGND VCC FB COMP EN MAX1951A FB 4 5 COMP ON PGND GND OFF SO ________________________________________________________________ Maxim Integrated Products For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com. 1 MAX1951A General Description MAX1951A 1MHz, 2A, 2.6V to 5.5V Input, PWM DC-DC Step-Down Regulator with Enable ABSOLUTE MAXIMUM RATINGS IN, VCC to GND ........................................................-0.3V to +6V COMP, FB, EN to GND...............................-0.3V to (VCC + 0.3V) LX Current (Note 1).............................................................±4.5A PGND to GND..............................................Internally connected Continuous Power Dissipation (TA = +70°C) 8-Pin SO (derate 12.2mW/°C above +70°C).................976mW Junction-to-Case Thermal Resistance (θJC) (Note 2) 8-Pin SO ........................................................................32°C/W Operating Temperature Range ...........................-40°C to +85°C Junction Temperature Range ............................-40°C to +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. Note 1: LX has internal clamp diodes to PGND and IN. Applications that forward bias these diodes should take care not to exceed the IC’s package power dissipation limits. Note 2: Package thermal resistances were obtained using the method described in JEDEC specification JESD51-7, using a fourlayer board. For detailed information on package thermal considerations, refer to www.maxim-ic.com/thermal-tutorial. ELECTRICAL CHARACTERISTICS (VIN = VCC = VEN = 3.3V, VPGND = VGND = 0V, FB in regulation, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note 3) PARAMETER CONDITIONS MIN TYP MAX UNITS IN AND VCC IN Voltage Range 2.6 Supply Current Switching with no load, LX floating Shutdown Current EN = GND VCC Undervoltage Lockout Threshold When LX starts/stops switching VIN = 5.5V VCC rising VCC falling 5.5 V 7 10 mA 0.1 0.4 mA 2.19 2.32 1.92 2.07 V COMP COMP Transconductance From FB to COMP, VCOMP = 0.8V 40 50 80 COMP Clamp Voltage, Low VIN = 2.6V to 5.5V, VFB = 0.9V 0.6 1 1.45 µS V COMP Clamp Voltage, High VIN = 2.6V to 5.5V, VFB = 0.7V 1.97 2.13 2.28 V Output Voltage Range When using external feedback resistors to drive FB 0.8 VIN V FB Regulation Voltage (Error Amplifier Only) IOUT = 0A to 1.5A, VIN = 2.6V to 5.5V FB Input Bias Current PNP input stage FB TA = 0°C to +85°C 0.789 TA = -40°C to +85°C 0.786 0.796 0.804 0.804 -0.1 +0.1 µA 266 mΩ 246 mΩ V LX LX On-Resistance, PMOS LX On-Resistance, NMOS 2 ILX = -180mA ILX = 180mA VIN = 5V 119 VIN = 3.3V 145 VIN = 2.6V 171 VIN = 5V 122 VIN = 3.3V 133 VIN = 2.6V 142 _______________________________________________________________________________________ 1MHz, 2A, 2.6V to 5.5V Input, PWM DC-DC Step-Down Regulator with Enable (VIN = VCC = VEN = 3.3V, VPGND = VGND = 0V, FB in regulation, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note 3) PARAMETER CONDITIONS MIN TYP MAX UNITS 0.16 0.24 0.35 Ω 2.2 3.1 4.5 LX Current-Sense Transimpedance From LX to COMP, VIN = 2.6V to 5.5V LX Current-Limit Threshold Duty = 100%, VIN = 2.6V to 5.5V LX Leakage Current VIN = 5.5V LX Switching Frequency VIN = 2.6V to 5.5V 0.8 LX Maximum Duty Cycle VCOMP = 1.5V, LX = Hi-Z, VIN = 2.6V to 5.5V 100 LX Minimum Duty Cycle VCOMP = 1V, IN = 2.6V to 5.5V High side Low side A -0.3 VLX = 5.5V 10 VLX = 0V µA -10 0.96 1.1 MHz % 15 % THERMAL Thermal Shutdown Threshold When LX starts/stops switching TJ rising 165 TJ falling 155 °C EN Enable Low Threshold (VIL) 0.8 V Enable High Threshold (VIH) EN Input Current 2.0 V 1 µA Note 3: Specifications to TA = -40°C are guaranteed by design and not production tested. Typical Operating Characteristics (Typical values are at VIN = VCC = 5V, VOUT = 1.5V, IOUT = 1.5A, and TA = +25°C, unless otherwise noted. See Figure 2.) EFFICIENCY vs. OUTPUT CURRENT (VCC = VIN = 5V) VOUT = 3.3V VOUT = 2.5V 60 VOUT = 1.5V 50 40 VOUT = 1.8V VOUT = 1.5V 60 50 40 30 30 20 20 10 10 0 VOUT = 2.5V 70 VOUT = 1.0V 0.4 0.8 1.2 OUTPUT CURRENT (A) 1.6 2.0 1025 1000 975 950 925 900 0 0 1050 MAX1951A toc03 80 EFFICIENCY (%) 70 90 SWITCHING FREQUENCY (kHz) 80 MAX1951A toc02 90 EFFICIENCY (%) 100 MAX1951A toc01 100 SWITCHING FREQUENCY vs. INPUT VOLTAGE EFFICIENCY vs. OUTPUT CURRENT (VCC = VIN = 3.3V) 0 0.4 0.8 1.2 OUTPUT CURRENT (A) 1.6 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 INPUT VOLTAGE (V) _______________________________________________________________________________________ 3 MAX1951A ELECTRICAL CHARACTERISTICS (continued) Typical Operating Characteristics (continued) (Typical values are at VIN = VCC = 5V, VOUT = 1.5V, IOUT = 1.5A, and TA = +25°C, unless otherwise noted. See Figure 2.) LOAD TRANSIENT (90% TRANSIENT) LOAD TRANSIENT (50% TRANSIENT) LOAD REGULATION 0.40 0.30 MAX1951A toc06 MAX1951A toc05 MAX1951A toc04 0.50 OUTPUT VOLTAGE DEVIATION (%) MAX1951A 1MHz, 2A, 2.6V to 5.5V Input, PWM DC-DC Step-Down Regulator with Enable IOUT 1A/div IOUT 1A/div VOUT (AC-COUPLED) 500mV/div VOUT (AC-COUPLED) 200mV/div VOUT = 2.5V 0.20 0.10 0 -0.10 -0.20 VOUT = 1.8V VOUT = 1.5V -0.30 VOUT = 2.5V -0.40 -0.50 0 0.4 0.8 1.2 1.6 2.0 40Fs/div 40Fs/div OUTPUT CURRENT (A) SOFT-START WAVEFORMS (VIN = 3.3V, VOUT = 1.8V) SWITCHING WAVEFORMS (VIN = 3.3V, VOUT = 1.8V, RL = 1I) MAX1951A toc08 MAX1951A toc07 ILX 500mA/div EN 2V/div LX 2V/div VOUT 1V/div VOUT (AC-COUPLED) 10mV/div 400ns/div 1ms/div SOFT-START WAVEFORMS (VIN = 3.3V, VOUT = 0.8V) STARTUP INTO PREBIASED OUTPUT MAX1951A toc09 MAX1951A toc10 EN 5V/div EN 2V/div LX 5V/div VOUT = 2.5V VOUT 500mV/div 1ms/div 4 VOUT = 1.5V 1ms/div _______________________________________________________________________________________ VOUT 2V/div 1MHz, 2A, 2.6V to 5.5V Input, PWM DC-DC Step-Down Regulator with Enable SHUTDOWN WAVEFORMS (VIN = 3.3V, VOUT = 2.5V, RL = 1.5I) MAX1951A toc11 SUPPLY CURRENT vs. INPUT VOLTAGE MAX1951A toc12 8 7 EN 5V/div SUPPLY CURRENT (mA) EN 2V/div LX 5V/div VOUT = 3.3V VOUT = 2.5V MAX1951A toc13 STARTUP INTO PREBIASED OUTPUT LX 2V/div VOUT 2V/div 6 5 4 3 2 VOUT 2V/div 1 0 1ms/div 2.5 20Fs/div 3.0 3.5 4.0 4.5 5.0 5.5 INPUT VOLTAGE (V) FEEDBACK VOLTAGE vs. TEMPERATURE CASE TEMPERATURE vs. AMBIENT TEMPERATURE 160 140 CASE TEMPERATURE (NC) 803 801 799 797 MAX1951A toc15 180 MAX1951A toc14 FEEDBACK VOLTAGE (mV) 805 120 100 80 60 40 20 0 -20 795 -40 -40 -15 10 35 TEMPERATURE (NC) 60 85 -40 -15 10 35 60 85 AMBIENT TEMPERATURE (NC) _______________________________________________________________________________________ 5 MAX1951A Typical Operating Characteristics (continued) (Typical values are at VIN = VCC = 5V, VOUT = 1.5V, IOUT = 1.5A, and TA = +25°C, unless otherwise noted. See Figure 2.) MAX1951A 1MHz, 2A, 2.6V to 5.5V Input, PWM DC-DC Step-Down Regulator with Enable Pin Description PIN NAME FUNCTION 1 VCC Supply Voltage. Bypass with a 0.1µF capacitor to ground and a 10Ω resistor to IN. 2 EN Enable Input. Connect to VCC for normal operation. Connect to GND to disable the MAX1951A. 3 GND 4 FB Feedback Input. Connect an external resistordivider from the output to FB and GND to set the output to a voltage between 0.8V and VIN. 5 COMP Regulator Compensation. Connect series RC network to GND. 6 Power Ground. Internally connected to GND. PGND Keep power ground and signal ground planes separate. 7 8 Signal Ground LX Inductor Connection. Connect an inductor between LX and the regulator output. IN Power-Supply Voltage. Input voltage range from 2.6V to 5.5V. Bypass with a 10µF (min) ceramic capacitor to GND and a 10Ω resistor to VCC. Detailed Description The MAX1951A high-efficiency switching regulator is a small, simple, current-mode DC-DC step-down converter capable of delivering up to 2A of output current. The device operates in pulse-width modulation (PWM) at a fixed frequency of 1MHz from a 2.6V to 5.5V input voltage and provides an output voltage from 0.8V to VIN, making the MAX1951A ideal for on-board postregulation applications. The high switching frequency allows for the use of smaller external components, and an internal synchronous rectifier improves efficiency and eliminates the typical Schottky free-wheeling diode. Using the on-resistance of the internal high-side MOSFET to sense switching currents eliminates currentsense resistors, further improving efficiency and cost. The MAX1951A total output error over load, line, and temperature (-40°C to +85°C) is less than 1.5%. Controller Block Function The MAX1951A step-down converter uses a PWM current-mode control scheme. An open-loop comparator compares the integrated voltage-feedback signal against the sum of the amplified current-sense signal and the slope compensation ramp. At each rising edge of the 6 internal clock, the internal high-side 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 the inductor. The currentmode feedback system regulates the peak inductor current as a function of the output-voltage error signal. Since the average inductor current is nearly the same as the peak inductor current (< 30% ripple current), the circuit acts as a switch-mode transconductance amplifier. To preserve inner-loop stability and eliminate inductor staircasing, a slope-compensation ramp is summed into the main PWM comparator. During the second half of the cycle, the internal high-side p-channel 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 (see the Current Limit section), the high-side MOSFET does not turn on at the rising edge of the clock and the low-side MOSFET remains on to let the inductor current ramp down. Current Sense An internal current-sense amplifier produces a current signal proportional to the voltage generated by the high-side MOSFET on-resistance and the inductor current (RDS(ON) x ILX). The amplified current-sense signal and the internal slope compensation signal are summed together into the comparator’s inverting input. The PWM comparator turns off the internal high-side MOSFET when this sum exceeds the output from the voltage-error amplifier. Current Limit The internal high-side MOSFET has a current limit of 3.1A (typ). If the current flowing out of LX exceeds this limit, the high-side MOSFET turns off and the synchronous rectifier turns on. This lowers the duty cycle and causes the output voltage to droop until the current limit is no longer exceeded. A synchronous rectifier current limit of -0.6A (typ) protects the device from current flowing into LX. If the negative current limit is exceeded, the synchronous rectifier turns off, forcing the inductor current 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. The MAX1951A utilizes a pulse-skip mode to prevent overheating during short-circuit output conditions. The device enters pulse-skip mode when the FB voltage drops below 300mV, limiting the current to 3A (typ) and reducing power dissipation. Normal operation resumes upon removal of the short-circuit condition. _______________________________________________________________________________________ 1MHz, 2A, 2.6V to 5.5V Input, PWM DC-DC Step-Down Regulator with Enable MAX1951A POSITIVE AND NEGATIVE CURRENT LIMITS VCC IN OSC CLOCK CURRENT SENSE PWM CONTROL RAMP GEN SLOPE COMP LX PREBIAS CLAMP ERROR SIGNAL THERMAL SHUTDOWN PGND ZEROCROSSING DETECTOR FB gm COMP SOFT-START/ UVLO EN DAC BANDGAP REF 1.25V MAX1951A GND Figure 1. Functional Diagram VCC Decoupling Due to the high switching frequency and tight output tolerance (1.5%), decouple VCC with a 0.1µF capacitor connected from VCC to GND, and a 10Ω resistor connected from VCC to IN. Place the capacitor as close as possible to VCC. Soft-Start The MAX1951A employs digital soft-start circuitry to reduce supply inrush current during startup conditions. When the device exits undervoltage lockout (UVLO) shutdown mode, or restarts following a thermal-overload event, or EN is driven high, the digital soft-start circuitry slowly ramps up the voltage to the error-amplifier noninverting input. Undervoltage Lockout If VCC drops below 2.07V, the UVLO circuit inhibits switching. Once V CC rises above 2.19V, the UVLO clear and the soft-start sequence activates. Shutdown Mode Use the enable input, EN, to turn on or off the MAX1951A. Connect EN to VCC for normal operation. Connect EN to GND to place the device in shutdown. Shutdown causes the internal switches to stop switching and forces LX into a high-impedance state. In shutdown, the MAX1951A draws 500µA of supply current. The device initiates a soft-start sequence when brought out of shutdown. Thermal-Overload Protection Thermal-overload protection limits total power dissipation in the device. When the junction temperature exceeds TJ = +165°C, a thermal sensor forces the device into shutdown, allowing the die to cool. The thermal sensor turns the device on again after the junction temperature cools by 9°C, resulting in a pulsed output during continuous overload conditions. Following a thermal-shutdown condition, the soft-start sequence begins. Safe Startup into Prebiased Output The MAX1951A can start up safely even with a prebiased output. A zero crossover detection (ZCD) circuit turns on the switches only after the soft-start ramping voltage equals the prebiased output voltage. If the prebiased output voltage is greater than the set voltage, the ZCD circuit turns on the low-side switch (after the soft-start period is over) to discharge the output capacitor until its voltage equals the set voltage. _______________________________________________________________________________________ 7 MAX1951A 1MHz, 2A, 2.6V to 5.5V Input, PWM DC-DC Step-Down Regulator with Enable Design Procedure Adjustable Output Voltage The MAX1951A provides an adjustable output voltage between 0.8V and VIN. Connect FB to output for 0.8V output. To set the output voltage of the MAX1951A to a voltage greater than VFB (0.8V typ), connect the output to FB and GND using a resistive divider, as shown in Figure 2. Choose R2 between 2kΩ and 20kΩ, and set R3 according to the following equation: R3 = R2 x [(VOUT/VFB) - 1] The MAX1951A PWM circuitry is capable of a stable minimum duty cycle of 18%. This limits the minimum output voltage that can be generated to 0.18 VIN with an absolute minimum of 0.8V. Instability may result for VIN/VOUT ratios below 0.18. Output Inductor Design Use a 2µH inductor with a minimum 2A-rated DC current for most applications. For best efficiency, use an inductor with a DC resistance of less than 20mΩ and a saturation current greater than 3A (min). See Table 2 for recommended inductors and manufacturers. For most designs, derive a reasonable inductor value (LINIT) from the following equation: LINIT = VOUT x (VIN - VOUT)/(VIN x LIR x IOUT(MAX) x fSW) where fSW is the switching frequency (1MHz typ) of the oscillator. Keep the inductor current ripple percentage LIR between 20% and 40% of the maximum load current for the best compromise of cost, size, and performance. Calculate the maximum inductor current as: IL(MAX) = (1 + LIR/2) x IOUT(MAX) Check the final values of the inductor with the output ripple voltage requirement. The output ripple voltage is given by: VRIPPLE = VOUT x (VIN - VOUT) x ESR/(VIN x LFINAL x fSW) where ESR is the equivalent series resistance of the output capacitors. Input Capacitor Design The input filter 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 capacitor must meet the ripple current requirement (IRMS) imposed by the switching currents defined by the following equation: IRMS = (1 VIN ) × (IOUT 2 × VOUT × (VIN − VOUT )) For duty ratios less than 0.5, the input capacitor RMS current is higher than the calculated current. Therefore, 8 use a +20% margin when calculating the RMS current at lower duty cycles. Use ceramic capacitors for their low ESR and equivalent series inductance (ESL). Choose a capacitor that exhibits less than 10°C temperature rise at the maximum operating RMS current for optimum long-term reliability. After determining the input capacitor, check the input ripple voltage due to capacitor discharge when the high-side MOSFET turns on. Calculate the input ripple voltage as follows: VIN_RIPPLE = (IOUT x VOUT)/(fSW x VIN x CIN) Keep the input ripple voltage less than 3% of the input voltage. Output Capacitor Design The key selection parameters for the output capacitor are capacitance, ESR, ESL, and the voltage rating requirements. These affect the overall stability, output ripple voltage, and transient response of the DC-DC converter. The output ripple occurs due to variations in the charge stored in the output capacitor, the voltage drop due to the capacitor’s ESR, and the voltage drop due to the capacitor’s ESL. Calculate the output voltage ripple due to the output capacitance, ESR, and ESL as: VRIPPLE = VRIPPLE(C) + VRIPPLE(ESR) + VRIPPLE(ESL) where the output ripple due to output capacitance, ESR, and ESL is: VRIPPLE(C) = IP-P/(8 x COUT x fSW) VRIPPLE(ESR) = IP-P x ESR VRIPPLE(ESL) = (IP-P/tON) x ESL or (IP-P/tOFF) x ESL, whichever is greater and IP-P the peak-to-peak inductor current is: IP-P = [(VIN – VOUT )/fSW x L)] x VOUT/VIN Use these equations for initial capacitor selection, but determine final values by testing a prototype or evaluation circuit. As a rule, a smaller ripple current results in less output-voltage ripple. Since the inductor ripple current is a factor of the inductor value, the outputvoltage ripple decreases with larger inductance. Use ceramic capacitors for their low ESR and ESL at the switching frequency of the converter. The low ESL of ceramic capacitors makes ripple voltages negligible. Load-transient response depends on the selected output capacitor. During a load transient, the output instantly changes by ESR x ∆ILOAD. Before the controller can respond, the output deviates further, depending on the inductor and output capacitor values. After a short time (see the Load Transient graph in the Typical Operating Characteristics), the controller responds by regulating the output voltage back to its _______________________________________________________________________________________ 1MHz, 2A, 2.6V to 5.5V Input, PWM DC-DC Step-Down Regulator with Enable Compensation Design The double pole formed by the inductor and output capacitor of most voltage-mode controllers introduces a large phase shift that requires an elaborate compensation network to stabilize the control loop. The MAX1951A utilizes a current-mode control scheme that regulates the output voltage by forcing the required current through the external inductor, eliminating the double pole caused by the inductor and output capacitor, and greatly simplifying the compensation network. A simple type 1 compensation with single compensation resistor (R1) and compensation capacitor (C2) in Figure 2 creates a stable and high-bandwidth loop. An internal transconductance error amplifier compensates the control loop. Connect a series resistor and capacitor between COMP (the output of the error amplifier) and GND to form a pole-zero pair. The external inductor, internal current-sensing circuitry, output capacitor, and the external compensation circuit determine the loop system stability. Choose the inductor and output capacitor based on performance, size, and cost. Additionally, select the compensation resistor and capacitor to optimize control-loop stability. The component values shown in the typical application circuit (Figure 2) yield stable operation over a broad range of input-to-output voltages. The basic regulator loop consists of a power modulator, an output feedback divider, and an error amplifier. The power modulator has DC gain set by gmc x RLOAD, with a pole-zero pair set by RLOAD, the output capacitor (COUT), and its ESR. The following equations define the power modulator: Modulator gain: GMOD = ∆VOUT/∆VCOMP = gmc x RLOAD Modulator pole frequency: fpMOD = 1/(2 x π x COUT x (RLOAD + ESR)) Modulator zero frequency: fzESR = 1/(2 x π x COUT x ESR) where RLOAD = VOUT/IOUT(MAX) and gmc = 4.2S. The feedback divider has a gain of GFB = VFB/VOUT, where VFB is equal to 0.8V. The transconductance error amplifier has a DC gain, GEA(DC), of 70dB. The compensation capacitor, C2, and the output resistance of the error amplifier, R OEA (20MΩ), set the dominant pole. C2 and R1 set a compensation zero. Calculate the dominant pole frequency as: fpEA = 1/(2π x C2 x ROEA) Determine the compensation zero frequency as: fzEA = 1/(2π x C2 x R1) For best stability and response performance, set the closed-loop unity-gain frequency much higher than the modulator pole frequency. In addition, set the closedloop crossover unity-gain frequency less than, or equal to 1/5 of the switching frequency. However, set the maximum zero crossing frequency to less than 1/3 of the zero frequency set by the output capacitance and its ESR when using POSCAP, SPCAP, OSCON, or other electrolytic capacitors. The loop-gain equation at the unity-gain frequency is: GEA(fc) x GMOD(fc) x VFB/VOUT = 1 where GEA(fc) = gmEA x R1, and GMOD(fc) = gmc x RLOAD x fpMOD/fC, where gmEA = 60µS. R1 calculated as: R1 = VOUT x K/(gmEA x VFB x GMOD(fc)) where K is the correction factor due to the extra phase introduced by the current loop at high frequencies (>100kHz). K is related to the value of the output capacitance (see Table 1 for values of K vs. C). Set the error-amplifier compensation zero formed by R1 and C2 at the modulator pole frequency at maximum load. C2 is calculated as follows: C2 = (2 x VOUT x COUT/(R1 x IOUT(MAX)) As the load current decreases, the modulator pole also decreases; however, the modulator gain increases accordingly, resulting in a constant closed-loop unitygain frequency. Use the following numerical example to calculate R1 and C2 values of the typical application circuit of Figure 2. VOUT = 1.5V IOUT(MAX) = 2A Table 1. K Value DESCRIPTION COUT (µF) 10 22 K Values are for output inductance from 1.2µH 0.55 to 2.2µH. Do not use output inductors larger 0.47 than 2.2µH. Use fC = 200kHz to calculate R1. COUT = 10µF RESR = 0.010Ω gmEA = 60µS gmc = 4.2S fSWITCH = 1MHz _______________________________________________________________________________________ 9 MAX1951A nominal state. The controller response time depends on the closed-loop bandwidth. A higher bandwidth yields a faster response time, thus preventing the output from deviating further from its regulating value. MAX1951A 1MHz, 2A, 2.6V to 5.5V Input, PWM DC-DC Step-Down Regulator with Enable RLOAD = VOUT/IOUT(MAX) = 1.5V/2A = 0.75Ω fpMOD = [1/(2π x COUT x (RLOAD + RESR)] = [1/(2 x π x10 x10-6 x (0.75 + 0.01)] = 20.9Hz. fzESR = [1/(2π x COUT x RESR)] = [1/(2 x π x 10 x10-6 x 0.01)] = 1.59MHz. For a 2µH output inductor, pick the closed-loop unitygain crossover frequency (fC) at 200kHz. Determine the power modulator gain at fC: GMOD(fc) = gmc x RLOAD x fpMOD/fC = 4.2 x 0.75 x 20.9kHz/200kHz = 0.33 then: R1 = VO x K/(gmEA x VFB x GMOD(fC)) = (1.5 x 0.55)/(60 x 10-6 x 0.8 x 0.33) ≈ 52.3kΩ (1%) C2 = (2 x VOUT x COUT)/R1 x IOUT(MAX) = (2 x 1.5 x 10 x 10-6)/(52.3kΩ x 2) ≈ 143pF, choose 150pF, 10% Applications Information PCB Layout Considerations Careful PCB layout is critical to achieve clean and stable operation. The switching power stage requires particular attention. Follow these guidelines for good PCB layout: 1) Place decoupling capacitors as close as possible to the IC. Keep the power ground plane (connected to PGND) and signal ground plane (connected to GND) separate. 10 2) Connect input and output capacitors to the power ground plane; connect all other capacitors to the signal ground plane. 3) Keep the high-current paths as short and wide as possible. Keep the path of switching current (C1 to IN and C1 to PGND) short. Avoid vias in the switching paths. 4) If possible, connect IN, LX, and PGND separately to a large copper area to help cool the IC to further improve efficiency and long-term reliability. 5) Ensure all feedback connections are short and direct. Place the feedback resistors as close as possible to the IC. 6) Route high-speed switching nodes away from sensitive analog areas (FB, COMP). Thermal Considerations See the MAX1951A Evaluation Kit for an optimized layout example. Thermal performance can be further improved with one of the following options: 1) Increase the copper areas connected to GND, LX, and IN. 2) Provide thermal vias next to GND and IN, to the ground plane and power plane on the back side of PCB with openings in the solder mask next to the vias to provide better thermal conduction. 3) Provide forced-air cooling to further reduce case temperature. ______________________________________________________________________________________ 1MHz, 2A, 2.6V to 5.5V Input, PWM DC-DC Step-Down Regulator with Enable IN C4 0.1µF MAX1951A L1 2µH 2.6V TO 5.5V 1.5V AT 2A LX R4 10Ω MAX1951A FB VCC R1 51.1kΩ EN ON R3 13.0kΩ 1% OFF COMP C1 10µF GND C2 220pF PGND R2 15.0kΩ 1% C3 10µF OUTPUT COMPONENT VALUES VOLTAGE (V) R1 (kΩ) R2 (kΩ) R3 (kΩ) C2 (pF) 470 SHORT OPEN 0.8 10 150 13 15 1.5 52.3 150 31.6 15 2.5 86.6 150 46.4 15 3.3 115 Figure 2. MAX1951A Adjustable Output Typical Application Circuit ______________________________________________________________________________________ 11 MAX1951A 1MHz, 2A, 2.6V to 5.5V Input, PWM DC-DC Step-Down Regulator with Enable Table 2. External Components List COMPONENT (FIGURE 2) FUNCTION DESCRIPTION L1 Output inductor C1 Input filtering capacitor 10µF ±20%, 6.3V X5R capacitor Taiyo Yuden JMK316BJ106ML or TDK C3216X5R0J106MT C2 Compensation capacitor 220pF ±10%, 50V capacitor Murata GRM1885C1HZZ1JA01 or Taiyo Yuden UMK107CH221KZ C3 Output filtering capacitor 10µF ±20%, 6.3V X5R capacitor Taiyo Yuden JMK316BJ106ML or TDK C3216X5R0J106MT C4 VCC bypass capacitor 0.1µF ±20%, 16V X7R capacitor Taiyo Yuden EMK107BJ104MA, TDK C1608X7R1C104K, or Murata GRM188R171C104KA01 R1 Loop compensation resistor Figure 2 R2 Feedback resistor Figure 2 R3 Feedback resistor Figure 2 R4 Bypass resistor 2µH ±20% inductor Sumida CDRH4D28-1R8 or TOKO A915AY-2R0M 10Ω ±5% resistor Table 3. Component Suppliers MANUFACTURER Murata Electronics North America, Inc. WEBSITE www.murata-northamerica.com Sumida Corp. www.sumida.com Taiyo Yuden www.t-yuden.com TDK Corp. www.component.tdk.com TOKO America, Inc. www.tokoam.com Chip Information PROCESS: BiCMOS Package Information For the latest package outline information and land patterns, go to www.maxim-ic.com/packages. PACKAGE TYPE PACKAGE CODE DOCUMENT NO. 8 SO S8-6F 21-0041 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 © 2009 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.