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PDF MAX8654 Data sheet ( Hoja de datos )
| Número de pieza | MAX8654 | |
| Descripción | 8A 1.2MHz Step-Down Regulator | |
| Fabricantes | Maxim Integrated Products | |
| Logotipo |  |
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19-0588; Rev 0; 8/06
EVAALVUAAILTAIOBNLEKIT
12V, 8A 1.2MHz
Step-Down Regulator
General Description
The MAX8654 high-efficiency switching regulator delivers
up to 8A of load current at output voltages from 0.6V to
0.85 x VIN. The IC operates from 4.5V to 14V, making it
ideal for on-board point-of-load and postregulation appli-
cations, with total output error less than ±1% over load,
line, and temperature ranges.
The MAX8654 is a fixed-frequency PWM mode regulator
with a switching frequency range of 250kHz to 1.2MHz
set by an external resistor or SYNC input. High-frequency
operation allows for an all-ceramic-capacitor solution. A
SYNCOUT output is provided to synchronize a second
regulator switching 180° out-of-phase with the first to
reduce the input ripple current and consequently reduce
the required input capacitance. The high operating fre-
quency minimizes the size of external components.
The on-board low RDS(ON) dual-nMOS design keeps
the board cooler at heavy loads while minimizing the
critical inductances, making the layout a much simpler
task with respect to the discrete solutions.
The MAX8654 comes with a high-bandwidth (20MHz)
voltage-error amplifier. The voltage-mode control archi-
tecture and the op-amp voltage-error amplifier permit a
type 3 compensation scheme to be utilized to achieve
maximum loop bandwidth, up to 20% of the switching
frequency. High loop bandwidth achieves fast transient
response resulting in less output capacitance required.
The MAX8654 offers programmable soft-start to accom-
modate different types of output capacitors and reduce
input inrush current. The MAX8654 is available in a 36-
lead thin QFN package.
Applications
POL Power Supplies
Servers
DDR Memory
RAID Power Supplies
Network Power Supplies
Graphic Cards
Pin Configuration appears at end of data sheet.
Features
♦ Internal 26mΩ RDS(ON) MOSFETs
♦ Guaranteed 8A Output Current
♦ Adjustable Overcurrent Protection
♦ 1% Output Accuracy Over Temperature
♦ Operates from 4.5V to 14V Supply
♦ Adjustable Output from 0.6V to 0.85 x VIN
♦ Soft-Start Reduces Inrush Supply Current
♦ 250kHz to 1.2MHz Adjustable Switching or SYNC
Input
♦ Compatible with Ceramic, Polymer, and
Electrolytic Output Capacitors
♦ SYNCOUT Synchronizes 2nd Regulator 180°
Out-of-Phase
♦ 36-Pin, Lead-Free, 6mm x 6mm Thin QFN Package
Ordering Information
PART
TEMP RANGE PIN-
PACKAGE
PKG
CODE
MAX8654ETX+ -40°C to +85°C
+Denotes lead-free package.
*EP = Exposed pad.
36 Thin QFN-EP*
(6mm x 6mm)
T3666-3
Typical Operating Circuit
INPUT
4.5V TO 14V
VL
IN
VP
BST
LX
OUTPUT
UP TO 8A
VL PGND
VDL
MAX8654 FB
FREQ
ILIM
REFIN
COMP
SS SYNC
SYNCOUT
IN
VL
EN PWRGD
GND
________________________________________________________________ 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.
1 page  
12V, 8A 1.2MHz
Step-Down Regulator
Typical Operating Characteristics
(Typical values are: VIN = VVP = 12V, VOUT = 3.3V, RFREQ = 100kΩ, and TA = +25°C, circuit of Figure 1.)
EFFICIENCY vs. LOAD CURRENT
100
90
80
70
60
VOUT = 1.8V
VOUT = 5V
50 VOUT = 3.3V
40
30
20
10
VIN = 12V, VVDL = VVP = 5V, fS = 500kHz
0
0.1 1
OUTPUT CURRENT (A)
10
EFFICIENCY vs. LOAD CURRENT
100
90
80
70
60
50 VOUT = 1.8V VOUT = 3.3V
40
30
20
10
VIN = VVP = 5V, fS = 500kHz
0
0.1 1
OUTPUT CURRENT (A)
10
EFFICIENCY vs. LOAD CURRENT
100
90 VOUT = 5V
80
70
60
50 VOUT = 3.3V
40 VOUT = 1.8V
30
20
10
VIN = VVP = 12V, fS = 500kHz
0
0.1 1
OUTPUT CURRENT (A)
10
REFERENCE VOLTAGE vs. TEMPERATURE
0.610
0.605
0.600
0.595
0.590
-50 -25
0 25 50 75 100 125
TEMPERATURE (°C)
LOAD REGULATION
0
VOUT = 3.3V
-0.05
-0.10
-0.15
VOUT = 1.8V
-0.20
-0.25
-0.30
-0.35
0 1 2 3 45 6 78
LOAD CURRENT (A)
SWITCHING FREQUENCY
vs. INPUT VOLTAGE
1200
1000
800
600
400
200
0
57
9 11 13 15
INPUT VOLTAGE (V)
SHUTDOWN SUPPLY CURRENT
vs. INPUT VOLTAGE
10
9
8
7
6
5
4
3
2
1
0
0 2 4 6 8 10 12 14
INPUT VOLTAGE (V)
SWITCHING FREQUENCY vs. RFREQ
1400
1200
1000
800
600
400
200
0
0 50 100 150 200 250
RFREQ (kΩ)
CURRENT LIMIT vs. OUTPUT VOLTAGE
9.0
8.8
8.6
8.4
8.2
8.0
7.8
7.6
7.4
7.2
7.0
1.5
.2.0 2.5 3.0
OUTPUT VOLTAGE (V)
3.5
_______________________________________________________________________________________ 5
5 Page 
12V, 8A 1.2MHz
Step-Down Regulator
Controller Function
The controller logic block is the central processor that
determines the duty cycle of the high-side MOSFET
under different line, load, and temperature conditions.
Under normal operation, where the current-limit and
temperature protection are not triggered, the controller
logic block takes the output from the PWM comparator
and generates the driver signals for both high-side and
low-side MOSFETs. The break-before-make logic and
the timing for charging the bootstrap capacitors are
calculated by the controller logic block. The error signal
from the voltage-error amplifier is compared with the
ramp signal generated by the oscillator at the PWM
comparator, and thus the required PWM signal is pro-
duced. The high-side switch is turned on at the begin-
ning of the oscillator cycle and turns off when the ramp
voltage exceeds the VCOMP signal or when the current-
limit threshold is exceeded. The low-side switch is then
turned on for the remainder of the oscillator cycle.
Current Limit
The MAX8654 adjustable current limit is set by a resis-
tor, RILIM, connected from ILIM to GND. The current
through RILIM determines the LX current-limit trip point:
RILIM (kΩ) = 800 / ILXLIM (A)
where ILXLIM is the LX current-limit threshold. The valid
RILIM range is 40kΩ to 200kΩ. RILIM of 100kΩ sets a
typical current limit of 8A, sourcing or sinking at LX.
When current flowing out of LX exceeds this limit,
the high-side MOSFET turns off and the synchronous
rectifier turns on. The synchronous rectifier remains on
until the inductor current falls below the low-side cur-
rent limit. This lowers the duty cycle and causes the
output voltage to droop until the current limit is no
longer exceeded.
When the negative current limit is exceeded, the device
turns off the synchronous rectifier, forcing the inductor
current to flow through the high-side MOSFET body
diode, back to the input, until the beginning of the next
cycle, or until the inductor current drops to zero.
The MAX8654 uses a hiccup mode to prevent over-
heating during short-circuit output conditions. The
device enters hiccup mode when VFB drops below
420mV for more than 12µs, pulling COMP and REFIN
low. The IC turns off for 900 clock cycles and then
enters soft-start for 110 clock cycles. If the short-circuit
condition remains, the IC shuts down for another 512
clock cycles. The IC repeats this behavior until the
short-circuit condition is removed.
Soft-Start and REFIN
The MAX8654 utilizes an adjustable soft-start function
to limit inrush current during startup. An 8µA (typ) cur-
rent source charges an external capacitor connected to
SS to increase the capacitor voltage in a controlled
manner. The soft-start time is adjusted by the value of
the external capacitor from SS to GND. The required
capacitance value is determined as:
C = 8μA × tSS
0.6V
where tSS is the required soft-start time in seconds.
The MAX8654 also features an external reference input
(REFIN). The IC regulates FB to the voltage applied to
REFIN. The internal soft-start is not available when
using an external reference. A method of soft-start
when using an external reference is shown in Figure 2.
When using an external reference, in order to avoid
current limit during soft-start, care should be taken to
ensure the following condition:
COUT
×
dVREFIN
dt
+ IOUT
< ILXLIM
−
IP−P
2
where IOUT is the maximum output current, COUT is the
output capacitance, and IP-P is the peak-to-peak induc-
tor ripple current.
Connect REFIN to SS to use the internal 0.6V reference.
R1
R2 C
REFIN
MAX8654
Figure 2. Typical Soft-Start Implementation with External
Reference
______________________________________________________________________________________ 11
11 Page | ||
| Páginas | Total 17 Páginas | |
| PDF Descargar | [ Datasheet MAX8654.PDF ] | |
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