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PDF MAX1970-MAX1972 Data sheet ( Hoja de datos )
| Número de pieza | MAX1970-MAX1972 | |
| Descripción | Dual / 180 Out-of-Phase / 1.4MHz / 750mA Step- Down Regulator with POR and RSI/PFO | |
| Fabricantes | Maxim Integrated | |
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19-2297; Rev 0; 1/02
Dual, 180° Out-of-Phase, 1.4MHz, 750mA Step-
Down Regulator with POR and RSI/PFO
General Description
The MAX1970/MAX1971/MAX1972 dual-output current-
mode PWM buck regulators operate from 2.6V to 5.5V
input and deliver a minimum of 750mA on each output.
The MAX1970 and MAX1972 operate at a fixed 1.4MHz
(MAX1971 operates at 700kHz) to reduce output induc-
tor and capacitor size and cost. Switching the regula-
tors 180° out-of-phase also reduces the input capacitor
size and cost. Ceramic capacitors can be used for
input and output.
The output voltages are programmable from 1.2V to VIN
using external feedback resistors, or can be preset to
1.8V or 3.3V for output 1 and 1.5V or 2.5V for output 2.
When one output is higher than 1.2V, the second can
be configured down to sub-1V levels. Output accuracy
is better than ±1% over variations in load, line, and tem-
perature. Internal soft-start reduces inrush current dur-
ing startup.
All devices feature power-on reset (POR). The
MAX1971 includes a reset input (RSI), which forces
POR low for 175ms after RSI goes low. The MAX1970
and MAX1972 include an open-drain power-fail output
(PFO) that monitors input voltage and goes high when
the input falls below 3.94V. For USB-powered xDSL
modems, this output can be used to detect USB power
failure. A minimum switching frequency of 1.2MHz
ensures operation outside the xDSL band.
Applications
xDSL Modems
xDSL Routers
Copper Gigabit SFP
and GBIC Modules
USB-Powered Devices
Dual LDO Replacement
Typical Operating Circuit
VIN 2.6V TO 5.5V
Features
o Current-Mode, 1.4MHz Fixed-Frequency PWM
Operation
o 180° Out-of-Phase Operation Reduces Input
Capacitor
o ±1% Output Accuracy Over Load, Line, and
Temperature Ranges
o 750mA Guaranteed Output Current
o 2.6V to 5.5V Input
o Power-On Reset Delay of 16.6ms (MAX1970) or
175ms (MAX1971 and MAX1972)
o Power-Fail Output (MAX1970 and MAX1972 Only)
o Power-On Reset Input (MAX1971 Only)
o Operation Outside xDSL Band
o Ultra-Compact Design with Smallest External
Components
o Outputs Adjustable from 0.8V to VIN or 1.8V/3.3V
and 1.5V/2.5V Preset
o All-Ceramic Capacitor Application
o Soft-Start Reduces Inrush Current
PART
MAX1970EEE
MAX1971EEE
MAX1972EEE
TOP VIEW
Ordering Information
TEMP RANGE
-40°C to +85°C
-40°C to +85°C
-40°C to +85°C
PIN-PACKAGE
16 QSOP
16 QSOP
16 QSOP
Pin Configuration
RSI
EN
VCC
RSI IN
VCC
POR
EN
MAX1971
COMP1
LX1
FB1
COMP2
FBSEL1
LX2
FB2
FBSEL2
REF
PGND
POR
OUT1
1.8V
750mA
OUT2
2.5V
750mA
LX1 1
VCC 2
COMP1 3
FB1 4
FB2 5
COMP2 6
REF 7
GND 8
MAX1970
MAX1971
MAX1972
QSOP
16 PGND
15 LX2
14 IN
13 FBSEL1
12 FBSEL2
11
PF0 (MAX1970/MAX1972)
RSI (MAX1971)
10 EN
9 POR
________________________________________________________________ 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  
Dual, 180° Out-of-Phase, 1.4MHz, 750mA Step-
Down Regulator with POR and RSI/PFO
ELECTRICAL CHARACTERISTICS (continued)
(VIN = VCC = VEN = 5V, VFB1 = VFB2 = 1.15V, R POR = 100kΩ to IN, RPFO = 100kΩ to IN, RSI = 0, CVCC = 0.1µF, CREF = 0.1µF,
FBSEL1 = unconnected, FBSEL2 = unconnected, TA = -40°C to +85°C.) (Note 2)
PARAMETER
FB1 Input Resistance
FB2 Input Resistance
FB_ Input Bias Current
COMP1 AND COMP2
CONDITIONS
Measured from FB1 to
GND
FBSEL1 = GND
FBSEL1 = VCC
Measured from FB2 to
GND
FBSEL2 = GND
FBSEL2 = VCC
FB1 or FB2, FBSEL_ = unconnected, VFB1 = VFB2 = 1.15V
MIN
30
30
22.5
22.5
TYP
MAX
120
120
90
90
0.1
UNITS
kΩ
kΩ
µA
COMP1
Transconductance
FB1 = COMP1,
VCOMP1 = 1.2V
FBSEL1 = unconnected
35
85 µS
COMP2
Transconductance
LX1 AND LX2
Internal High-Side
MOSFET On-Resistance
Internal Low-Side
MOSFET On-Resistance
LX_ Current-Sense
Transresistance
FB2 = COMP2,
VCOMP2 = 1.2V
ILX = -180mA
ILX = 180mA
FBSEL2 = unconnected
VIN = 5.0V
VIN = 3.3V
VIN = 5.0V
VIN = 3.3V
35
0.4
85 µS
0.32
Ω
0.37
0.23
Ω
0.25
0.6 V/A
LX_ Current-Limit
Threshold
LX_ Leakage Current
LX_ Switching Frequency
LX_ Minimum Duty Cycle
POR
POR Thresholds
POR Delay Time (TD)
POR Output Current, High
POR Output Voltage, Low
POR Start-Up Voltage
Duty cycle = 100%,
VIN = 2.6V to 5.5V
VIN = 5.5V
VIN = 2.6V to 5.5V
VIN = 2.6V to 5.5V
High side
Low side
VLX1 = VLX2 = 5.5V
VLX1 = VLX2 = 0
MAX1970/MAX1972
MAX1971
MAX1970/MAX1972
MAX1971
Percentage of VOUT,
VIN = 2.6V to 5.5V
MAX1970
VOUT rising
VOUT falling
MAX1971/MAX1972
V POR = VIN = 5.5V, VFB1 = VFB2 = 1.15V
VFB1 = 1.05V or VFB2 = 1.05V or RSI = IN (MAX1971 only),
IPOR = 1mA
FB1 = FB2 = GND, IPOR = 100µA, VIN = 1.2V
0.76
-1.6
-20
1.2
0.60
87
13.3
140
-1
1.60
-0.40
20
1.6
0.80
20
15
A
µA
MHz
%
94
%
20
ms
210
1 µA
0.05 V
0.05 V
_______________________________________________________________________________________ 5
5 Page 
Dual, 180° Out-of-Phase, 1.4MHz, 750mA Step-
Down Regulator with POR and RSI/PFO
NAME
PIN
MAX1970/MAX1972 MAX1971
9 POR
POR
10 EN
PFO
11
—
12 FBSEL2
13 FBSEL1
14 IN
15 LX2
16 PGND
EN
—
RSI
FBSEL2
FBSEL1
IN
LX2
PGND
Pin Description (continued)
FUNCTION
Active-Low Power-On Reset Output. Open-drain output goes high 16.6ms
(MAX1970) or 175ms (MAX1971 or MAX1972) after both outputs reach 92% of
nominal value, and RSI (MAX1971 only) is low.
Enable Input. Drive high to turn on both OUT1 and OUT2. Drive low to place the
device in shutdown.
Power-Fail Output. Open-drain output goes high when VCC drops below 3.94V.
Useful for detecting a valid USB input voltage.
Noninverting Reset Input. Causes POR to go low when RSI is high. Allows POR to
go high 175ms after RSI falls, if outputs are in regulation.
Regulator 2 Feedback Select. Connect to VCC to set VOUT2 to 2.5V. Connect to
GND to set VOUT2 to 1.5V. Leave unconnected to use external feedback resistors.
Regulator 1 Feedback Select. Connect to VCC to set VOUT1 to 3.3V. Connect to
GND to set VOUT1 to 1.8V. Leave unconnected to use external feedback resistors.
Power-Supply Voltage. Input range from 2.6V to 5.5V. Bypass with 10µF capacitor
to PGND.
Inductor Connection 2. Connect an inductor between LX2 and OUT2.
Power Ground
Detailed Description
The MAX1970/MAX1971/MAX1972 are dual-output,
fixed-frequency, current-mode, PWM, step-down
DC/DC converters. The MAX1970 and MAX1972 switch
at 1.4 MHz while the MAX1971 switches at 700kHz. The
two converters on each IC switch 180° out of phase
with each other to reduce input ripple current. The
high-switching frequency allows use of smaller capaci-
tors for filtering and decoupling. Internal synchronous
rectifiers improve efficiency and eliminate the typical
Schottky freewheeling diode. The on-resistances of the
internal MOSFETs are used to sense the switch cur-
rents for controlling and protecting the MOSFETs, elimi-
nating current-sensing resistors to further improve
efficiency and cost.
The input voltage range is 2.6V to 5.5V. Each converter
has a three-mode feedback input. Internally, OUT1 is
set to either 3.3V or 1.8V, and OUT2 to 2.5V or 1.5V by
connecting FBSEL1 and FBSEL2 to VCC or GND,
respectively. When FBSEL1 or FBSEL2 are floating,
each output can be set to any voltage between 1.2V
and VIN through an external resistive divider. Having an
output below 1.2V is also possible (see the Output
Voltage Selection section).
DC-DC Controller
The MAX1970/MAX1971/MAX1972 family of step-down
converters uses a pulse-width-modulating (PWM) current-
mode control scheme. The heart of the current-mode
PWM controller is an open-loop comparator that com-
pares the integrated voltage-feedback signal against
the sum of the amplified current-sense signal and the
slope compensation ramp. At each rising edge of the
internal clock, the internal high-side MOSFET turns on
until the PWM comparator trips. During this on time,
current ramps up through the inductor, sourcing cur-
rent to the output and storing energy in a magnetic
field. The current-mode feedback system regulates the
peak inductor current as a function of the output volt-
age error signal. Since the average inductor current is
nearly the same as the peak inductor current (assum-
ing that the inductor value is relatively high to minimize
ripple current), the circuit acts as a switch-mode
transconductance amplifier. It pushes the output LC filter
pole, normally found in a voltage-mode PWM, to a higher
frequency. To preserve inner loop stability and eliminate
inductor stair casing, a slope-compensation ramp is
summed into the main PWM comparator. During the
second half of the cycle, the internal high-side MOSFET
______________________________________________________________________________________ 11
11 Page | ||
| Páginas | Total 20 Páginas | |
| PDF Descargar | [ Datasheet MAX1970-MAX1972.PDF ] | |
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