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PDF MAX8646 Data sheet ( Hoja de datos )
| Número de pieza | MAX8646 | |
| Descripción | 2MHz Step-Down Regulator | |
| Fabricantes | Maxim Integrated Products | |
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19-3971; Rev 2; 10/07
6A, 2MHz Step-Down Regulator
with Integrated Switches
General Description
The MAX8646 high-efficiency switching regulator deliv-
ers up to 6A load current at output voltages from 0.6V
to 0.9 x VIN. The IC operates from 2.35V to 3.6V, mak-
ing it ideal for on-board point-of-load and postregula-
tion applications. Total output error is less than ±1%
over load, line, and temperature ranges.
The MAX8646 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 MAX8646 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 MAX8646 provides two tri-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%
www.DraetsaiSshtoeerst4.UI.ncoamddition, 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 MAX8646 offers programmable soft-start
time using one capacitor to reduce input inrush current.
The MAX8646 is available in a lead-free, 24-pin, 4mm x
4mm, thin QFN package.
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
Features
♦ Internal 23mΩ RDS(ON) MOSFETs
♦ Continuous 6A Output Current
♦ ±1% Output Accuracy over Load, Line,
and Temperature
♦ Operates from 2.35V to 3.6V Supply
♦ Adjustable Output from 0.6V to (0.9 x VIN)
♦ Soft-Start Reduces Inrush Supply Current
♦ 500kHz to 2MHz Adjustable Switching Frequency
♦ Compatible with Ceramic, Polymer, and
Electrolytic Output Capacitors
♦ VID-Selectable Output Voltages
0.6V, 0.7V, 0.8V, 1.0V, 1.2V, 1.5V, 1.8V, 2.0V,
and 2.5V
♦ Fully Protected Against Overcurrent and
Overtemperature
♦ Safe-Start into Prebiased Output
♦ Sink/Source Current in DDR Applications
♦ Lead-Free, 24-Pin, 4mm x 4mm Thin QFN Package
Ordering Information
PART
TEMP
RANGE
PIN-PACKAGE
PKG
CODE
MAX8646ETG+ -40°C to +85°C
+Denotes a lead-free package.
*EP = Exposed pad.
24 Thin QFN-EP*
4mm x 4mm
T2444-4
Typical Operating Circuit
INPUT
2.4V, 3.6V
IN BST
MAX8646
EN
LX
VDD
OUT
OUTPUT
1.8V, 6A
CTL1
CTL2
FREQ
REFIN
SS
PGND
FB
COMP
VDD
Pin Configuration appears at end of data sheet.
PREBIAS
PWRGD
GND
________________________________________________________________ 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.
1 page  
6A, 2MHz Step-Down Regulator
with Integrated Switches
Typical Operating Characteristics
(Typical values are: VIN = VDD = 3.3V; VOUT = 1.8V, RFREQ = 50kΩ, IOUT = 6A, and TA = +25°C, unless otherwise noted.)
EFFICIENCY vs. OUTPUT CURRENT
100
90
80
VOUT = 2.5V
70 VOUT = 1.8V
60 VOUT = 1.2V
50
VIN = VDD = 3.3V
40
0.1
1
OUTPUT CURRENT (A)
10
FREQUENCY vs. INPUT VOLTAGE
1950
1800
- 40°C + 25°C
+ 85°C
1650
1500
1350
1200 - 40°C + 25°C
www.DataSheet4U.com
1050
+ 85°C
900
2.2
2.6 3.0 3.4
INPUT VOLTAGE (V)
3.8
EFFICIENCY vs. OUTPUT CURRENT
100
95
90
85
80
VOUT = 1.5V
75 VOUT = 1.8V
70 VOUT = 1.2V
65
VIN = VDD = 2.5V
60
0.1
1
OUTPUT CURRENT (A)
10
LOAD REGULATION
0
-0.05
-0.10
VIN = VDD = 3.3V
VOUT = 2.5V
-0.15 VOUT = 1.8V
-0.20
-0.25
VOUT = 1.2V
-0.30
-0.35
-0.40
0
246
LOAD CURRENT (A)
8
EFFICIENCY vs. OUTPUT CURRENT
100
95
90
85
80 VOUT = 1.8V
VOUT = 1.5V
75
70 VOUT = 1.2V
65 VIN = 2.5V
VDD = 3.3V
60
0.1
1
OUTPUT CURRENT (A)
10
LOAD TRANSIENT
MAX8646 toc06
VIN = VDD = 3.3V
AC-COUPLED
VOUT 50mV/div
IOUT
1A/div
40µs/div
0A
SWITCHING WAVEFORMS
MAX8646 toc07
SOFT-START WAVEFORM
MAX8646 toc08
SHUTDOWN WAVEFORMS
MAX8646 toc09
VOUT
AC-COUPLED VEN
20mV/div
2V/div
VEN
2V/div
0V 0V
2A/div
ILX
0A VOUT 1V/div
VLX
2V/div
VOUT
1V/div
0V
0V
0V
RLOAD = 0.5Ω
RLOAD = 0.5Ω
100ns/div
400µs/div
10µs/div
_______________________________________________________________________________________ 5
5 Page 
6A, 2MHz Step-Down Regulator
with Integrated Switches
Shutdown Mode
Drive EN to GND to shut down the IC and reduce quies-
cent current to less than 12µA. During shutdown, the LX
is high impedance. Drive EN high to enable the
MAX8646.
Thermal Protection
Thermal-overload protection limits total power dissipation
in the device. When the junction temperature exceeds TJ
= +165°C, a thermal sensor forces the device into shut-
down, allowing the die to cool. The thermal sensor turns
the device on again after the junction temperature cools
by 20°C, causing a pulsed output during continuous
overload conditions. The soft-start sequence begins after
recovery from a thermal-shutdown condition.
Applications Information
IN and VDD Decoupling
To decrease the noise effects due to the high switching
frequency and maximize the output accuracy of
the MAX8646, decouple VIN with a 22µF capacitor from
VIN to PGND. Also decouple VDD with a 1µF from VDD
to GND. Place these capacitors as close to the IC
as possible.
Table 1. CTL1 and CTL2 Output Voltage
Selection
CTL1
GND
VDD
GND
GND
Unconnected
Unconnected
Unconnected
VDD
VDD
CTL2
GND
VDD
Unconnected
VDD
GND
Unconnected
VDD
GND
Unconnected
VOUT (V)
0.6
0.7
0.8
1.0
1.2
1.5
1.8
2.0
2.5
where the output ripple due to output capacitance,
ESR, and ESL is:
VRIPPLE(C) =
IP−P
8 x COUT x fS
VRIPPLE(ESR) = IP−P x ESR
Inductor Selection
Choose an inductor with the following equation:
L = VOUT × (VIN − VOUT)
fS × VIN × LIR × IOUT(MAX)
where LIR is the ratio of the inductor ripple current to full
www.DlaotaaSdheceut4rrUe.cnotmat the minimum duty cycle. Choose LIR
between 20% to 40% for best performance and stability.
Use an inductor with the lowest possible DC resistance
that fits in the allotted dimensions. Powdered iron ferrite
core types are often the best choice for performance.
With any core material, the core must be large enough
not to saturate at the current limit of the MAX8646.
Output-Capacitor Selection
The key selection parameters for the output capacitor are
capacitance, ESR, ESL, and voltage-rating requirements.
These affect the overall stability, output ripple voltage,
and transient response of the DC-DC converter. The out-
put ripple occurs due to variations in the charge stored
in the output capacitor, the voltage drop due to the
capacitor’s ESR, and the voltage drop due to the
capacitor’s ESL. Calculate the output voltage ripple
due to the output capacitance, ESR, and ESL:
VRIPPLE = VRIPPLE(C) +
VRIPPLE(ESR) + VRIPPLE(ESL)
VRIPPLE(ESL) = IP−P x ESL
tON
or: VRIPPLE(ESL) = IP−P x ESL
tOFF
or whichever is larger.
The peak inductor current (IP-P) is:
IP−P
=
VIN − VOUT
fS × L
x
VOUT
VIN
Use these equations for initial capacitor selection.
Determine final values by testing a prototype or an
evaluation circuit. A smaller ripple current results in less
output-voltage ripple. Since the inductor ripple current
is a factor of the inductor value, the output voltage rip-
ple decreases with larger inductance. Use ceramic
capacitors for low ESR and low ESL at the switching
frequency of the converter. The ripple voltage due to
ESL is negligible when using ceramic capacitors.
Load-transient response depends on the selected out-
put capacitance. During a load transient, the output
instantly changes by ESR x ∆ILOAD. Before the con-
troller can respond, the output deviates further,
depending on the inductor and output capacitor val-
ues. After a short time, the controller responds by regu-
lating the output voltage back to its predetermined
______________________________________________________________________________________ 11
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet MAX8646.PDF ] | |
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