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PDF MAX724 Data sheet ( Hoja de datos )
| Número de pieza | MAX724 | |
| Descripción | 5V/3.3V/3V 5A Step-Down / PWM / Switch-Mode DC-DC Regulators | |
| Fabricantes | Maxim Integrated | |
| Logotipo |  |
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19-0107; Rev 3; 9/95
5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
_______________General Description
The MAX724/MAX726 are monolithic, bipolar, pulse-
width modulation (PWM), switch-mode DC-DC regula-
tors optimized for step-down applications. The
MAX724 is rated at 5A, and the MAX726 at 2A. Few
external components are needed for standard opera-
tion because the power switch, oscillator, and control
circuitry are all on-chip. Employing a classic buck
topology, these regulators perform high-current step-
down functions, but can also be configured as invert-
ers, negative boost converters, or flyback converters.
These regulators have excellent dynamic and transient
response characteristics, while featuring cycle-by-cycle
current limiting to protect against overcurrent faults and
short-circuit output faults. The MAX724/MAX726 also
have a wide 8V to 40V input range in the buck step-
down configuration. In inverting and boost configura-
tions, the input can be as low as 5V.
The MAX724/MAX726 are available in a 5-pin TO-220
package. The devices have a preset 100kHz oscillator
frequency and a preset current limit of 6.5A (MAX724)
or 2.6A (MAX726).
_______________________Applications
Distributed Power from High-Voltage Buses
High-Current, High-Voltage Step-Down Applications
High-Current Inverter
Negative Boost Converter
Multiple-Output Buck Converter
Isolated DC-DC Conversion
___________________________Features
o Input Range: Up to 40V
o 5A On-Chip Power Switch (MAX724)
2A On-Chip Power Switch (MAX726)
o Adjustable Output: 2.5V to 35V
o 100kHz Switching Frequency
o Excellent Dynamic Characteristics
o Few External Components
o 8.5mA Quiescent Current
o TO-220 Package
______________Ordering Information
PART
MAX724CCK
MAX724ECK
MAX726CCK
MAX726ECK
TEMP. RANGE
0°C to +70°C
-40°C to +85°C
0°C to +70°C
-40°C to +85°C
PIN-PACKAGE
5 TO-220
5 TO-220
5 TO-220
5 TO-220
__________Typical Operating Circuit
INPUT
8V TO 40V
220µF
VIN VSW
MAX724
VC FB
2.7k
0.01µF
GND
50µH
MBR745
OUTPUT
5V AT 5A
2.8k
2.21k
470µF
5A STEP-DOWN CONVERTER
__________________Pin Configuration
FRONT VIEW
MAX724
MAX726
5
4
3
2
1
5-PIN TO-220
CASE IS CONNECTED TO GROUND.
STANDARD PACKAGE HAS STAGGERED LEADS.
CONTACT FACTORY FOR STRAIGHT LEADS.
VIN
VSW
GND
VC
FB
________________________________________________________________ Maxim Integrated Products 1
Call toll free 1-800-998-8800 for free samples or literature.
1 page  
5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
______________________________________________________________Pin Description
PIN NAME
FUNCTION
Feedback Input is the error amplifier's inverting input, and controls output voltage by adjusting switch duty cycle.
1 FB Input bias current is typically 0.5µA when the error amplifier is balanced (IOUT = 0V). FB also aids current limiting
by reducing the oscillator frequency when the output voltage is low. (See the Applications Information section.)
Error-Amplifier Output. A series RC network connected to this pin compensates the MAX724/MAX726. Output
2 VC swing is limited to about 5.8V in the positive direction and -0.7V in the negative direction. VC can also synchro-
nize the MAX724/MAX726 to an external clock. (See the Applications Information section).
Ground requires a short low-noise connection to ensure good load regulation. The internal reference is referred
3 GND to GND, so errors at this pin are multiplied by the error amplifier. See the Applications Information section for
grounding details.
4
VSW
Internal Power Switch Output. The Switch output can swing 35V below ground and is rated for 5A (MAX724), 2A
(MAX726).
5
VIN
VIN supplies power to the MAX724/MAX726's internal circuitry and also connects to the collector. VIN must be
bypassed with a low-ESR capacitor, typically 200µF or 220µF.
_________________Detailed Description
The MAX724/MAX726 are complete, single-chip, pulse-
width modulation (PWM), step-down DC-DC converters
(Figure 1). All oscillator (100kHz), control, and current-
limit circuitry, including a 5A power switch (2A for
MAX726), are included on-chip. The oscillator turns on
the switch (VSW) at the beginning of each clock cycle.
The switch turns off at a point later in the clock cycle,
which is a function of the signal provided by the error
amplifier. The maximum switch duty cycle is approxi-
mately 93% at the MAX724/MAX726's 100kHz switch-
ing frequency.
Both the input (FB) and output (VC) of the error
amplifier are brought out to simplify compensation.
Most applications require only a single series RC
network connected from VC to ground. The error
amplifier is a transconductance amplifier with a gM of
approximately 5000µmho. When slewing, VC can
source about 140µA, and sink about 1.1mA. This
asymmetry helps minimize start-up overshoot by
allowing the amplifier output to slew more quickly in
the negative direction.
Current limiting is provided by the current-limit com-
parator. If the current-limit threshold is exceeded, the
switch cycle terminates within about 600ns. The cur-
rent-limit threshold is internally set to approximately
6.5A (2.6A for MAX726). VSW is a power NPN, internally
driven by the PWM controller circuitry. VSW can swing
35V below ground and is rated for 5A (2A for MAX726).
Basic Step-Down Application
Figure 2 shows the MAX724/MAX726 in a basic step-
down DC-DC converter. Typical MAX724 waveforms
are shown in Figure 3 for VIN = 20V, VOUT = 5V, L =
50µH, and IOUT = 3A and 0.16A. Two sets of wave-
forms are shown. One set shows high load current (3A)
where inductor current never falls to zero during the
switch "off-cycle" (continuous-conduction mode, CCM).
The second set of waveforms, at low output current
(0.16A), shows inductor current at zero during the latter
half of the switch off-cycle (discontinuous-conduction
mode, DCM). The transition from CCM to DCM occurs
at an output current (IDCM) that can be derived with the
following equation:
IDCM = (VOUT + VD) [(VIN - VSW) - (VOUT + VD)]
2 (VIN - VSW) fOSCL
where VD is the diode forward voltage drop, VSW is the
voltage drop across the switch, and fOSC = 100kHz. In
most applications, the distinction between CCM and
DCM is academic since actual performance differences
are minimal. All CCM designs can be expected to exhibit
DCM behavior at some level of reduced load current.
_______________________________________________________________________________________ 5
5 Page 
5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
___________________Typical Applications
Positive-to-Negative DC-DC Inverter
The MAX724/MAX726 can convert positive input volt-
ages to negative outputs if the sum of input and output
voltage is greater than 8V, and the minimum positive
supply is 4.5V. The connection in Figure 7 shows the
MAX724 generating -5V. The device's GND pin is con-
nected to the negative output, which allows the feed-
back divider, R3, and R4 to be connected normally. If
the GND pin were tied to circuit ground, a level shift
and inversion would be required to generate the proper
feedback signal.
Component values in Figure 8 are shown for input volt-
ages up to 35V and for a 1A output. If the maximum
input voltage is lower, a Schottky diode with lower
reverse breakdown than the MBR745 (D1) may be
used. If lower output current is needed, then the cur-
rent rating of both D1 and L1 may be reduced. In addi-
tion, if the minimum input voltage is higher than 4.5V,
then greater output current can be supplied.
R1, R2, and C4 provide compensation for low input
voltages, but R1 and R2 also figure in the output-volt-
age calculation because they are effectively connected
in parallel with R3. For larger negative outputs,
increase R1, R2, and R3 proportionally while maintain-
ing the following relationships. If VIN does not fall below
2VOUT, then R1, R2, and C4 can be omitted and only R3
and R4 set the output voltage.
R4 = 1.82kΩ
R3 = |VOUT| - 2.37 (in kΩ)
R1 = 1.86 (R3)
R2 = 3.65 (R3)
Negative Boost DC-DC Converter
The MAX724/MAX726 can also work as a negative
boost converter (Figure 8) by tying the GND pin to the
negative output. This allows the regulator to operate
from input voltages as low as -4.5V. If the regulated
output is at least -8V, R1 and R2 set the output volt-
age as in a conventional connection, with R1 selected
from:
R1
=
VOUT R2
2.21
-
R2
L1 must be a low value to maintain stability, but if VIN is
greater than -10V, L1 can be increased to 50µH. Since
this is a boost configuration, if the input voltage
exceeds the output voltage, D1 will pull the output more
negative and out of regulation. Also, if the output is
pulled toward ground, D1 will drag down the input sup-
ply. For this reason, this configuration is not short-cir-
cuit protected.
VIN
+4.5V TO +35V
220µH
C1 50V
VIN
VSW
MAX724
L1
50µH
5A
R1
5.1k
R2
10k
R3
2.74k
C2
1000µF
10V
VC
C3
0.1µF
FB
GND
D1 - MOTOROLA MBR745
C1 - NICHICON UPL1C221MRH6
D1
C4
0.01µF
R4
1.82k
C2 - NICHICON UPL1A102MRH6
L1 - COILTRONICS CTX25-5-52
-5V
1A
ALL RESISTORS HAVE 1% TOLERANCE
Figure 7. Positive-to-Negative DC-DC Inverter
C3
100µF
25V
-VIN
-4.5V TO -15V
VIN
FB
MAX724
VSW
GND VC
1000pF
R1
12.7k
R2
2.21k C1
1000µF
25V
0.01µF
C2
1µF
R3
750Ω
L1
25µH
D1
MBR735
VOUT
-15V
Figure 8. Negative Step-Up DC-DC Converter
______________________________________________________________________________________ 11
11 Page | ||
| Páginas | Total 12 Páginas | |
| PDF Descargar | [ Datasheet MAX724.PDF ] | |
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