MAX1821EUB ,WCDMA Cellular Phone 600mA Buck RegulatorsMAX1820/MAX182119-2011; Rev 3; 4/05WCDMA Cellular Phone 600mABuck Regulators
MAX1821EUB+T ,WCDMA Cellular Phone 600mA Buck RegulatorsELECTRICAL CHARACTERISTICS(V = 3.6V, SHDN = BATT, SKIP = SYNC = GND, V = 1.25V (MAX1820 only), T = ..
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MAX1820XEBC-MAX1820XEBC+T-MAX1820XEUB-MAX1820XEUB+T-MAX1820ZEBC-MAX1820ZEBC+T-MAX1821EUB-MAX1821EUB+T-MAX1821EUB-T
WCDMA Cellular Phone 600mA Buck Regulators
General DescriptionThe MAX1820/MAX1821 low-dropout, pulse-width-mod-
ulated (PWM) DC-DC buck regulators are optimized to
provide power to the power amplifier (PA) in WCDMA
cell phones; 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 guar-
anteed output current is 600mA; 1MHz PWM switching
allows for small external components, while skip mode
reduces quiescent current to 180µA with light loads.
The MAX1820 is dynamically controlled to provide vary-
ing 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.
________________________ApplicationsWCDMA Cell Phone Power Amplifiers
PDA, Palmtop, and Notebook Computers
Microprocessor Core Supplies
Digital Cameras
PCMCIA and Network Cards
Hand-Held Instruments
FeaturesDynamically Adjustable Output from 0.4V to 3.4V
(MAX1820)Programmable Output from 1.25V to 5.5V
(MAX1821)SYNC to 13MHz External Clock (MAX1820X)SYNC to 19.8MHz External Clock (MAX1820Y)NO SYNC, Internal 1MHz Oscillator (MAX1820Z)Low Quiescent Current
180µA (typ) in Skip Mode
0.1µA (typ) in Shutdown ModeNo External Schottky Diode Required600mA Guaranteed Output Current0% to 100% Duty-Cycle Operation150mV Dropout at 600mA Load (Including RDC
of External Inductor)µMAX or UCSP Packaging
MAX1820/MAX1821
WCDMA Cellular Phone 600mA
Buck Regulators19-2011; Rev 3; 4/05
EVALUATION KIT
AVAILABLE
Ordering Information
PARTSYNC
FREQ (MHz)OUTPUT VOLTAGETEMP RANGEPIN-PACKAGEUCSP MARK
MAX1820ZEBC*No SyncDynamic-40°C to +85°C3 ✕ 4 UCSPAAB
MAX1820YEBC*19.8Dynamic-40°C to +85°C3 ✕ 4 UCSPAAL
MAX1820XEBC*13Dynamic-40°C to +85°C3 ✕ 4 UCSPAAM
MAX1820ZEUBNo SyncDynamic-40°C to +85°C10 µMAX—
MAX1820YEUB19.8Dynamic-40°C to +85°C10 µMAX—
MAX1820XEUB13Dynamic-40°C to +85°C10 µMAX—
Pin Configurations appear at end of data sheet.
Typical Operating Circuits continued at end of data sheet.SYNC
GND
SHDN
BATT
PGND
COMP
VOUT CONTROL
DACREF
OUT
13MHz
19.8MHz
MAX1820
SKIP
4.7μH
4.7μF
INPUT
2.6V TO
5.5V
DYNAMIC
OUTPUT
0.4V TO 3.4V
Typical Operating Circuits*UCSP reliability is integrally linked to the user’s assembly methods, circuit board material, and environment. See the UCSP Reliability
Notice in the UCSP Reliability section of this data sheet for more information.
UCSP is a trademark of Maxim Integrated Products, Inc.
µMAX is a registered trademark of Maxim Integrated Products, Inc.
Ordering Information continued at end of data sheet.
MAX1820/MAX1821
WCDMA Cellular Phone 600mA
Buck Regulators
ABSOLUTE MAXIMUM RATINGSStresses 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.
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 (derate10.4mW/°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 Ranges
3 ✕4 UCSP ....................................................-40°C to +150°C
10-Pin µMAX..................................................-65°C to +150°C
Solder Profile (UCSP)......................................................(Note 1)
Lead Temperature (soldering, 10s).................................+300°C
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITSInput BATT VoltageVIN2.65.5V
Undervoltage Lockout
ThresholdVUVLOVBATT rising, 1% hysteresis2.202.352.55V
SKIP = GND (MAX1820Z/MAX1821)180300
SKIP = BATT, no switching4502000
SKIP = GND (MAX1820Y, MAX1820X, and
MAX1821X)240360Quiescent CurrentIQ
SKIP = BATT, 1MHz switching3300
SKIP = GND5301000Quiescent Current in DropoutSKIP = BATT, no switching5501000µA
Shutdown Supply CurrentISHDNSHDN = GND0.16µA
ERROR AMPLIFIERVREF = 1.932 ±0.005V, load = 0 to 600mA,
SKIP = BATT or GND3.333.43.47
OUT Voltage Accuracy
(MAX1820)VOUT
VREF = 0.227 ±0.005V, load = 0 to 30mA,
SKIP = BATT, VBATT ≤ 4.2V0.350.400.45
OUT Input Resistance
(MAX1820)ROUT250400kΩ
REF Input Current (MAX1820)IREF0.11µA
FB Voltage Accuracy
(MAX1821)VFBFB = COMP1.2251.251.275V
FB Input Current (MAX1821)IFBVFB = 1.4V0.0150nA
Transconductancegm305085µS
COMP Clamp Low Voltage0.20.451.0V
COMP Clamp High Voltage2.042.152.28V
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.) (Note 2)
Note 1:For UCSP solder profile information, visit /1st_pages/UCSP.htm.
MAX1820/MAX1821
WCDMA Cellular Phone 600mA
Buck Regulators
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITS
CONTROLLERILX = 180mA, VBATT = 3.6V0.150.3P-Channel On-ResistancePRDSILX = 180mA, VBATT = 2.6V0.2Ω
ILX = 180mA, VBATT = 3.6V0.20.35N-Channel On-ResistanceNRDSILX = 180mA, VBATT = 2.6V0.3Ω
Current-Sense TransresistanceRCS0.250.500.75V/A
P-Channel Current-Limit
ThresholdDuty factor = 100%0.751.21.55A
P-Channel Pulse-Skipping
Current ThresholdSKIP = GND0.040.130.24A
SKIP = BATT-1.6-0.85-0.45N-Channel Current-Limit
ThresholdSKIP = GND0.020.080.14A
LX Leakage CurrentILXVBATT = 5.5V, LX = GND or BATT-10.11µA
Maximum Duty CycledutyMAX100%
SKIP = GND0Minimum Duty CycledutyMINSKIP = BATT, VBATT = 4.2VP-P10%
SYNC AND OSCILLATORY N C = si ne w ave, S Y N C i np ut = 200m V P-P1313SYNC Divide Ratio
(MAX1820X)S Y N C = si ne w ave, S Y N C i np ut = 800m V P-P1313Hz/Hz
SYNC Capture Range
(MAX1820X)
SYNC = sine wave, AC-coupled,
SYNC input = 500mVP-P101316MHz
VSYNC = 1V (MAX1820Z, MAX1821)-1+1
SYNC Leakage Current
FrequencyISYNCVSYNC = 1V (MAX1820X, MAX1820Y, and
MAX1821X)-5+5µAY N C = si ne w ave, S Y N C i np ut = 200m V P-P1818SYNC Divide Ratio
(MAX1820Y)S Y N C = si ne w ave, S Y N C i np ut = 800m V P-P1818Hz/Hz
SYNC Capture Range
(MAX1820Y)
SYNC = sine wave, AC-coupled,
SYNC input = 500mVP-P1519.821MHz
Internal Oscillator Frequency
(MAX1820Z, MAX1821)fOSCSYNC = GND0.811.2MHz
LOGIC INPUTS (SKIP, SHDN)Logic Input HighVIH1.6V
Logic Input LowVIL0.4V
Logic Input Current-10.11µA
ELECTRICAL CHARACTERISTICS (continued)(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.) (Note 2)
MAX1820/MAX1821
WCDMA Cellular Phone 600mA
Buck Regulators
PARAMETERSYMBOLCONDITIONSMINMAXUNITSInput BATT VoltageVIN2.65.5V
Undervoltage Lockout
ThresholdVUVLOVBATT rising, 1% hysteresis2.152.55V
SKIP = GND (MAX1820Z, MAX1821)300
SKIP = GND (MAX1820X, MAX1820Y, and
MAX1821X)360Quiescent CurrentIQ
SKIP = BATT, no switching2000
SKIP = GND1000Quiescent Current in DropoutSKIP = BATT, no switching1000µA
Shutdown Supply CurrentISHDNSHDN = GND6µA
ERROR AMPLIFIERVREF = 1.932 ±0.005V, load = 0 to 600mA,
SKIP = BATT or GND3.333.47
OUT Voltage Accuracy
(MAX1820)VOUT
VREF = 0.227 ±0.005V, load = 0 to 30mA,
SKIP = BATT, VBATT ≤ 4.2V0.350.45
OUT Input Resistance
(MAX1820)ROUT250kΩ
REF Input Current (MAX1820)IREF1µA
FB Voltage Accuracy
(MAX1821)VFBFB = COMP1.2251.275V
FB Input Current (MAX1821)IFBVFB = 1.4V50nA
Transconductancegm3085µS
COMP Clamp Low Voltage0.21.0V
COMP Clamp High Voltage2.042.28V
CONTROLLERP-Channel On-ResistancePRDSILX = 180mA, VBATT = 3.6V0.3Ω
N-Channel On-ResistanceNRDSILX = 180mA, VBATT = 3.6V0.35Ω
Current-Sense TransresistanceRCS0.250.75V/A
P-Channel Current-Limit
ThresholdDuty factor = 100%0.751.55A
P-Channel Pulse-Skipping
Current ThresholdSKIP = GND0.040.24A
SKIP = BATT-1.6-0.45N-Channel Current-Limit
ThresholdSKIP = GND0.010.14A
ELECTRICAL CHARACTERISTICS(VBATT = 3.6V, SHDN= BATT, SKIP= SYNC = GND, VREF= 1.25V (MAX1820 only), TA = -40°C to +85°C, unless otherwise noted.)
(Notes 2, 3)
MAX1820/MAX1821
WCDMA Cellular Phone 600mA
Buck Regulators
Note 2:Limits are 100% production tested at TA= +25°C for UCSP parts. Limits over the entire operating temperature range are
guaranteed by design and characterization but are not production tested.
Note 3:Specifications to -40°C are guaranteed by design and not subject to production test.
PARAMETERSYMBOLCONDITIONSMINMAXUNITSLX Leakage CurrentILXVBATT = 5.5V, LX = GND or BATT-11µA
Maximum Duty CycledutyMAX100%
SKIP = GND0Minimum Duty CycledutyMINSKIP = BATT, VBATT = 4.2V10%
SYNC AND OSCILLATORY N C = si ne w ave, S Y N C i np ut = 200m V P-P1313SYNC Divide Ratio
(MAX1820X)S Y N C = si ne w ave, S Y N C i np ut = 800m V P-P1313Hz/Hz
SYNC Capture Range
(MAX1820X)
SYNC = sine wave, AC-coupled,
SYNC input = 500mVP-P1016MHzY N C = si ne w ave, S Y N C i np ut = 200m V P-P1818SYNC Divide Ratio
(MAX1820Y)S Y N C = si ne w ave, S Y N C i np ut = 800m V P-P1818Hz/Hz
SYNC Capture Range
(MAX1820Y)
SYNC = sine wave, AC-coupled,
SYNC input = 500mVP-P1521MHzSYNC = IV ( M AX 1820Z , M AX 1821) -1+1
SYNC Leakage CurrentISYNCV SYNC = IV ( M AX 1820X , M AX 1820Y , and AX 1821X ) -5+5µA
Internal Oscillator Frequency
(MAX1820Z, MAX1821)fOSCSYNC = GND0.81.2MHz
LOGIC INPUTS (SKIP, SHDN)Logic Input HighVIH1.6V
Logic Input LowVIL0.4V
Logic Input Current1µA
ELECTRICAL CHARACTERISTICS (continued)(VBATT = 3.6V, SHDN= BATT, SKIP= SYNC = GND, VREF= 1.25V (MAX1820 only), TA = -40°C to +85°C, unless otherwise noted.)
(Notes 2, 3)
EFFICIENCY vs. OUTPUT VOLTAGE
(NORMAL MODE, VIN = 3.6V)
MAX1820/21 toc01
OUTPUT VOLTAGE (V)
EFFICIENCY (%)
RLOAD = 10Ω
RLOAD = 15Ω
RLOAD = 5Ω
EFFICIENCY vs. OUTPUT VOLTAGE
(PWM MODE, VIN = 3.6V)
MAX1820/21 toc02
OUTPUT VOLTAGE (V)
EFFICIENCY (%)
RLOAD = 10Ω
RLOAD = 15Ω
RLOAD = 5Ω
MAX1820/21 toc03
VIN (V)
EFFICIENCY (%)
EFFICIENCY vs. INPUT VOLTAGE
NORMAL MODE, RLOAD = 10ΩVOUT = 1.8VVOUT = 3.4V
VOUT = 0.4V
Typical Operating Characteristics(TA = +25°C, unless otherwise noted.)
MAX1820/MAX1821
WCDMA Cellular Phone 600mA
Buck Regulators
Typical Operating Characteristics (continued)(TA = +25°C, unless otherwise noted.)100010010
MAX1821 EFFICIENCY vs. LOAD CURRENT
(VOUT = 3.3V)MAX1820/21 toc04
LOAD CURRENT (mA)
EFFICIENCY (%)VIN = 5.0V
VIN = 3.6V
VIN = 5.0V
SKIP = GND (DASHED LINE)
SKIP = BATT (SOLID LINE)
VIN = 3.6V100010010
MAX1821 EFFICIENCY vs. LOAD CURRENT
(VOUT = 2.5V)MAX1820/21 toc05
LOAD CURRENT (mA)
EFFICIENCY (%)
VIN = 5.0V
VIN = 3.6V
VIN = 3.6V
VIN = 5.0V
SKIP = GND (DASHED LINE)
SKIP = BATT (SOLID LINE)
VIN = 2.7V
VIN = 2.7V100010010
MAX1821 EFFICIENCY vs. LOAD CURRENT
(VOUT = 1.5V)MAX1820/21 toc06
LOAD CURRENT (mA)
EFFICIENCY (%)
VIN = 5.0V
VIN = 3.6V
SKIP = GND (DASHED LINE)
SKIP = BATT (SOLID LINE)
VIN = 2.7V
DROPOUT VOLTAGE vs. LOAD CURRENT
MAX1820/21 toc07
LOAD CURRENT (mA)
DROPOUT VOLTAGE (mV)
VOUT = 3.4V
RL = 57mΩ
SUPPLY CURRENT vs. SUPPLY VOLTAGE
MAX1820/21 toc08
SUPPLY VOLTAGE (V)
SUPPLY CURRENT (mA)
VOUT = 1.5V
SKIP = BATT
2.02.53.03.5 4.04.55.05.5
SUPPLY CURRENT vs. SUPPLY VOLTAGE MAX1820/21 toc09
SUPPLY VOLTAGE (V)
SUPPLY CURRENT (
VOUT = 1.5V
SKIP = GND
MAX1820/21 toc10
400ns/div
A: VLX, 5V/div
B: INDUCTOR CURRENT, 500mA/div
HEAVY-LOAD SWITCHING WAVEFORMS
(VIN = 3.8V, VOUT = 3.4V,
ILOAD = 600mA, SKIP = BATT)MAX1820/21 toc11
400ns/div
A: VLX, 5V/div
B: INDUCTOR CURRENT, 500mA/div
MEDIUM-LOAD SWITCHING WAVEFORMS
(VIN = 3.8V, VOUT = 1.8V,
ILOAD = 300mA, SKIP = BATT)MAX1820/21 toc12
400ns/div
A: VLX, 5V/div
B: INDUCTOR CURRENT, 100mA/div
LIGHT-LOAD PWM SWITCHING WAVEFORMS
(VIN = 3.8V, VOUT = 0.45V,
ILOAD = 30mA, SKIP = BATT)
MAX1820/MAX1821
WCDMA Cellular Phone 600mA
Buck RegulatorsMAX1820/21 toc13
2μs/div
A: VLX, 5V/div
B: INDUCTOR CURRENT, 500mA/div
C: VOUT (AC-COUPLED), 20mV/div
LIGHT-LOAD SKIP-SWITCHING WAVEFORMS
(VIN = 4.2V, VOUT = 1.5V,
LOAD = 30mA, SKIP = GND)MAX1820/21 toc14
2ms/div
EXITING AND ENTERING SHUTDOWN
(VIN = 3.6V, VOUT = 3.4V, RLOAD = 15Ω)VOUT
2V/div
IBATT
0.5A/div
VSHDN
5V/div
MAX1820/21 toc15
40μs/div
LOAD TRANSIENT (ILOAD = 20mA TO 420mA,
VOUT = 1.5V, VIN = 3.6V, SKIP = BATT)IOUT
200mA/div
VOUT
AC-COUPLED
100mV/div
COUT = 10μF
MAX1820/21 toc16
40μs/div
LOAD TRANSIENT (ILOAD = 20mA TO 420mA,
VOUT = 1.5V, VIN = 3.6V, SKIP = GND)IOUT
200mA/div
VOUT
AC-COUPLED
100mV/div
COUT = 10μF
MAX1820/21 toc17
20μs/div
MAX1820
REF TRANSIENT (VREF = 0.23V TO 1.932V,
RLOAD = 10Ω, VIN = 3.6V, SKIP = BATT)VREF
1V/div
VOUT
1V/div
MAX1820/21 toc18
40μs/div
LINE TRANSIENT (VIN = 3.6V TO 4.0V,
VOUT = 1.5V, ILOAD = 300mA)VIN
200mV/div
VOUT
AC-COUPLED
20mV/div
COUT = 10μF
Typical Operating Characteristics (continued)(TA = +25°C, unless otherwise noted.)
MAX1820/MAX1821
WCDMA Cellular Phone 600mA
Buck RegulatorsOUTPUT SWITCHING HARMONICS
vs. FREQUENCY
(VIN = 3.8V, VOUT = 3.4V, ILOAD = 600mA)
MAX1820/21 toc19
FREQUENCY (MHz)
HARMONICS (mV
RMS
OUTPUT SWITCHING HARMONICS
vs. FREQUENCY
(VIN = 3.8V, VOUT = 1.8V, ILOAD = 300mA)
MAX1820/21 toc20
FREQUENCY (MHz)
HARMONICS (mV
RMS
Pin Description
PIN
MAX1820
UCSP
MAX1820
µMAX
MAX1821
UCSP
MAX1821
µMAX
NAMEFUNCTION1A11SKIP
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.2A22COMP
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.3——OUTOutput Voltage Sense Input. Connect OUT directly to the output.
OUTPUT SWITCHING HARMONICS
vs. FREQUENCY
(VIN = 4.2V, VOUT = 0.4V, ILOAD = 30mA)
MAX1820/21 toc21
FREQUENCY (MHz)
HARMONICS (mV
RMS
FREQUENCY (MHz)
OUTPUT NOISE (VIN = 3.6V,
VOUT = 1.8V, IOUT = 300mA)MAX1820/21 toc22
NOISE (
Hz)
Typical Operating Characteristics (continued)(TA = +25°C, unless otherwise noted.)
MAX1820/MAX1821
WCDMA Cellular Phone 600mA
Buck Regulators
_______________Detailed DescriptionThe MAX1820/MAX1821 PWM step-down DC-DC con-
verters 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 VBATT, and the MAX1820 is
specifically intended to power a linear 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 rec-
tifier that provides for high efficiency and eliminates the
need for an external Schottky diode. The normal operat-
ing 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 con-
stant frequency, regardless of load, to provide a well-
controlled 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 ControlThe 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 superi-
or 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
Pin Description (continued)
PIN
MAX1820
UCSP
MAX1820
µMAX
MAX1821
UCSP
MAX1821
µMAX
NAMEFUNCTION—A33FB
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.4——REF
External Reference Input. Connect REF to the output of a D/A
converter for dynamic adjustment of the output voltage. REF-to-
OUT gain is 1.76.—A44REFInternal Reference Bypass. Connect a 0.047µF capacitor from
REF to GND.5B45GNDGround6C46PGNDPower Ground7C37LXInductor Connection. LX connects to the drains of the internal
power MOSFETs. LX is high impedance in shutdown mode.8C28BATTSupply Voltage Input. Connect BATT to a 2.6V to 5.5V source.
Bypass BATT to PGND with a low-ESR 10µF capacitor.9C19SHDNActive-Low, Shutdown Control Input10B110SYNC
Clock Synchronization Input. Drive SYNC with a 13MHz
(MAX1820X, MAX1821X) 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.
