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PDF MAX1744 Data sheet ( Hoja de datos )
| Número de pieza | MAX1744 | |
| Descripción | High-Voltage / Step-Down DC-DC Controller | |
| Fabricantes | Maxim Integrated | |
| Logotipo |  |
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19-1776; Rev 4; 8/09
EVAALVUAAILTAIOBNLEKIT
High-Voltage, Step-Down DC-DC
Controllers in µMAX
General Description
The MAX1744/MAX1745 are step-down DC-DC con-
trollers capable of handling up to 36V inputs. These
parts use a proprietary current-limited control scheme
for excellent light- and full-load efficiency, while their
330kHz (max) switching frequency permits small exter-
nal components for space-critical applications.
Operation to 100% duty cycle permits the lowest possi-
ble dropout voltage.
The MAX1744 contains an internal feedback network
that provides a pin-selectable output voltage of either
3.3V or 5V. The MAX1745 uses an external feedback
network to generate an adjustable output voltage
between 1.25V and 18V.
The MAX1744/MAX1745 are available in a space-sav-
ing 10-pin μMAX® package.
________________________Applications
Automotive Electronics
Telecom Systems
Wall-Cube-Powered Devices
Industrial Control Systems
Firewire®/IEEE® 1394
μMAX is a registered trademark of Maxim Integrated Products, Inc.
Firewire is a registered trademark of Apple, Inc.
IEEE is a registered service mark of the Institute of Electrical
and Electronics Engineers, Inc.
Typical Operating Circuit
____________________________Features
♦ High-Voltage Operation (Up to 36V IN)
♦ Efficiency > 90%
♦ Output Power Capability Exceeds 50W
♦ 10-Pin µMax Package
♦ Low-Dropout Voltage
♦ 100% (max) Duty Cycle
♦ 90µA Quiescent Current
♦ 4µA Shutdown Current
♦ Up to 330kHz Switching Frequency
♦ Output Voltage
5V or 3.3V (MAX1744)
Adjustable 1.25V to 18V (MAX1745)
♦ Current-Limited Control Scheme
Ordering Information
PART
TEMP RANGE
PIN-PACKAGE
MAX1744EUB+
-40°C to +85°C
10 μMAX
MAX1744AUB+
-40°C to +125°C 10 μMAX
MAX1744EUB/V+ -40°C to +85°C
10 μMAX
MAX1745EUB+
-40°C to +85°C
10 μMAX
MAX1745AUB+
-40°C to +125°C 10 μMAX
MAX1745EUB/V+ -40°C to +85°C
10 μMAX
+Denotes a lead(Pb)-free/RoHS-compliant package.
/V Denotes an automotive qualified part.
Pin Configuration
IN
4.5V TO 36V
TOP VIEW
ON IN VH EXT
OFF SHDN
5V
3.3V 3/5
MAX1744
VL
REF CS
GND OUT
P
OUT
3.3V
OR 5V
+
GND 1
VL 2
REF 3
3/5 (FB) 4
MAX1744
MAX1745
OUT 5
( ) ARE FOR MAX1745 ONLY.
μMAX
10 IN
9 EXT
8 VH
7 SHDN
6 CS
________________________________________________________________ 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  
High-Voltage, Step-Down DC-DC
Controllers in µMAX
(Circuit of Figure 1, TA = +25°C, unless otherwise specified.)
Typical Operating Characteristics
EFFICIENCY vs. LOAD CURRENT
(VOUT = +3.3V)
100
B
A
80
60 C D
40
20
0
0.0001 0.001 0.01
A: VIN = +5.5V
B: VIN = +12.0V
C: VIN = +24.0V
D: VIN = +36.0V
0.1 1 10
LOAD CURRENT (A)
IN PIN QUIESCENT CURRENT
vs. INPUT VOLTAGE (3.5V TO 5.5V)
6
5
4
3
2
1
VOUT = 3.3V
0
3.5
4.5
INPUT VOLTAGE (V)
5.5
EFFICIENCY vs. LOAD CURRENT
(VOUT = +5.0V)
100
B
A
80
CD
60
40
20
0
0.0001
140
A: VIN = +7.2V
B: VIN = +12.0V
C: VIN = +24.0V
D: VIN = +36.0V
0.001 0.01 0.1
1
LOAD CURRENT (A)
SWITCHING FREQUENCY
vs. INPUT VOLTAGE
10
120
100
80
60
40
20
0
0
VOUT = 3.3V
IOUT = 2.0A
10 20 30
INPUT VOLTAGE (V)
40
IN PIN QUIESCENT CURRENT
vs. INPUT VOLTAGE (5.5V TO 36V)
110
105
100
95
90
85
80
0
95
94
93
92
91
90
89
88
87
86
85
-50
10 20 30
INPUT VOLTAGE (V)
IN PIN QUIESCENT CURRENT
vs. TEMPERATURE
-25 0 25 50 75 100
TEMPERATURE (°C)
40
125
EXT RISE AND FALL TIMES
vs. CAPACITANCE
120
VIN = +5V
100
80
60
tFALL
40
20
0
0
tRISE
1000 2000 3000 4000
CAPACITANCE (pF)
5000
EXT RISE AND FALL TIMES
vs. TEMPERATURE
50
45
VIN = +5V
CL = 1000pF
40 tRISE
35
30
25 tFALL
20
15
10
5
0
-50 -25
0 25 50 75 100 125
TEMPERATURE (°C)
CURRENT-SENSE TRIP LEVEL
vs. TEMPERATURE
115
110
105
100
95
90
85
-50 -25
0 25 50 75 100 125
TEMPERATURE (°C)
_______________________________________________________________________________________ 5
5 Page 
High-Voltage, Step-Down DC-DC
Controllers in µMAX
loss. For 1% error, the current through R2 should be at
least 100 times FB’s input bias current.
Current-Sense-Resistor Selection
The current-sense comparator limits the peak switching
current to VCS/RSENSE, where RSENSE is the value of
the current-sense resistor and VCS is the current-sense
threshold. VCS is typically 100mV. Minimizing the peak
switching current will increase efficiency and reduce
the size and cost of external components. However,
since available output current is a function of the peak
switching current, the peak current limit must not be set
too low.
Set the peak current limit to 1.3 times the maximum
load current by setting the current-sense resistor to:
R CS
=
1.3
VCS(MIN)
x IOUT(MAX)
Inductor Selection
The essential parameters for inductor selection are induc-
tance and current rating. The MAX1744/MAX1745 ope-
rate with a wide range of inductance values. In many
applications, values between 4.7μH and 100μH take best
advantage of the controller’s high switching frequency.
Calculate the minimum inductance value as follows:
( )L (MIN) = VIN - VOUT x 1μs
VCS(MIN)
RCS
where 1μs is the minimum on-time. Inductor values
between 2 and 10 times L(MIN) are recommended. With
high inductor values, the MAX1744/MAX1745 begin
continuous-conduction operation at a lower fraction of
the full load (see the Detailed Description section).
The inductor’s saturation and heating current ratings
must be greater than the peak switching current to pre-
vent overheating and core saturation. Saturation occurs
when the inductor’s magnetic flux density reaches the
maximum level the core can support, and inductance
starts to fall. The heating current rating is the maximum
DC current the inductor can sustain without overheating.
For optimum efficiency, the inductor windings’ resis-
tance should be less than the current-sense resistance.
If necessary, use a toroid, pot-core, or shielded-core
inductor to minimize radiated noise. Table 1 lists induc-
tor types and suppliers for various applications.
FROM
OUTPUT
R2
TO FB
R3
Figure 5. Adjustable-Output Operation Using the MAX1745
External Switching Transistor
The MAX1744/MAX1745 drive a P-channel enhance-
ment-mode MOSFET. The EXT output swings from VH
to IN. Be sure that the MOSFET’s on-resistance is spec-
ified for 5V gate drive or less. Table 1 recommends
MOSFET suppliers.
Four important parameters for selecting a P-channel
MOSFET are drain-to-source breakdown voltage, cur-
rent rating, total gate charge (Qg), and RDS(ON). The
drain-to-source breakdown voltage rating should be at
least a few volts higher than VIN. Choose a MOSFET
with a maximum continuous drain-current rating higher
than the peak current limit:
ID(MAX)
≥
ILIM(MAX)
=
VCS(MAX)
R SENSE
The Qg specification should be 80nC or less to ensure
fast drain voltage rise and fall times, and reduce power
losses during transition through the linear region. Qg
specifies all of the capacitances associated with charging
the MOSFET gate. EXT pin rise and fall times vary with dif-
ferent capacitive loads, as shown in the Typical Operating
Characteristics. RDS(ON) should be as low as practical to
reduce power losses while the MOSFET is on. It should
be equal to or less than the current-sense resistor.
______________________________________________________________________________________ 11
11 Page | ||
| Páginas | Total 15 Páginas | |
| PDF Descargar | [ Datasheet MAX1744.PDF ] | |
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