MAXIM MAX1821EBC

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.