19-2011; Rev 0; 5/01 KIT ATION EVALU E L B A AVAIL WCDMA Cellular Phone 600mA Buck Regulators Features ♦ Dynamically Adjustable Output from 0.4V to 3.4V (MAX1820) ♦ Programmable Output from 1.25V to 5.5V (MAX1821) ♦ SYNC to 13MHz External Clock (MAX1820X) The MAX1820 is dynamically controlled to provide varying output voltages from 0.4V to 3.4V. The circuit is designed such that the output voltage settles in <30µs for a full-scale change in voltage and current. The MAX1821 is set with external resistors to provide any fixed output voltage in the 1.25V to 5.5V range. The MAX1820/MAX1821 include a low on-resistance internal MOSFET switch and synchronous rectifier to maximize efficiency and minimize external component count. 100% duty-cycle operation allows for low dropout of only 150mV at 600mA load, including the external inductor resistance. The devices are offered in 10-pin µMAX and tiny 3 ✕4 chip-scale (UCSP™) packages. ♦ Low Quiescent Current 180µA (typ) in Skip Mode 0.1µA (typ) in Shutdown Mode ♦ No External Schottky Diode Required ________________________Applications WCDMA Cell Phone Power Amplifiers PDA, Palmtop, and Notebook Computers Microprocessor Core Supplies Digital Cameras PCMCIA and Network Cards Hand-Held Instruments ♦ SYNC to 19.8MHz External Clock (MAX1820Y) ♦ NO SYNC, Internal 1MHz Osculator (MAX1820Z) ♦ 600mA Guaranteed Output Current ♦ 0% to 100% Duty-Cycle Operation ♦ 150mV Dropout at 600mA Load (including RDC of external inductor) ♦ µMAX or UCSP Packaging Typical Operating Circuits 4.7µH INPUT 2.6V TO 5.5V BATT LX OUT SHDN DYNAMIC OUTPUT 0.4V TO 3.4V 4.7µF PGND MAX1820 COMP SYNC 13MHz OR 19.8MHz VOUT CONTROL DAC REF SKIP Typical Operating Circuits continued at end of data sheet. GND Pin Configurations appear at end of data sheet. UCSP is a trademark of Maxim Integrated Products, Inc. Ordering Information PART SYNC FREQ. (MHz) OUTPUT VOLTAGE TEMP. RANGE PIN-PACKAGE UCSP MARK MAX1820ZEBC* No Sync Dynamic -40°C to +85°C 3x4 UCSP AAB MAX1820YEBC* 19.8 Dynamic -40°C to +85°C 3x4 UCSP AAL MAX1820XEBC* 13 Dynamic -40°C to +85°C 3x4 UCSP AAM MAX1820ZEUB No Sync Dynamic -40°C to +85°C 10 µMAX — MAX1820YEUB* 19.8 Dynamic -40°C to +85°C 10 µMAX — MAX1820XEUB* 13 Dynamic -40°C to +85°C 10 µMAX — MAX1821EBC* No Sync Programmable -40°C to +85°C 3x4 UCSP AAC MAX1821EUB No Sync Programmable -40°C to +85°C 10 µMAX — *Future Product Specification subject to change prior to release. Contact Factory for Availability. ________________________________________________________________ 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 MAX1820/MAX1821 General Description The MAX1820/MAX1821 low-dropout, pulse-width-modulated (PWM) DC-DC buck regulators are optimized to provide power to the PA in WCDMA cellphones; however, they may be applied in many other applications where high efficiency is a priority. The supply voltage range is from 2.6V to 5.5V, and the guaranteed output current is 600mA. 1MHz PWM switching allows for small external components, while skip mode reduces quiescent current to 180µA with light loads. MAX1820/MAX1821 WCDMA Cellular Phone 600mA Buck Regulators ABSOLUTE MAXIMUM RATINGS BATT, OUT (FB), SHDN, SYNC, SKIP, REF to GND .......................................................-0.3V to +6.0V PGND to GND .......................................................-0.3V to +0.3V LX, COMP to GND...................................-0.3V to (VBATT + 0.3V) Output Short-Circuit Duration ............................................Infinite Continuous Power Dissipation (TA = +70°C) 3✕4 UCSP (derate 10.4mW/°C above +70°C) .............832mW 10-Pin µMAX (derate 5.6mW/°C above +70°C) ...........444mW Operating Temperature Range ...........................-40°C to +85°C Junction Temperature ......................................................+150°C Storage Temperature Range 3✕4 UCSP .....................................................-40°C to +150°C 10-Pin µMAX ..................................................-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 (VBATT = 3.6V, SHDN = BATT, SKIP = SYNC = GND, VREF = 1.25V (MAX1820 only), TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER Input BATT Voltage Undervoltage Lockout Threshold Quiescent Current SYMBOL VIN VUVLO IQ Quiescent Current in Dropout Shutdown Supply Current CONDITIONS ISHDN MIN TYP 2.6 VBATT rising, 1% hysteresis 2.20 2.35 MAX UNITS 5.5 V 2.55 V SKIP = GND 180 300 SKIP = BATT, no switching 450 2000 SKIP = BATT, 1MHz switching 3300 SKIP = GND 530 1000 SKIP = BATT, no switching 550 1000 SHDN = GND 0.1 6 µA µA µA ERROR AMPLIFIER OUT Voltage Accuracy (MAX1820) 2 VOUT VREF = 1.932 ±0.005V, load = 0 to 600mA, SKIP = BATT or GND 3.33 3.4 3.47 VREF = 0.227 ±0.005V, load = 0 to 30mA, SKIP = BATT, VBATT ≤ 4.2V 0.35 0.40 0.45 250 400 OUT Input Resistance (MAX1820) ROUT REF Input Current (MAX1820) IREF FB Voltage Accuracy (MAX1821) VFB FB = COMP FB Input Current (MAX1821) IFB VFB = 1.4V Transconductance gm V 1.225 kΩ 0.1 1 µA 1.25 1.275 V 0.01 50 nA 30 50 85 µS COMP Clamp Low Voltage 0.2 0.45 1.0 V COMP Clamp High Voltage 2.04 2.15 2.28 V _______________________________________________________________________________________ WCDMA Cellular Phone 600mA Buck Regulators (VBATT = 3.6V, SHDN = BATT, SKIP = SYNC = GND, VREF = 1.25V (MAX1820 only), TA = 0°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX ILX = 180mA, VBATT = 3.6V 0.15 0.3 ILX = 180mA, VBATT = 2.6V 0.2 ILX = 180mA, VBATT = 3.6V 0.2 ILX = 180mA, VBATT = 2.6V 0.3 UNITS CONTROLLER P-Channel On-Resistance PRDS N-Channel On-Resistance NRDS Current-Sense Transresistance RCS 0.35 Ω Ω 0.5 0.75 0.9 V/A P-Channel Current-Limit Threshold Duty factor = 100% 0.75 1.2 1.55 A P-Channel Pulse-Skipping Current Threshold SKIP = GND 0.04 0.13 0.24 A N-Channel Current-Limit Threshold SKIP = BATT -1.6 -0.85 -0.45 SKIP = GND 0.02 0.08 0.14 -1 0.1 1 LX Leakage Current ILX Maximum Duty Cycle dutyMAX Minimum Duty Cycle dutyMIN VBATT = 5.5V, LX = GND or BATT 100 A µA % SKIP = GND 0 SKIP = BATT, VBATT = 4.2V 10 % SYNC AND OSCILLATOR SYNC Divide Ratio (MAX1820X) SYNC = sine wave, SYNC input = 200mVp-p 13 13 SYNC = sine wave, SYNC input = 800mVp-p 13 13 SYNC Capture Range (MAX1820X) SYNC = sine wave, AC-coupled, SYNC input = 500mVp-p 10 SYNC Leakage Current ISYNC 13 Hz/Hz 16 MHz µA VSYNC = 1V -1 +1 SYNC Divide Ratio (MAX1820Y) SYNC = sine wave, SYNC input = 200mVp-p 18 18 SYNC = sine wave, SYNC input = 800mVp-p 18 18 SYNC Capture Range (MAX1820Y) SYNC = sine wave, AC-coupled, SYNC input = 500mVp-p 15 19.8 21 MHz SYNC = GND 0.8 1 1.2 MHz Internal Oscillator Frequency (MAX1820Z, MAX1821) fOSC Hz/Hz LOGIC INPUTS (SKIP, SHDN) Logic Input High VIH Logic Input Low VIL Logic Input Current 1.6 -1 V 0.1 0.4 V 1 µA _______________________________________________________________________________________ 3 MAX1820/MAX1821 ELECTRICAL CHARACTERISTICS (continued) MAX1820/MAX1821 WCDMA Cellular Phone 600mA Buck Regulators ELECTRICAL CHARACTERISTICS (VBATT = 3.6V, SHDN = BATT, SKIP = SYNC = GND, VREF = 1.25V (MAX1820 only), TA = -40°C to +85°C, unless otherwise noted.) (Note 1) PARAMETER Input BATT Voltage Undervoltage Lockout Threshold Quiescent Current SYMBOL VIN VUVLO IQ Quiescent Current in Dropout Shutdown Supply Current CONDITIONS ISHDN VBATT rising, 1% hysteresis MIN MAX UNITS 2.6 5.5 V 2.15 2.55 V SKIP = GND 300 SKIP = BATT, no switching 2000 SKIP = GND 1000 SKIP = BATT, no switching 1000 SHDN = GND 6 µA µA µA ERROR AMPLIFIER OUT Voltage Accuracy (MAX1820) VOUT VREF = 1.932 ±0.005V, load = 0 to 600mA, SKIP = BATT or GND 3.33 3.47 VREF = 0.227 ±0.005V, load = 0 to 30mA, SKIP = BATT, VBATT ≤ 4.2V 0.35 0.45 OUT Input Resistance (MAX1820) ROUT REF Input Current (MAX1820) IREF FB Voltage Accuracy (MAX1821) VFB FB = COMP FB Input Current (MAX1821) IFB VFB = 1.4V Transconductance gm V 250 1.225 30 kΩ 1 µA 1.275 V 50 nA 85 µS COMP Clamp Low Voltage 0.2 1.0 V COMP Clamp High Voltage 2.04 2.28 V 0.3 Ω CONTROLLER P-Channel On-Resistance PRDS ILX = 180mA, VBATT = 3.6V N-Channel On-Resistance NRDS ILX = 180mA, VBATT = 3.6V Current-Sense Transresistance RCS 0.35 Ω 0.5 0.9 V/A P-Channel Current-Limit Threshold Duty factor = 100% 0.75 1.55 A P-Channel Pulse-Skipping Current Threshold SKIP = GND 0.04 0.24 A SKIP = BATT -1.6 -0.45 SKIP = GND 0.01 0.14 N-Channel Current-Limit Threshold 4 _______________________________________________________________________________________ A WCDMA Cellular Phone 600mA Buck Regulators (VBATT = 3.6V, SHDN = BATT, SKIP = SYNC = GND, VREF = 1.25V (MAX1820 only), TA = -40°C to +85°C, unless otherwise noted.) (Note 1) PARAMETER SYMBOL LX Leakage Current ILX Maximum Duty Cycle dutyMAX Minimum Duty Cycle CONDITIONS VBATT = 5.5V, LX = GND or BATT MIN MAX UNITS -1 1 µA 100 dutyMIN % SKIP = GND 0 SKIP = BATT, VBATT = 4.2V 10 % SYNC AND OSCILLATOR SYNC Divide Ratio (MAX1820X) SYNC = sine wave, SYNC input = 200mVp-p 13 13 SYNC = sine wave, SYNC input = 800mVp-p 13 13 SYNC Capture Range (MAX1820X) SYNC = sine wave, AC-coupled, SYNC input = 500mVp-p 10 16 SYNC Divide Ratio (MAX1820Y) SYNC = sine wave, SYNC input = 200mVp-p 18 18 SYNC = sine wave, SYNC input = 800mVp-p 18 18 SYNC Capture Range (MAX1820Y) SYNC = sine wave, AC-coupled, SYNC input = 500mVp-p 15 21 MHz Hz/Hz MHz Hz/Hz SYNC Leakage Current ISYNC VSYNC = IV -1 +1 µA Internal Oscillator Frequency (MAX1820Z, MAX1821) fOSC SYNC = GND 0.8 1.2 MHz LOGIC INPUTS (SKIP, SHDN) Logic Input High VIH Logic Input Low VIL 1.6 V Logic Input Current 0.4 V 1 µA Note 1: Specifications to -40°C are guaranteed by design and not subject to production test. Typical Operating Characteristics (TA = +25°C, unless otherwise noted.) 80 90 EFFICIENCY (%) RLOAD = 10Ω RLOAD = 5Ω RLOAD = 15Ω 70 60 RLOAD = 15Ω 70 60 50 40 40 80 RLOAD = 10Ω 80 50 90 EFFICIENCY (%) 90 100 MAX1820/21 toc02 RLOAD = 5Ω EFFICIENCY (%) 100 MAX1820/21 toc01 100 EFFICIENCY vs. INPUT VOLTAGE NORMAL MODE, RLOAD = 10Ω EFFICIENCY vs. OUTPUT VOLTAGE (PWM MODE, VIN = 3.6V) MAX1820/21 toc03 EFFICIENCY vs. OUTPUT VOLTAGE (NORMAL MODE, VIN = 3.6V) VOUT = 1.8V VOUT = 3.4V 70 60 VOUT = 0.4V 50 40 30 20 10 0 0.5 1.0 1.5 2.0 2.5 3.0 OUTPUT VOLTAGE (V) 3.5 4.0 0 0 0.5 1.0 1.5 2.0 2.5 3.0 OUTPUT VOLTAGE (V) 3.5 4.0 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 VIN (V) _______________________________________________________________________________________ 5 MAX1820/MAX1821 ELECTRICAL CHARACTERISTICS (continued) Typical Operating Characteristics (continued) (TA = +25°C, unless otherwise noted.) 50 VIN = +5.0V VIN = +5.0V 50 VIN = +2.7V 40 30 20 20 SKIP = GND (DASHED LINE) SKIP = BATT (SOLID LINE) VIN = +3.6V 60 30 10 VIN = +2.7V 70 VIN = +3.6V 80 SKIP = GND (DASHED LINE) SKIP = BATT (SOLID LINE) 100 DROPOUT VOLTAGE vs. LOAD CURRENT 10 100 60 40 MAX1820/21 toc08 7 6 5 4 3 2 VOUT = 3.4V RL = 57mΩ 10 600 SUPPLY CURRENT vs. SUPPLY VOLTAGE 200 180 160 140 120 100 80 VOUT = 1.5V SKIP = GND 20 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 LOAD CURRENT (mA) SUPPLY VOLTAGE (V) SUPPLY VOLTAGE (V) HEAVY-LOAD SWITCHING WAVEFORMS (VIN = 3.8V, VOUT = 3.4V, ILOAD = 600mA, SKIP = BATT) MEDIUM-LOAD SWITCHING WAVEFORMS (VIN = 3.8V, VOUT = 1.8V, ILOAD = 300mA, SKIP = BATT) LIGHT-LOAD PWM SWITCHING WAVEFORMS (VIN = 3.8V, VOUT = 0.45V, ILOAD = 30mA, SKIP = BATT) MAX1820/21 toc10 MAX1820/21 toc12 MAX1820/21 toc11 A A A B B B C C 400ns/div A: VLX, 5V/div B: INDUCTOR CURRENT, 500mA/div C: VOUT (AC-COUPLED), 5mV/div 6 1000 220 40 0 500 100 60 VOUT = 1.5V SKIP = BATT 1 0 400 SKIP = GND (DASHED LINE) SKIP = BATT (SOLID LINE) LOAD CURRENT (mA) 8 SUPPLY CURRENT (mA) 80 300 VIN = +5.0V 1 SUPPLY CURRENT vs. SUPPLY VOLTAGE 100 200 40 1000 9 MAX1820/21 toc07 120 100 VIN = +3.6V LOAD CURRENT (mA) 140 0 VIN = +2.7V 50 0 1 1000 LOAD CURRENT (mA) 20 60 10 SUPPLY CURRENT (µA) 10 VIN = +5.0V 20 0 1 VIN = +3.6V 70 30 VIN = +5.0V 10 0 VIN = +2.7V 90 MAX1820/21 toc09 40 80 100 EFFICIENCY (%) VIN = +3.6V 60 90 EFFICIENCY (%) EFFICIENCY (%) VIN = +5.0V VIN = +3.6V 70 MAX1821 EFFICIENCY vs. LOAD CURRENT (VOUT = 1.5V) MAX1820/21 toc05 90 80 100 MAX1820/21 toc04 100 MAX1821 EFFICIENCY vs. LOAD CURRENT (VOUT = 2.5V) MAX1820/21 toc06 MAX1821 EFFICIENCY vs. LOAD CURRENT (VOUT = 3.3V) DROPOUT VOLTAGE (mV) MAX1820/MAX1821 WCDMA Cellular Phone 600mA Buck Regulators C 400ns/div A: VLX, 5V/div B: INDUCTOR CURRENT, 500mA/div C: VOUT (AC-COUPLED), 5mV/div 400ns/div A: VLX, 5V/div B: INDUCTOR CURRENT, 100mA/div C: VOUT (AC-COUPLED), 5mV/div _______________________________________________________________________________________ WCDMA Cellular Phone 600mA Buck Regulators LIGHT-LOAD SKIP-SWITCHING WAVEFORMS (VIN = 4.2V, VOUT = 1.5V, LOAD = 30mA, SKIP = GND) EXITING AND ENTERING SHUTDOWN (VIN = 3.6V, VOUT = 3.4V, RLOAD = 15Ω) MAX1820/21 toc14 MAX1820/21 toc13 VSHDN 5V/div A B VOUT 2V/div C IBATT 0.5A/div 2ms/div 2µs/div A: VLX, 5V/div B: INDUCTOR CURRENT, 500mA/div C: VOUT (AC-COUPLED), 20mV/div LOAD TRANSIENT (ILOAD = 20mA TO 420mA, VOUT = 1.5V, VIN = 3.6V, SKIP = BATT) LOAD TRANSIENT (ILOAD = 20mA TO 420mA, VOUT = 1.5V, VIN = 3.6V, SKIP = GND) MAX1820/21 toc15 MAX1820/21 toc16 IOUT 200mA/div IOUT 200mA/div VOUT AC-COUPLED 100mV/div VOUT AC-COUPLED 100mV/div COUT = 10µF COUT = 10µF 40µs/div 40µs/div MAX1820 REF TRANSIENT (VREF = 0.23V TO 1.932V, RLOAD = 10Ω, VIN = 3.6V, SKIP = BATT) LINE TRANSIENT (VIN = 3.6V TO 4.0V, VOUT = 1.5V, ILOAD = 300mA) MAX1820/21 toc18 MAX1820/21 toc17 VREF 1V/div VIN 200mV/div VOUT 1V/div VOUT AC-COUPLED 200mV/div COUT = 10µF 20µs/div 40µs/div _______________________________________________________________________________________ 7 MAX1820/MAX1821 Typical Operating Characteristics (continued) (TA = +25°C, unless otherwise noted.) Typical Operating Characteristics (continued) (TA = +25°C, unless otherwise noted.) OUTPUT SWITCHING HARMONICS vs. FREQUENCY (VIN = 3.8V, VOUT = 3.4V, ILOAD = 600mA) 1.2 0.8 0.4 MAX1820/21 toc20 MAX1820/21 toc19 OUTPUT SWITCHING HARMONICS vs. FREQUENCY (VIN = 3.8V, VOUT = 1.8V, ILOAD = 300mA) 1.6 HARMONICS (mVRMS) 0 1.2 0.8 0.4 0 0.1 1 10 0.1 1 10 FREQUENCY (MHz) OUTPUT SWITCHING HARMONICS vs. FREQUENCY (VIN = 4.2V, VOUT = 0.4V, ILOAD = 30mA) OUTPUT NOISE (VIN = 3.6V, VOUT = 1.8V, IOUT = 300mA) MAX1820/21 toc21 FREQUENCY (MHz) 1.6 4.0 NOISE (µV/√Hz) 1.2 0.8 3.0 2.0 0.4 1.0 0 0 0.1 1 10 MAX1820/21 toc22 HARMONICS (mVRMS) 1.6 HARMONICS (mVRMS) MAX1820/MAX1821 WCDMA Cellular Phone 600mA Buck Regulators 0.1 1 10 100 250 FREQUENCY (MHz) FREQUENCY (MHz) Pin Description PIN 8 MAX1820 UCSP MAX1820 µMAX MAX1821 UCSP MAX1821 µMAX NAME FUNCTION A1 1 A1 1 SKIP PWM/Skip-Mode Input. Drive with logic 0 to use PWM at medium and heavy loads and pulse skipping at light loads. Drive with logic 1 to force PWM at all loads. A2 2 A2 2 COMP A3 3 — — OUT Compensation. Typically, connect an 82kΩ (for MAX1821) or 43kΩ (for MAX1820) series resistor and 330pF capacitor from this pin to GND to stabilize the regulator. Output Voltage Sense Input. Connect OUT directly to the output. _______________________________________________________________________________________ WCDMA Cellular Phone 600mA Buck Regulators PIN MAX1820 UCSP MAX1820 µMAX MAX1821 UCSP MAX1821 µMAX NAME FUNCTION — — A3 3 FB Output Feedback Sense Input. To set the output voltage, connect FB to the center of an external resistive-divider between the output and GND. FB voltage regulates to 1.25V. A4 4 — — REF External Reference Input. Connect REF to the output of a D/A converter for dynamic adjustment of the output voltage. REF-toOUT gain is 1.76. — — A4 4 REF Internal Reference Bypass. Connect a 0.047µF capacitor from REF to GND. B4 5 B4 5 GND Ground C4 6 C4 6 PGND C3 7 C3 7 LX Inductor Connection. LX connects to the drains of the internal power MOSFETs. LX is high impedance in shutdown mode. C2 8 C2 8 BATT Supply Voltage Input. Connect BATT to a 2.6V to 5.5V source. Bypass BATT to PGND with a low-ESR 10µF capacitor. C1 9 C1 9 SHDN Active-Low, Shutdown Control Input SYNC Clock Synchronization Input. Drive SYNC with a 13MHz (MAX1820X) or 19.8MHz (MAX1820Y) AC-coupled sine-wave input to synchronize power switching at 1MHz. MAX1820Z and MAX1821 do not have SYNC capability. Connect SYNC to GND to use the internally generated, free-running 1MHz clock. MAX1820Z and MAX1821 SYNC pin must be connected to GND. B1 10 B1 10 _______________Detailed Description The MAX1820/MAX1821 PWM step-down DC-DC converters are optimized for low-voltage, battery-powered applications where high-efficiency and small size are priorities. The MAX1821 is a general-purpose device that uses external feedback resistors to set the output voltage from 1.25V to V BATT , and the MAX1820 is specifically intended to power a linear power amplifier (PA) in WCDMA handsets. An analog control signal dynamically adjusts the MAX1820’s output voltage from 0.4V to 3.4V with a settling time <30µs. The MAX1820/MAX1821 operate at a high 1MHz switching frequency that reduces external component size. Each device includes an internal synchronous rectifier that provides for high efficiency and eliminates the need for an external Schottky diode. The normal operating mode uses constant frequency PWM switching at medium and heavy loads, and automatically pulse skips at light loads to reduce supply current and extend Power Ground battery life. An additional forced PWM mode (with optional external synchronization) switches at a constant frequency, regardless of load, to provide a wellcontrolled spectrum in noise-sensitive applications. Battery life is maximized by low-dropout operation at 100% duty-cycle and a 0.1µA (typ) logic-controlled shutdown mode. PWM Control The MAX1820/MAX1821 use a slope-compensated, current-mode PWM controller capable of achieving 100% duty cycle. The current-mode control design is capable of minimum duty cycles of less than 10%, ensuring a constant switching frequency with outputs as low as 0.4V when powered from a single lithium ion (Li+) cell. Current-mode feedback provides stable switching and cycle-by-cycle current limiting for superior load- and line-response and protection of the internal MOSFET and synchronous rectifier. The output voltage is regulated by switching at a constant frequency and _______________________________________________________________________________________ 9 MAX1820/MAX1821 Pin Description (continued) MAX1820/MAX1821 WCDMA Cellular Phone 600mA Buck Regulators BATT TO IC BIAS GND TRANSIMPEDANCE ERROR AMP OUT ERROR SIGNAL 0.45V TO 2.15V 1.25V VOLTAGE REFERENCE CLAMP PWM COMPARATOR SLOPE COMP CURRENT SENSE SKIP COMPARATOR PWM CONTROL AND SKIP LOGIC LX SKIP THRESHOLD REF PGND SKIP SYNC COMP 1MHz OSCILLATOR SHDN ÷13 OR ÷18 MAX1820 Figure 1. MAX1820 Simplified Functional Diagram (No SYNC for MAX1820Z) BATT 1.25V TO IC BIAS ERROR SIGNAL GND CLAMP PWM COMPARATOR SLOPE COMP CURRENT SENSE SKIP COMPARATOR PWM CONTROL AND SKIP LOGIC LX SKIP THRESHOLD REF PGND SHDN SKIP COMP MAX1821 1MHz OSCILLATOR SYNC FB TRANSIMPEDANCE ERROR AMP 0.45V TO 2.15V VOLTAGE REFERENCE Figure 2. MAX1821 Simplified Functional Diagram (No SYNC for MAX1821) then modulating the power transferred to the load during each cycle, using the PWM comparator. The power transferred to the load is adjusted by changes in the inductor peak current limit during the first half of each cycle, based on the output error voltage. A new cycle begins at each falling edge of the internal oscillator. The controller turns on the P-channel MOSFET to increase the inductor current, and the slope compensation block initiates a new reference current 10 ramp that is summed with the internal P-channel MOSFET current (Figures 1 and 2). The second half of the cycle begins when the reference ramp is greater than the error voltage. The P-channel MOSFET is turned off, the synchronous rectifier is turned on, and inductor current continues to flow to the output capacitor. The output capacitor stores charge when the current is high and releases it when the inductor current is low, smoothing the voltage across ______________________________________________________________________________________ WCDMA Cellular Phone 600mA Buck Regulators BATT 10µF MAX1820/MAX1821 4.7µH VIN = 2.6V TO 5.5V VOUT = 1.5V LX 0.1µF SHDN MAX1821 4.7µF R1 6kΩ PGND SYNC FB REF 0.047µF R2 30kΩ COMP SKIP RC 82kΩ GND C1 330pF C2* 1pF * CAN BE OMMITTED IF CERAMIC OUTPUT CAPACITOR IS USED. Figure 3. Standard Operating Circuit the load. The duty cycle of a buck step-down converter is ideally a ratio of the output voltage to input voltage in steady-state condition. The MAX1820/MAX1821 have internal switch current limits of 1.2A (typ). If ILX exceeds this maximum, the high-side FET turns off and the synchronous rectifier turns on. This will lower the duty cycle and cause the output voltage to droop as long as the load current remains excessive. There is also a synchronous rectifier current limit of -0.85A when the device is operating in forced PWM mode (see Forced PWM Mode). If the negative current limit is exceeded, the synchronus rectifier is turned off, and the inductor current continues to flow through its body diode until the beginning of the next cycle or the inductor current drops to zero. This means there is a limit on how much current the device is allowed to shuttle in response to output power reduction. Normal Mode Operation Connecting SKIP to GND enables MAX1820/MAX1821 normal operation (Figure 3). This allows automatic PWM control at medium and heavy loads and skip mode at light loads to improve efficiency and reduce quiescent current to 180µA. Operating in normal mode also allows the MAX1820/MAX1821 to pulse-skip when the peak inductor current drops below 130mA, corresponding to a load current of approximately 65mA. During skip operation, the MAX1820/MAX1821 switch only as needed to service the load, reducing the switching frequency and associated losses in the internal switch, the synchronous rectifier, and the external inductor. There are three steady-state operating conditions for the MAX1820/MAX1821 in normal mode. The device performs in continuous conduction for heavy loads in a manner identical to forced PWM mode. The inductor current becomes discontinuous at medium loads, requiring the synchronous rectifier to be turned off before the end of a cycle as the inductor current reaches zero. The device enters into skip mode when the converter output voltage exceeds its regulation limit before the inductor current reaches its skip threshold level. During skip mode, a switching cycle initiates when the output voltage has dropped out of regulation. The Pchannel MOSFET switch turns on and conducts current to the output-filter capacitor and load until the inductor current reaches the skip peak current limit. Then the main switch turns off, and the magnetic field in the inductor collapses, while current flows through the synchronous rectifier to the output filter capacitor and the load. The synchronous rectifier is turned off when the inductor current reaches zero. The MAX1820/ MAX1821 wait until the skip comparator senses a low output voltage again. Forced PWM Operation Connect SKIP to BATT for forced PWM operation. Forced PWM operation is desirable in sensitive RF and data-acquisition applications to ensure that switching harmonics do not interfere with sensitive IF and datasampling frequencies. A minimum load is not required during forced PWM operation since the synchronous rectifier passes reverse-inductor current as needed to allow constant-frequency operation with no load. ______________________________________________________________________________________ 11 MAX1820/MAX1821 WCDMA Cellular Phone 600mA Buck Regulators Forced PWM operation uses higher supply current with no load (3.3mA typ) compared to skip mode. 100% Duty-Cycle Operation The on-time can exceed one internal oscillator cycle, which permits operation up to 100% duty cycle. As the input voltage drops, the duty cycle increases until the P-channel MOSFET is held on continuously. Dropout voltage in 100% duty cycle is the output current multiplied by the on-resistance of the internal switch and inductor, approximately 150mV (IOUT = 600mA). Near dropout, the on-time may exceed 1 PWM clock cycle; therefore, small amplitude subharmonic ripple may occur. COMP Clamp The MAX1820/MAX1821 compensation network has a 0.45V to 2.15V error regulation range. The clamp prevents COMP from rising too high or falling too low to optimize transient response. Dropout Dropout occurs when the input voltage is less than the desired output voltage plus the IR drops in the circuit components. The duty cycle is 100% during this condition, and the main switch remains on, continuously delivering current to the output up to the current limit. IR drops in the circuit are primarily caused by the onresistance of the main switch and the resistance in the inductor. During dropout, the high-side P-channel MOSFET turns on, and the controller enters a low-current consumption mode. Every 6µs (6 cycles), the MAX1820/MAX1821 check to see if the device is still in dropout. The device remains in this mode until the MAX1820/MAX1821 are no longer in dropout. Undervoltage Lockout (UVLO) The MAX1820/MAX1821 do not operate with battery voltages below the UVLO threshold of 2.35V (typ). The BATT input remains high impedance until the supply voltage exceeds the UVLO threshold. This guarantees the integrity of the output voltage regulation and prevents excessive current during startup and as the battery supply voltage drops during usage. Synchronous Rectification An N-channel synchronous rectifier eliminates the need for an external Schottky diode and improves efficiency. The synchronous rectifier turns on during the second half of each cycle (off-time). During this time, the voltage across the inductor is reversed, and the inductor current falls. In normal mode, the synchronous rectifier is turned off when either the output falls out of regula12 tion (and another on-time begins) or when the inductor current approaches zero. In forced PWM mode, the synchronous rectifier remains active until the beginning of a new cycle. SYNC Input and Frequency Control The MAX1820Z and MAX1821 internal oscillator is set to fixed 1MHz switching frequency. The MAX1820Z and MAX1821 do not have synchronizing capability and SYNC pin must be connected to GND. The MAX1820Y and MAX1820X are capable of synchronizing to external signal. For external synchronization, drive the SYNC pin with a 13MHz (MAX1820X) or 19.8MHz (MAX1820Y) AC-coupled sine wave. SYNC has a perfect 13:1 (MAX1820X) or 18:1 (MAX1820Y) clock divider for 1MHz (MAX1820X) or 1.1MHz (MAX1820Y) switching from common system clocks. The input frequency range for SYNC is 10MHz to 16MHz (MAX1820X) or 15MHz to 21MHz (MAX1820Y). Connect SYNC to GND to use the internal free-running oscillator at 1MHz. Shutdown Mode Drive SHDN to GND to place the MAX1820/MAX1821 in shutdown mode. In shutdown, the reference, control circuitry, internal switching MOSFET, and the synchronous rectifier turn off, reducing the supply current to 0.1µA, and the output goes high impedance. Connect SHDN to BATT for normal operation. Current-Sense Comparators The MAX1820/MAX1821 use several internal currentsense comparators. In PWM operation, the PWM comparator terminates the cycle-by-cycle on-time (Figures 1 and 2) and provides improved load and line response. This allows tighter specification of the inductor-saturation current limit to reduce inductor cost. A second current-sense comparator used across the Pchannel switch controls entry into skip mode. A third current-sense comparator monitors current through the internal N-channel MOSFET to prevent excessive reverse currents and determine when to turn off the synchronous rectifier. A fourth comparator used at the P-channel MOSFET detects overcurrent. This protects the system, external components, and internal MOSFETs under overload conditions. ___ _____ _Applications Information Setting the Output Voltage (MAX1820) The MAX1820 is optimized for highest system efficiency when applying power to a linear power amplifier (PA) in WCDMA handsets. When transmitting at less than full power, the supply voltage to the PA is reduced (from 3.4V to as low as 0.4V) to greatly reduce battery ______________________________________________________________________________________ WCDMA Cellular Phone 600mA Buck Regulators WCDMA PA SUPPLY VOLTAGE (V) 3.4 3.0 rent regulation loop. The resistor sets the proportional gain of the output error voltage by a factor gm ✕ RC. Increasing this resistor also increases the sensitivity of the control loop to the output capacitor ripple. This resistor and capacitor set a compensation zero that defines the system’s transient response. The load pole is a dynamic pole, shifting the pole frequency with changes in load. As the load decreases, the pole frequency will shift to the left. System stability requires that the compensation zero must be placed properly to ensure adequate phase margin (at least 30° at unity gain). The following is a design procedure for the compensation network: 1) Select an appropriate converter bandwidth (fC) to stabilize the system while maximizing transient response. This bandwidth should not exceed 1/5 of the switching frequency. Use 100kHz as a reasonable starting point. 2) Calculate the compensation capacitor, C1, based on this bandwidth: VO(MAX) 1 R2 1 C1 = gm × R1+R2 2 × π × f I R O(MAX) CS C 1.0 0.4 30 300 600 WCDMA PA SUPPLY CURRENT (mA) Figure 4. Typical WCDMA PA Load Profile Setting the Output Voltage (MAX1821) The MAX1821 is intended for general-purpose stepdown applications where high efficiency is a priority. Select an output voltage between 1.25V and VBATT by connecting FB to a resistive-divider between the output and GND (Figure 3). Select feedback resistor R2 in the 5kΩ to 30kΩ range. R1 is then given by: V R1 = R2 OUT - 1 VFB Resistors R1 and R2 are internal to the MAX1820; use R1 = 151kΩ and R2 = 199kΩ as nominal values for calculations. These resistors are external to the MAX1821 (see Setting the Output Voltage). Using VOMAX = 3.4V, IOMAX = 0.6A, gm = 50µs, RCS = 0.75Ω, C1 is evaluated as: 199kΩ 3.4V 1 TIONC1 3 = 50µs × 0.6A 0.75Ω 151kΩ +199kΩ 1 × = 341pF 2 × 3.14 × 100kHz Selecting the nearest standard value of 330pF corresponds to a 103kHz bandwidth, which is still acceptable per the above criteria. 3) Calculate the equivalent load impedance, RL, by: where VFB = 1.25V. Compensation and Stability The MAX1820/MAX1821 are externally compensated by placing a resistor and a capacitor (RC and C1) in series, from COMP to GND (Figure 3). The capacitor integrates the current from the transimpedance amplifier, averaging output capacitor ripple. This sets the device speed for transient responses and allows the use of small ceramic output capacitors because the phase shifted capacitor ripple does not disturb the cur- RL ≈ VOUT(MAX) IOUT(MAX) 4) Calculate the compensation resistance (RC) value to cancel out the dominant pole created by the output load and the output capacitance: 1 2 × π × RL × COUT = 1 2 × π × RC × C1 ______________________________________________________________________________________ 13 MAX1820/MAX1821 current. Figure 4 shows the typical WCDMA PA load profile. The use of a DC-DC converter such as the MAX1820 will dramatically reduce battery drain in these applications. The MAX1820’s output voltage is dynamically adjustable from 0.4V to VBATT by the use of the REF input. The gain from VREF to VOUT is internally set to 1.76. VOUT can be adjusted during operation by driving REF with an external DAC. The MAX1820 output responds to full-scale change in voltage and current in <30µs. MAX1820/MAX1821 WCDMA Cellular Phone 600mA Buck Regulators Table 1. Suggested Inductors MANUFACTURER PART NUMBER INDUCTANCE (µH) ESR (mΩ) SATURATION CURRENT (A) DIMENSIONS (mm) Sumida CDRH4D18-4R7 4.7 125 0.84 5x5x2 Coilcraft DO1606 4.7 120 1.2 5.3x5.3x2 Sumida CR43 4.7 108.7 1.15 4.5x4x3.5 Sumida CDRH5D18-4R1 4.1 57 1.95 5.5x5.5x2 Coilcraft LPT1606-472 4.7 240 (max) 1.2 6.5x5.3x2.0 Solving for RC gives: RL × COUT 3.4V 4.7µF = = 80.8kΩ 0.6A 330pF C1 RC = 5) Calculate the high-frequency compensation pole to cancel the zero created by the output capacitor’s equivalent series resistance (ESR): 1 1 = ` 2 × π × RESR × COUT 2 × π × R3 × C2 Solving for C2 gives: C2 = 4.7µF × 0.01Ω RESR × COUT = = 0.55pF R3 80.8kΩ In this case, C2 can be omitted due to the use of ceramic capacitors. Larger output capacitors and higher ESR may require the use of capacitor C2. Inductor Selection A 4µH to 6µH inductor with a saturation current of at least 800mA is recommended for most applications. For best efficiency, the inductor’s DC resistance should be <200mΩ, and saturation current should be >1A. See Table 1 for recommended inductors and manufacturers. For most designs, a reasonable inductor value (LIDEAL) can be derived from the following equation: LIDEAL = VOUT (VBATT - VOUT ) VBATT × LIR × IOUT(MAX) × ƒ OSC where LIR is the inductor current ripple as a percentage. LIR should be kept between 20% and 40% of the maximum load current for best performance and stability. The maximum inductor current is: LIR IL(MAX) = 1+ IOUT(MAX) 2 14 The inductor current will become discontinuous if IOUT decreases to LIR/2 from the output current value used to determine LIDEAL. Input Capacitor Selection The input capacitor reduces the current peaks drawn from the battery or input power source and reduces switching noise in the IC. The impedance of the input capacitor at the switching frequency should be less than that of the input source so high-frequency switching currents do not pass through the input source. The input capacitor must meet the ripple-current requirement (IRMS) imposed by the switching currents. Nontantalum chemistries (ceramic, POSCAP, or OSCON) are preferred due to their resistance to power-up surge currents. V (V OUT BATT - VOUT ) IRMS = ILOAD VBATT For optimal circuit reliability, choose a capacitor that has less than 10°C temperature rise at the peak ripple current. Output Capacitor Selection The output capacitor is required to keep the output voltage ripple small and to ensure regulation control loop stability. The output capacitor must have low impedance at the switching frequency. Ceramic capacitors are recommended. The output ripple is approximately: VRIPPLE ≈ LIR ✕ IOUT(MAX) 1 × ESR + (2 × ƒ OSC × COUT ) See Compensation Design for a discussion of the influence of output capacitance and ESR on regulation control-loop stability. The capacitor voltage rating must exceed the maximum applied capacitor voltage. Consult the manufacturer’s ______________________________________________________________________________________ WCDMA Cellular Phone 600mA Buck Regulators PC Board Layout and Routing High switching frequencies and large peak currents make PC board layout a very important part of design. Good design minimizes excessive EMI on the feedback paths and voltage gradients in the ground plane, both of which can result in instability or regulation errors. Connect the inductor, input filter capacitor, and output filter capacitor as close together as possible, and keep their traces short, direct, and wide. Connect their ground pins at a single common node in a star-ground configuration. The external voltage-feedback network should be very close to the FB pin, within 0.2in (5mm). Keep noisy traces (from the LX pin, for example) away from the voltage-feedback network; also, keep them separate, using grounded copper. Connect GND and PGND at a single point, as close as possible to the MAX1820/MAX1821. The MAX1820/MAX1821 evaluation kit manual illustrates an example PC board layout and routing scheme. Table 2. Capacitor Selection CAPACITOR VALUE (µF) ESR (mΩ) CAPACITOR TYPE CBATT 4.7 to 10 <150 Ceramic COUT (MAX1820) 2.2 to 4.7 <50 Ceramic COUT (MAX1821) 4.7 to 10 <150 Ceramic CAPACITOR Table 3. Component Manufacturers MANUFACTURER USA PHONE NUMBER Coilcraft 847-639-6400 www.coilcraft.com Kemet 408-986-0424 www.kemet.com WEBSITE Panasonic 847-468-5624 www.panasonic.com Sumida 847-956-0666 www.sumida.com Taiyo Yuden 408-573-4150 www.t-yuden.com ____________________Chip Information TRANSISTOR COUNT: 2722 ______________________________________________________________________________________ 15 MAX1820/MAX1821 specifications for proper capacitor derating. Avoid Y5V and Z5U dielectric types due to their huge voltage and temperature coefficients of capacitance and ESR. WCDMA Cellular Phone 600mA Buck Regulators MAX1820/MAX1821 Typical Operating Circuits (continued) INPUT 2.6V TO 5.5V BATT SHDN OUTPUT 1.25V TO 5.5V LX PGND FB MAX1821 COMP SYNC REF SKIP GND Pin Configurations TOP VIEW AFTER ASSEMBLED ON PC BOARD (BUMPS AT THE BOTTOM) A B 1 2 3 4 SKIP A1 COMP A2 OUT (FB) A3 REF A4 SYNC GND B1 B4 SHDN C C1 ( ) ARE FOR MAX1821 ONLY. 16 BATT C2 LX C3 PGND C4 TOP VIEW SKIP 1 COMP 10 SYNC 2 MAX1820 MAX1821 9 SHDN OUT (FB) 3 8 BATT REF 4 7 LX GND 5 6 PGND µMAX ( ) ARE FOR MAX1821 ONLY. UCSP ______________________________________________________________________________________ WCDMA Cellular Phone 600mA Buck Regulators 12L, USPC.EPS ______________________________________________________________________________________ 17 MAX1820/MAX1821 Package Information WCDMA Cellular Phone 600mA Buck Regulators 10LUMAX.EPS MAX1820/MAX1821 Package Information (continued) 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. 18 ____________________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.