19-0767; Rev 2; 10/09
3A, 2MHz Step-Down Regulator
with Integrated Switches
General Description
The MAX8643A high-efficiency switching regulator
delivers up to 3A load current at output voltages from
0.6V to (0.9 x V
IN
). The IC operates from 2.35V to 3.6V,
making it ideal for on-board point-of-load and postregu-
lation applications. Total output error is less than ±1%
over load, line, and temperature.
The MAX8643A features fixed-frequency PWM mode
operation with a switching frequency range of 500kHz
to 2MHz set by an external resistor. High-frequency
operation allows for an all-ceramic capacitor design.
The high operating frequency also allows for small-size
external components.
The low-resistance on-chip nMOS switches ensure high
efficiency at heavy loads while minimizing critical induc-
tances, making the layout a much simpler task with
respect to discrete solutions. Following a simple layout
and footprint ensures first-pass success in new designs.
The MAX8643A comes with a high-bandwidth (> 14MHz)
voltage-error amplifier. The voltage-mode control archi-
tecture and the voltage-error amplifier permit a type III
compensation scheme to be utilized to achieve maxi-
mum loop bandwidth, up to 20% of the switching fre-
quency. High loop bandwidth provides fast transient
response, resulting in less required output capacitance
and allowing for all-ceramic capacitor designs.
The MAX8643A provides two three-state logic inputs to
select one of nine preset output voltages. The preset
output voltages allow customers to achieve ±1% out-
put-voltage accuracy without using expensive 0.1%
resistors. In addition, the output voltage can be set to
any customer value by either using two external resis-
tors at the feedback with 0.6V internal reference or
applying an external reference voltage to the REFIN
input. The MAX8643A offers programmable soft-start
time using one capacitor to reduce input inrush current.
The MAX8643A is available in a lead-free, 24-pin, 4mm
x 4mm thin QFN package.
Features
o
Internal 37mΩ R
DSON
MOSFETs
o
Continuous 3A Output Current
o
±1% Output Accuracy Over Load, Line,
and Temperature
o
Operates from 2.35V to 3.6V Supply
o
Adjustable Output from 0.6V to (0.9 x V
IN
)
o
Soft-Start Reduces Inrush Supply Current
o
500kHz to 2MHz Adjustable Switching Frequency
o
Compatible with Ceramic, Polymer, and
Electrolytic Output Capacitors
o
VID-Selectable Output Voltages
0.6, 0.7, 0.8, 1.0, 1.2, 1.5, 1.8, 2.0, and 2.5V
o
Fully Protected Against Overcurrent
and Overtemperature
o
Safe-Start into Prebiased Output
o
Sink/Source Current in DDR Applications
o
Lead-Free, 24-Pin, 4mm x 4mm Thin QFN Package
MAX8643A
Ordering Information
PART
MAX8643AETG+
TEMP RANGE
-40°C to +85°C
PIN-PACKAGE
24 Thin QFN-EP*
MAX8643AETG/V+
-40°C to +85°C
24 Thin QFN-EP*
+Denotes
a lead(Pb)-free/RoHS-compliant package.
/V Denotes an automotive qualified part.
*EP
= Exposed pad.
Typical Operating Circuit
INPUT
2.4V, 3.6V
IN
BST
EN
V
DD
MAX8643A
LX
OUT
OUTPUT
1.8V, 3A
Applications
POLs
ASIC/CPU/DSP Core and I/O Voltages
DDR Power Supplies
Base-Station Power Supplies
Telecom and Networking Power Supplies
RAID Control Power Supplies
PGND
CTL1
CTL2
FREQ
REFIN
SS
COMP
V
DD
FB
PREBIAS
GND
PWRGD
Pin Configuration appears at end of data sheet.
________________________________________________________________
Maxim Integrated Products
1
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,
or visit Maxim's website at www.maxim-ic.com.
3A, 2MHz Step-Down Regulator
with Integrated Switches
MAX8643A
ABSOLUTE MAXIMUM RATINGS
IN, V
DD
, PWRGD to GND ......................................-0.3V to +4.5V
COMP, FB, REFIN, OUT,
CTL_, EN, SS, FREQ to GND...................-0.3V to (V
DD
+ 0.3V)
LX Current (Note 1) .....................................................-4A to +4A
BST to LX..................................................................-0.3V to +4V
PGND to GND .......................................................-0.3V to +0.3V
Continuous Power Dissipation (T
A
= +70°C)
24-Pin TQFN-EP
(derated 27.8mW/°C above +70°C)........................2222.2mW
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature ......................................................+150°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
Note 1:
LX has internal clamp diodes to GND and IN. Applications that forward bias these diodes should take care not to exceed
the IC’s package power dissipation limits.
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
(V
IN
= V
DD
= 3.3V, V
FB
= 0.5V, T
A
= -40°C to +85°C. Typical values are at T
A
= +25°C, circuit of Figure 1, unless otherwise noted.) (Note 2)
PARAMETER
IN/V
DD
IN and V
DD
Voltage Range
IN Supply Current
V
DD
Supply Current
Total Shutdown Current from IN
and V
DD
V
DD
Undervoltage Lockout
Threshold
BST
BST Supply Current
PWM COMPARATOR
PWM Comparator Propagation
Delay
COMP
COMP Clamp Voltage, High
COMP Slew Rate
PWM Ramp Amplitude
COMP Shutdown Resistance
ERROR AMPLIFIER
Preset Output-Voltage Accuracy
FB Regulation Accuracy Using
External Resistors
FB to OUT Resistor
REFIN = SS
-1
Select
from
Table 1
0.600
8
+1
%
From COMP to GND, V
EN
= V
SS
= 0V
V
IN
= 2.35V to 3.6V
2
1.4
1
8
V
V/µs
V
Ω
10mV overdrive
20
ns
V
BST
= V
DD
= V
IN
= 3.6V,
V
LX
= 3.6V or 0V, V
EN
= 0V
T
A
= +25°C
T
A
= +85°C
10
5
µA
f
S
= 1MHz, no load
(includes gate-drive current)
f
S
= 1MHz
V
IN
= 2.5V
V
IN
= 3.3V
V
IN
= 2.5V
V
IN
= 3.3V
2.35
4
5.5
1.4
2
13
2
1.8
1.9
2
2.1
2.3
3.60
4.6
V
mA
mA
µA
V
µs
CONDITIONS
MIN
TYP
MAX
UNITS
V
IN
= V
DD
= V
BST
- V
LX
= 3.6V, V
EN
= 0V
V
DD
rising
LX starts/stops switching
V
DD
falling
Deglitching
CTL1 = CTL2 = GND
All VID settings except CTL1 = CTL2 = GND
0.594
5
0.606
11
V
kΩ
2
_______________________________________________________________________________________
3A, 2MHz Step-Down Regulator
with Integrated Switches
ELECTRICAL CHARACTERISTICS (continued)
(V
IN
= V
DD
= 3.3V, V
FB
= 0.5V, T
A
= -40°C to +85°C. Typical values are at T
A
= +25°C, circuit of Figure 1, unless otherwise noted.) (Note 2)
PARAMETER
Open-Loop Voltage Gain
Error-Amplifier Unity-Gain
Bandwidth
Error-Amplifier Common-Mode
Input Range
Error-Amplifier Minimum Output
Current
FB Input Bias Current
CTL_
CTL_ Input Bias Current
V
CTL_
= 0V
V
CTL_
= V
DD
Rising
High-Impedance Threshold
Hysteresis
REFIN
REFIN Input Bias Current
REFIN Common-Mode Range
REFIN Offset Voltage
LX (ALL PINS COMBINED)
LX On-Resistance, High Side
LX On-Resistance, Low Side
LX Current-Limit Threshold
I
LX
= -2A
I
LX
= 2A
V
IN
= 2.5V, high-side sourcing
T
A
= +25°C
LX Leakage Current
V
IN
= 3.6V, V
EN
= V
SS
= 0V
T
A
= +85°C
LX Switching Frequency
Frequency Range
LX Minimum Off-Time
LX Maximum Duty Cycle
LX Minimum On-Time
RMS LX Output Current
3
V
IN
= 2.5V to 3.3V
R
FREQ
= 50kΩ, V
IN
= 2.5V to 3.3V
93
V
IN
= 2.5V to 3.3V
V
LX
= 0V
V
LX
= 3.6V
V
LX
= 0V
V
LX
= 3.6V
0.9
1.8
500
40
96
80
1
1
1
2.0
1.1
2.2
2000
75
MHz
kHz
ns
%
ns
A
V
IN
= V
BST
- V
LX
= 2.5V
V
IN
= V
BST
- V
LX
= 3.3V
V
IN
= 2.5V
V
IN
= 3.3V
4
-2
+2
µA
39
37
36
34
5.5
55
58
mΩ
mΩ
A
V
REFIN
= 0.6V
V
DD
= 2.3V to 2.6V
V
DD
= 2.6V to 3.6V
CTL1 = CTL2 = GND, T
A
= +25°C
T
A
= +25°C
-500
0
0
-3
-100
V
DD
- 1.65
V
DD
- 1.7
+3
nA
V
mV
Falling
All VID transitions
V
DD
- 1.2V
50
-7
+7
0.75
V
mV
µA
CONDITIONS
1kΩ from COMP to GND
Parallel 10kΩ, 40pF from COMP to GND (Note 3)
V
DD
= 2.35V to 2.6V
V
DD
= 2.6V to 3.6V
V
COMP
= 1V
V
FB
= 0.7V, CTL1 = CTL2 =
Sourcing
Sinking
T
A
= +25°C
14
0
0
1000
-500
-200
-40
MIN
TYP
115
26
V
DD
- 1.65
V
DD
- 1.7
MAX
UNITS
dB
MHz
V
µA
nA
MAX8643A
R
FREQ
= 50kΩ
R
FREQ
= 23.2kΩ
_______________________________________________________________________________________
3
3A, 2MHz Step-Down Regulator
with Integrated Switches
MAX8643A
ELECTRICAL CHARACTERISTICS (continued)
(V
IN
= V
DD
= 3.3V, V
FB
= 0.5V, T
A
= -40°C to +85°C. Typical values are at T
A
= +25°C, circuit of Figure 1, unless otherwise noted.) (Note 2)
PARAMETER
ENABLE
EN Input Logic-Low, Falling
EN Input Logic-High, Rising
EN Hysteresis
EN, Input Current
SS
SS Charging Current
SS Discharge Resistance
THERMAL SHUTDOWN
Thermal-Shutdown Threshold
Thermal-Shutdown Hysteresis
POWER-GOOD (PWRGD)
Power-Good Threshold Voltage
Power-Good Falling-Edge Deglitch
PWRGD Output-Voltage Low
PWRGD Leakage Current
OVERCURRENT LIMIT
Current-Limit Startup Blanking
Restart Time
128
1024
Clock
cycles
Clock
cycles
I
PWRGD
= 4mA
V
DD
= V
PWRGD
= 3.6V, V
FB
= 0.9V
V
FB
falling, 3mV hysteresis
87
90
48
0.03
0.01
0.15
93
%
Clock
cycles
V
µA
+165
20
°C
°C
V
SS
= 0.45V
7
8
500
9
µA
Ω
V
EN
= 0V or 3.6V,
V
DD
= 3.6V
T
A
= +25°C
T
A
= +85°C
0.01
1.7
1.2
1.4
200
1
0.7
V
V
mV
µA
CONDITIONS
MIN
TYP
MAX
UNITS
Note 2:
Specifications are 100% production tested at T
A
= +25°C. Limits over the operating temperature range are guaranteed by
design and characterization.
Note 3:
Guaranteed by design.
Typical Operating Characteristics
(Typical values are at V
IN
= V
DD
= 3.3V, V
OUT
= 1.8V, R
FREQ
= 50kΩ, I
OUT
= 3A, and T
A
= +25°C, unless otherwise noted.)
EFFICIENCY vs. OUTPUT CURRENT
MAX8643A toc01
EFFICIENCY vs. OUTPUT CURRENT
95
90
EFFICIENCY (%)
85
80
75
70
V
OUT
= 1.2V
V
IN
= V
DD
= 2.5V
0.1
1
OUTPUT CURRENT (A)
10
V
OUT
= 1.88V
V
OUT
= 1.5V
MAX8643A toc02
100
90
80
70
60
50
V
IN
= V
DD
= 3.3V
40
0.1
1
OUTPUT CURRENT (A)
V
OUT
= 1.8V
V
OUT
= 1.2V
V
OUT
= 2.5V
100
EFFICIENCY (%)
65
60
10
4
_______________________________________________________________________________________
3A, 2MHz Step-Down Regulator
with Integrated Switches
Typical Operating Characteristics (continued)
(Typical values are at V
IN
= V
DD
= 3.3V, V
OUT
= 1.8V, R
FREQ
= 50kΩ, I
OUT
= 3A, and T
A
= +25°C, unless otherwise noted.)
EFFICIENCY vs. OUTPUT CURRENT
MAX8643A toc03
MAX8643A
FREQUENCY vs. INPUT VOLTAGE
MAX8643A toc04
LOAD REGULATION
-0.02
OUTPUT VOLTAGE CHARGE (%)
-0.04
-0.06
-0.08
-0.10
-0.12
-0.14
-0.16
V
OUT
= 1.2V
0
1
2
3
LOAD CURRENT (A)
4
5
V
OUT
= 1.8V
V
IN
= V
DD
= 3.3V
V
OUT
= 2.5V
MAX8643A toc05
100
95
90
0
1950
1800
FREQUENCY (kHz)
1650
1500
1350
1200
-40°C
1050
900
+25°C
+85°C
-40°C
+25°C
+85°C
EFFICIENCY (%)
85
V
OUT
= 1.8V
80
V
OUT
= 1.5V
75
70
65
60
0.1
V
OUT
= 1.2V
V
IN
= 2.5V
V
DD
= 3.3V
1
OUTPUT CURRENT (A)
10
2.2
2.6
3.0
3.4
INPUT VOLTAGE (V)
3.8
LOAD TRANSIENT
MAX8643A toc06
SWITCHING WAVEFORMS
MAX8643A toc07
V
IN
= V
DD
= 3.3V
V
OUT
AC-COUPLED
50mV/div
V
OUT
AC-COUPLED
20mV/div
2A/div
I
LX
0A
I
OUT
1A/div
0A
40µs/div
100ns/div
V
LX
2V/div
0V
SOFT-START WAVEFORMS
MAX8643A toc08
SHUTDOWN WAVEFORMS
MAX8643A toc09
V
EN
2V/div
0V
V
EN
2V/div
0V
V
OUT
V
OUT
R
LOAD
= 1Ω
400µs/div
1V/div
0V
R
LOAD
= 1Ω
10µs/div
1V/div
0V
_______________________________________________________________________________________
5
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