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19-2011; Rev 0; 5/01
KIT ATION EVALU ABLE AVAIL
WCDMA Cellular Phone 600mA Buck Regulators
Features
o Dynamically Adjustable Output from 0.4V to 3.4V (MAX1820) o Programmable Output from 1.25V to 5.5V (MAX1821) o SYNC to 13MHz External Clock (MAX1820X) o SYNC to 19.8MHz External Clock (MAX1820Y) o NO SYNC, Internal 1MHz Osculator (MAX1820Z) o Low Quiescent Current 180A (typ) in Skip Mode 0.1A (typ) in Shutdown Mode o No External Schottky Diode Required o 600mA Guaranteed Output Current o 0% to 100% Duty-Cycle Operation o 150mV Dropout at 600mA Load (including RDC of external inductor) o MAX or UCSP Packaging
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 180A with light loads. 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 <30s 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 MAX and tiny 3 4 chip-scale (UCSPTM) packages.
MAX1820/MAX1821
Typical Operating Circuits
INPUT 2.6V TO 5.5V 4.7H BATT SHDN LX OUT PGND 13MHz OR 19.8MHz VOUT CONTROL DAC MAX1820 COMP SYNC DYNAMIC OUTPUT 0.4V TO 3.4V 4.7F
________________________Applications
WCDMA Cell Phone Power Amplifiers PDA, Palmtop, and Notebook Computers Microprocessor Core Supplies Digital Cameras PCMCIA and Network Cards Hand-Held Instruments
Typical Operating Circuits continued at end of data sheet. Pin Configurations appear at end of data sheet. UCSP is a trademark of Maxim Integrated Products, Inc.
REF SKIP GND
Ordering Information
PART MAX1820ZEBC* MAX1820YEBC* MAX1820XEBC* MAX1820ZEUB MAX1820YEUB* MAX1820XEUB* MAX1821EBC* MAX1821EUB SYNC FREQ. (MHz) No Sync 19.8 13 No Sync 19.8 13 No Sync No Sync OUTPUT VOLTAGE Dynamic Dynamic Dynamic Dynamic Dynamic Dynamic Programmable Programmable TEMP. RANGE -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C PIN-PACKAGE 3x4 UCSP 3x4 UCSP 3x4 UCSP 10 MAX 10 MAX 10 MAX 3x4 UCSP 10 MAX UCSP MARK AAB AAL AAM -- -- -- AAC --
*Future Product Specification subject to change prior to release. Contact Factory for Availability. ________________________________________________________________ Maxim Integrated Products 1
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.
WCDMA Cellular Phone 600mA Buck Regulators MAX1820/MAX1821
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 = +70C) 34 UCSP (derate 10.4mW/C above +70C) .............832mW 10-Pin MAX (derate 5.6mW/C above +70C) ...........444mW Operating Temperature Range ...........................-40C to +85C Junction Temperature ......................................................+150C Storage Temperature Range 34 UCSP .....................................................-40C to +150C 10-Pin MAX ..................................................-65C to +150C Lead Temperature (soldering, 10s) .................................+300C
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 = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER Input BATT Voltage Undervoltage Lockout Threshold Quiescent Current SYMBOL VIN VUVLO VBATT rising, 1% hysteresis SKIP = GND IQ SKIP = BATT, no switching SKIP = BATT, 1MHz switching Quiescent Current in Dropout Shutdown Supply Current ERROR AMPLIFIER OUT Voltage Accuracy (MAX1820) VREF = 1.932 0.005V, load = 0 to 600mA, SKIP = BATT or GND VOUT VREF = 0.227 0.005V, load = 0 to 30mA, SKIP = BATT, VBATT 4.2V 3.33 0.35 250 3.4 0.40 400 0.1 FB = COMP VFB = 1.4V 30 0.2 2.04 1.225 1.25 0.01 50 0.45 2.15 1 1.275 50 85 1.0 2.28 3.47 V 0.45 k A V nA S V V ISHDN SKIP = GND SKIP = BATT, no switching SHDN = GND CONDITIONS MIN 2.6 2.20 2.35 180 450 3300 530 550 0.1 1000 1000 6 A A TYP MAX 5.5 2.55 300 2000 A UNITS V V
OUT Input Resistance (MAX1820) REF Input Current (MAX1820) FB Voltage Accuracy (MAX1821) FB Input Current (MAX1821) Transconductance COMP Clamp Low Voltage COMP Clamp High Voltage
ROUT IREF VFB IFB gm
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WCDMA Cellular Phone 600mA Buck Regulators
ELECTRICAL CHARACTERISTICS (continued)
(VBATT = 3.6V, SHDN = BATT, SKIP = SYNC = GND, VREF = 1.25V (MAX1820 only), TA = 0C to +85C, unless otherwise noted. Typical values are at TA = +25C.)
PARAMETER CONTROLLER P-Channel On-Resistance N-Channel On-Resistance Current-Sense Transresistance P-Channel Current-Limit Threshold P-Channel Pulse-Skipping Current Threshold N-Channel Current-Limit Threshold LX Leakage Current Maximum Duty Cycle Minimum Duty Cycle SYNC AND OSCILLATOR SYNC Divide Ratio (MAX1820X) SYNC Capture Range (MAX1820X) SYNC Leakage Current SYNC Divide Ratio (MAX1820Y) SYNC Capture Range (MAX1820Y) Internal Oscillator Frequency (MAX1820Z, MAX1821) LOGIC INPUTS (SKIP, SHDN) Logic Input High Logic Input Low Logic Input Current VIH VIL -1 0.1 1.6 0.4 1 V V A fOSC ISYNC SYNC = sine wave, SYNC input = 200mVp-p SYNC = sine wave, SYNC input = 800mVp-p SYNC = sine wave, AC-coupled, SYNC input = 500mVp-p VSYNC = 1V SYNC = sine wave, SYNC input = 200mVp-p SYNC = sine wave, SYNC input = 800mVp-p SYNC = sine wave, AC-coupled, SYNC input = 500mVp-p SYNC = GND 13 13 10 -1 18 18 15 0.8 19.8 1 13 13 13 16 +1 18 18 21 1.2 Hz/Hz MHz A Hz/Hz MHz MHz ILX dutyMAX dutyMIN SKIP = GND SKIP = BATT, VBATT = 4.2V PRDS NRDS RCS Duty factor = 100% SKIP = GND SKIP = BATT SKIP = GND VBATT = 5.5V, LX = GND or BATT ILX = 180mA, VBATT = 3.6V ILX = 180mA, VBATT = 2.6V ILX = 180mA, VBATT = 3.6V ILX = 180mA, VBATT = 2.6V 0.5 0.75 0.04 -1.6 0.02 -1 100 0 10 0.15 0.2 0.2 0.3 0.75 1.2 0.13 -0.85 0.08 0.1 0.9 1.55 0.24 -0.45 0.14 1 0.35 0.3 V/A A A A A % % SYMBOL CONDITIONS MIN TYP MAX UNITS
MAX1820/MAX1821
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WCDMA Cellular Phone 600mA Buck Regulators MAX1820/MAX1821
ELECTRICAL CHARACTERISTICS
(VBATT = 3.6V, SHDN = BATT, SKIP = SYNC = GND, VREF = 1.25V (MAX1820 only), TA = -40C to +85C, unless otherwise noted.) (Note 1)
PARAMETER Input BATT Voltage Undervoltage Lockout Threshold Quiescent Current Quiescent Current in Dropout Shutdown Supply Current ERROR AMPLIFIER OUT Voltage Accuracy (MAX1820) VREF = 1.932 0.005V, load = 0 to 600mA, SKIP = BATT or GND VREF = 0.227 0.005V, load = 0 to 30mA, SKIP = BATT, VBATT 4.2V 3.33 0.35 250 1 FB = COMP VFB = 1.4V 30 0.2 2.04 PRDS NRDS RCS Duty factor = 100% SKIP = GND SKIP = BATT SKIP = GND ILX = 180mA, VBATT = 3.6V ILX = 180mA, VBATT = 3.6V 0.5 0.75 0.04 -1.6 0.01 1.225 1.275 50 85 1.0 2.28 0.3 0.35 0.9 1.55 0.24 -0.45 0.14 3.47 V 0.45 k A V nA S V V V/A A A A ISHDN SYMBOL VIN VUVLO IQ VBATT rising, 1% hysteresis SKIP = GND SKIP = BATT, no switching SKIP = GND SKIP = BATT, no switching SHDN = GND CONDITIONS MIN 2.6 2.15 MAX 5.5 2.55 300 2000 1000 1000 6 UNITS V V A A A
VOUT
OUT Input Resistance (MAX1820) REF Input Current (MAX1820) FB Voltage Accuracy (MAX1821) FB Input Current (MAX1821) Transconductance COMP Clamp Low Voltage COMP Clamp High Voltage CONTROLLER P-Channel On-Resistance N-Channel On-Resistance Current-Sense Transresistance P-Channel Current-Limit Threshold P-Channel Pulse-Skipping Current Threshold N-Channel Current-Limit Threshold
ROUT IREF VFB IFB gm
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WCDMA Cellular Phone 600mA Buck Regulators
ELECTRICAL CHARACTERISTICS (continued)
(VBATT = 3.6V, SHDN = BATT, SKIP = SYNC = GND, VREF = 1.25V (MAX1820 only), TA = -40C to +85C, unless otherwise noted.) (Note 1)
PARAMETER LX Leakage Current Maximum Duty Cycle Minimum Duty Cycle SYNC AND OSCILLATOR SYNC Divide Ratio (MAX1820X) SYNC Capture Range (MAX1820X) SYNC Divide Ratio (MAX1820Y) SYNC Capture Range (MAX1820Y) SYNC Leakage Current Internal Oscillator Frequency (MAX1820Z, MAX1821) LOGIC INPUTS (SKIP, SHDN) Logic Input High Logic Input Low Logic Input Current VIH VIL 1.6 0.4 1 V V A ISYNC fOSC SYNC = sine wave, SYNC input = 200mVp-p SYNC = sine wave, SYNC input = 800mVp-p SYNC = sine wave, AC-coupled, SYNC input = 500mVp-p SYNC = sine wave, SYNC input = 200mVp-p SYNC = sine wave, SYNC input = 800mVp-p SYNC = sine wave, AC-coupled, SYNC input = 500mVp-p VSYNC = IV SYNC = GND 13 13 10 18 18 15 -1 0.8 13 13 16 18 18 21 +1 1.2 Hz/Hz MHz Hz/Hz MHz A MHz SYMBOL ILX dutyMAX dutyMIN SKIP = GND SKIP = BATT, VBATT = 4.2V CONDITIONS VBATT = 5.5V, LX = GND or BATT MIN -1 100 0 10 MAX 1 UNITS A % %
MAX1820/MAX1821
Note 1: Specifications to -40C are guaranteed by design and not subject to production test.
Typical Operating Characteristics
(TA = +25C, unless otherwise noted.)
EFFICIENCY vs. OUTPUT VOLTAGE (NORMAL MODE, VIN = 3.6V)
MAX1820/21 toc01
EFFICIENCY vs. OUTPUT VOLTAGE (PWM MODE, VIN = 3.6V)
RLOAD = 5 90 EFFICIENCY (%) 80 70 60 50 40 RLOAD = 10 EFFICIENCY (%) RLOAD = 15
MAX1820/21 toc02
EFFICIENCY vs. INPUT VOLTAGE NORMAL MODE, RLOAD = 10
90 80 70 60 50 40 30 20 10 VOUT = 0.4V VOUT = 1.8V VOUT = 3.4V
MAX1820/21 toc03
100 RLOAD = 5 90 EFFICIENCY (%) 80 70 60 50 40 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 RLOAD = 10 RLOAD = 15
100
100
0 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 2.0 2.5 3.0 3.5 4.0 VIN (V) 4.5 5.0 5.5 6.0 OUTPUT VOLTAGE (V)
4.0
OUTPUT VOLTAGE (V)
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WCDMA Cellular Phone 600mA Buck Regulators MAX1820/MAX1821
Typical Operating Characteristics (continued)
(TA = +25C, unless otherwise noted.)
MAX1821 EFFICIENCY vs. LOAD CURRENT (VOUT = 3.3V)
MAX1820/21 toc04
MAX1821 EFFICIENCY vs. LOAD CURRENT (VOUT = 2.5V)
MAX1820/21 toc05
MAX1821 EFFICIENCY vs. LOAD CURRENT (VOUT = 1.5V)
90 80 EFFICIENCY (%) 70 60 50 40 30 20 VIN = +2.7V VIN = +3.6V VIN = +5.0V VIN = +2.7V VIN = +3.6V VIN = +5.0V SKIP = GND (DASHED LINE) SKIP = BATT (SOLID LINE) 1 10 100 1000
MAX1820/21 toc06
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 1 10 100 SKIP = GND (DASHED LINE) SKIP = BATT (SOLID LINE) VIN = +5.0V VIN = +3.6V VIN = +3.6V VIN = +5.0V
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 SKIP = GND (DASHED LINE) SKIP = BATT (SOLID LINE) 1 10 100 VIN = +2.7V VIN = +3.6V VIN = +5.0V VIN = +2.7V VIN = +3.6V VIN = +5.0V
100
10 0 1000
1000
LOAD CURRENT (mA)
LOAD CURRENT (mA)
LOAD CURRENT (mA)
DROPOUT VOLTAGE vs. LOAD CURRENT
MAX1820/21 toc07
SUPPLY CURRENT vs. SUPPLY VOLTAGE
MAX1820/21 toc08
SUPPLY CURRENT vs. SUPPLY VOLTAGE
200 180 SUPPLY CURRENT (A) 160 140 120 100 80 60 VOUT = 1.5V SKIP = GND 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
MAX1820/21 toc09
140 120 DROPOUT VOLTAGE (mV) 100 80 60 40 20 0 0 100 200 300 400 500 VOUT = 3.4V RL = 57m
9 8 SUPPLY CURRENT (mA) 7 6 5 4 3 2 1 0 VOUT = 1.5V SKIP = BATT 2.0 2.5 3.0 3.5 4.0 4.5 5.0
220
40 20 5.5
600
LOAD CURRENT (mA)
SUPPLY VOLTAGE (V)
SUPPLY VOLTAGE (V)
HEAVY-LOAD SWITCHING WAVEFORMS (VIN = 3.8V, VOUT = 3.4V, ILOAD = 600mA, SKIP = BATT)
MAX1820/21 toc10
MEDIUM-LOAD SWITCHING WAVEFORMS (VIN = 3.8V, VOUT = 1.8V, ILOAD = 300mA, SKIP = BATT)
MAX1820/21 toc11
LIGHT-LOAD PWM SWITCHING WAVEFORMS (VIN = 3.8V, VOUT = 0.45V, ILOAD = 30mA, SKIP = BATT)
MAX1820/21 toc12
A A B B C A B
C
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, 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
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WCDMA Cellular Phone 600mA Buck Regulators MAX1820/MAX1821
Typical Operating Characteristics (continued)
(TA = +25C, unless otherwise noted.)
LIGHT-LOAD SKIP-SWITCHING WAVEFORMS (VIN = 4.2V, VOUT = 1.5V, LOAD = 30mA, SKIP = GND)
MAX1820/21 toc13
EXITING AND ENTERING SHUTDOWN (VIN = 3.6V, VOUT = 3.4V, RLOAD = 15)
MAX1820/21 toc14
A
VSHDN 5V/div
B VOUT 2V/div C IBATT 0.5A/div 2s/div A: VLX, 5V/div B: INDUCTOR CURRENT, 500mA/div C: VOUT (AC-COUPLED), 20mV/div 2ms/div
LOAD TRANSIENT (ILOAD = 20mA TO 420mA, VOUT = 1.5V, VIN = 3.6V, SKIP = BATT)
MAX1820/21 toc15
LOAD TRANSIENT (ILOAD = 20mA TO 420mA, VOUT = 1.5V, VIN = 3.6V, SKIP = GND)
MAX1820/21 toc16
IOUT 200mA/div
IOUT 200mA/div
VOUT AC-COUPLED 100mV/div COUT = 10F 40s/div
VOUT AC-COUPLED 100mV/div COUT = 10F 40s/div
MAX1820 REF TRANSIENT (VREF = 0.23V TO 1.932V, RLOAD = 10, VIN = 3.6V, SKIP = BATT)
MAX1820/21 toc17
LINE TRANSIENT (VIN = 3.6V TO 4.0V, VOUT = 1.5V, ILOAD = 300mA)
MAX1820/21 toc18
VREF 1V/div
VIN 200mV/div
VOUT 1V/div
VOUT AC-COUPLED 200mV/div COUT = 10F 20s/div 40s/div
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WCDMA Cellular Phone 600mA Buck Regulators MAX1820/MAX1821
Typical Operating Characteristics (continued)
(TA = +25C, unless otherwise noted.)
OUTPUT SWITCHING HARMONICS vs. FREQUENCY (VIN = 3.8V, VOUT = 3.4V, ILOAD = 600mA)
MAX1820/21 toc19
OUTPUT SWITCHING HARMONICS vs. FREQUENCY (VIN = 3.8V, VOUT = 1.8V, ILOAD = 300mA)
1.6 HARMONICS (mVRMS)
MAX1820/21 toc20
1.6 HARMONICS (mVRMS)
1.2
1.2
0.8
0.8
0.4
0.4
0 0.1 1 FREQUENCY (MHz) 10
0 0.1 1 FREQUENCY (MHz) 10
OUTPUT SWITCHING HARMONICS vs. FREQUENCY (VIN = 4.2V, VOUT = 0.4V, ILOAD = 30mA)
MAX1820/21 toc21
OUTPUT NOISE (VIN = 3.6V, VOUT = 1.8V, IOUT = 300mA)
4.0
MAX1820/21 toc22
1.6 HARMONICS (mVRMS)
NOISE (V/Hz)
1.2
3.0
0.8
2.0
0.4
1.0
0 0.1 1 FREQUENCY (MHz) 10
0 0.1 1 10 FREQUENCY (MHz) 100 250
Pin Description
PIN MAX1820 UCSP A1 MAX1820 MAX 1 MAX1821 UCSP A1 MAX1821 MAX 1 NAME FUNCTION
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. Compensation. Typically, connect an 82k (for MAX1821) or 43k (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.
A2 A3
2 3
A2 --
2 --
COMP OUT
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WCDMA Cellular Phone 600mA Buck Regulators
Pin Description (continued)
PIN MAX1820 UCSP -- MAX1820 MAX -- MAX1821 UCSP A3 MAX1821 MAX 3 NAME FUNCTION
MAX1820/MAX1821
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. 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. Internal Reference Bypass. Connect a 0.047F capacitor from REF to GND. Ground Power Ground Inductor Connection. LX connects to the drains of the internal power MOSFETs. LX is high impedance in shutdown mode. Supply Voltage Input. Connect BATT to a 2.6V to 5.5V source. Bypass BATT to PGND with a low-ESR 10F capacitor. Active-Low, Shutdown Control Input 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.
A4
4
--
--
REF
-- B4 C4 C3 C2 C1
-- 5 6 7 8 9
A4 B4 C4 C3 C2 C1
4 5 6 7 8 9
REF GND PGND LX BATT SHDN
B1
10
B1
10
SYNC
_______________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 <30s. 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
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 0.1A (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
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WCDMA Cellular Phone 600mA Buck Regulators MAX1820/MAX1821
BATT
VOLTAGE REFERENCE GND
1.25V
TO IC BIAS 0.45V TO 2.15V
ERROR SIGNAL SLOPE COMP CURRENT SENSE
PWM COMPARATOR
OUT REF
TRANSIMPEDANCE ERROR AMP CLAMP
SKIP COMPARATOR SKIP THRESHOLD
PWM CONTROL AND SKIP LOGIC
LX
PGND /13 OR /18 COMP SYNC 1MHz OSCILLATOR SHDN SKIP PWM COMPARATOR
MAX1820
Figure 1. MAX1820 Simplified Functional Diagram (No SYNC for MAX1820Z)
BATT
VOLTAGE REFERENCE GND
1.25V
TO IC BIAS 0.45V TO 2.15V
ERROR SIGNAL SLOPE COMP CURRENT SENSE
FB REF
TRANSIMPEDANCE ERROR AMP CLAMP
SKIP COMPARATOR SKIP THRESHOLD
PWM CONTROL AND SKIP LOGIC
LX
PGND 1MHz OSCILLATOR COMP SYNC SHDN SKIP
MAX1821
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
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WCDMA Cellular Phone 600mA Buck Regulators MAX1820/MAX1821
VIN = 2.6V TO 5.5V BATT 10F 0.1F SHDN SYNC FB REF 0.047F SKIP COMP GND RC 82k C1 330pF C2* 1pF R2 30k MAX1821 PGND LX 4.7F R1 6k 4.7H VOUT = 1.5V
* 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 180A. 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.
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WCDMA Cellular Phone 600mA Buck Regulators MAX1820/MAX1821
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.
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.
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 6s (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.
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 0.1A, 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.
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
___ _____ _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
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WCDMA Cellular Phone 600mA Buck Regulators
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 <30s. 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:
1.0 0.4 30 300 WCDMA PA SUPPLY CURRENT (mA) 600
MAX1820/MAX1821
3.4 WCDMA PA SUPPLY VOLTAGE (V) 3.0
VO(MAX) 1 R2 1 C1 = gm x IO(MAX) RCS R1+R2 2 x x fC Resistors R1 and R2 are internal to the MAX1820; use R1 = 151k and R2 = 199k 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 = 50s, RCS = 0.75, C1 is evaluated as: 199k 3.4V 1 C1 TION 3 = 50s x 0.6A 0.75 151k +199k 1 x = 341pF 2 x 3.14 x 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:
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 5k to 30k range. R1 is then given by: V R1 = R2 OUT - 1 VFB 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 x x RL x COUT = 1 2 x x RC x C1
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WCDMA Cellular Phone 600mA Buck Regulators MAX1820/MAX1821
Table 1. Suggested Inductors
MANUFACTURER Sumida Coilcraft Sumida Sumida Coilcraft PART NUMBER CDRH4D18-4R7 DO1606 CR43 CDRH5D18-4R1 LPT1606-472 INDUCTANCE (H) 4.7 4.7 4.7 4.1 4.7 ESR (m) 125 120 108.7 57 240 (max) SATURATION CURRENT (A) 0.84 1.2 1.15 1.95 1.2 DIMENSIONS (mm) 5x5x2 5.3x5.3x2 4.5x4x3.5 5.5x5.5x2 6.5x5.3x2.0
Solving for RC gives: RC = RL x COUT 3.4V 4.7F = = 80.8k 0.6A 330pF C1
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 10C temperature rise at the peak ripple current.
5) Calculate the high-frequency compensation pole to cancel the zero created by the output capacitor's equivalent series resistance (ESR): 1 1 = 2 x x RESR x COUT 2 x x R3 x C2 Solving for C2 gives: C2 = 4.7F x 0.01 RESR x COUT = = 0.55pF R3 80.8k
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 4H to 6H inductor with a saturation current of at least 800mA is recommended for most applications. For best efficiency, the inductor's DC resistance should be <200m, 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 x LIR x IOUT(MAX) x OSC
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 x ESR + (2 x OSC x 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
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
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WCDMA Cellular Phone 600mA Buck Regulators
specifications for proper capacitor derating. Avoid Y5V and Z5U dielectric types due to their huge voltage and temperature coefficients of capacitance and ESR.
MAX1820/MAX1821
Table 2. Capacitor Selection
CAPACITOR CBATT COUT (MAX1820) COUT (MAX1821) CAPACITOR VALUE (F) 4.7 to 10 2.2 to 4.7 4.7 to 10 ESR (m) <150 <50 <150 CAPACITOR TYPE Ceramic Ceramic Ceramic
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 3. Component Manufacturers
MANUFACTURER Coilcraft Kemet Panasonic Sumida Taiyo Yuden USA PHONE NUMBER 847-639-6400 408-986-0424 847-468-5624 847-956-0666 408-573-4150 WEBSITE www.coilcraft.com www.kemet.com www.panasonic.com www.sumida.com www.t-yuden.com
____________________Chip Information
TRANSISTOR COUNT: 2722
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WCDMA Cellular Phone 600mA Buck Regulators MAX1820/MAX1821
Typical Operating Circuits (continued)
INPUT 2.6V TO 5.5V BATT SHDN LX PGND FB MAX1821 COMP SYNC OUTPUT 1.25V TO 5.5V
REF SKIP GND
Pin Configurations
TOP VIEW AFTER ASSEMBLED ON PC BOARD (BUMPS AT THE BOTTOM) 1 SKIP A1 2 COMP A2 3 OUT (FB) A3 4 REF A4 SKIP 1 COMP OUT (FB) SYNC B B1 GND B4 REF GND 2 3 4 5 10 SYNC 9 SHDN BATT LX PGND
TOP VIEW
A
MAX1820 MAX1821
8 7 6
SHDN C C1
BATT C2
LX C3
PGND C4
MAX
( ) ARE FOR MAX1821 ONLY.
( ) ARE FOR MAX1821 ONLY.
UCSP
16
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WCDMA Cellular Phone 600mA Buck Regulators
Package Information
12L, USPC.EPS
MAX1820/MAX1821
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17
WCDMA Cellular Phone 600mA Buck Regulators MAX1820/MAX1821
Package Information (continued)
10LUMAX.EPS
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 (c) 2001 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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