max724(2).pdf

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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).
___________________________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
MAX724/MAX726
______________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
_______________________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
__________Typical Operating Circuit
__________________Pin Configuration
FRONT VIEW
INPUT
8V TO 40V
V
IN
220µF
V
SW
50µH
MBR745
FB
2.21k
GND
0.01µF
OUTPUT
5V AT 5A
2.8k
470µF
MAX724
V
C
2.7k
MAX724
MAX726
5
4
3
2
1
V
IN
V
SW
GND
V
C
FB
5-PIN TO-220
CASE IS CONNECTED TO GROUND.
STANDARD PACKAGE HAS STAGGERED LEADS.
CONTACT FACTORY FOR STRAIGHT LEADS.
5A STEP-DOWN CONVERTER
________________________________________________________________
Maxim Integrated Products
1
Call toll free 1-800-998-8800 for free samples or literature.
5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
MAX724/MAX726
ABSOLUTE MAXIMUM RATINGS
Input Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45V
Switch Voltage with Respect to Input Voltage. . . . . . . . . . . . . . . . 50V
Switch Voltage with Respect to Ground Pin (V
SW
Negative)
(Note 1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35V
Feedback Pin Voltage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V, +10V
Operating Temperature Ranges
MAX72_CCK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0°C to +70°C
MAX72_ECK. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -40°C to +85°C
Junction Temperature Ranges
MAX72_CCK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0°C to +125°C
MAX72_ECK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - 40°C to +125°C
Storage Temperature Range . . . . . . . . . . . . . . . . . . . -65°C to +160°C
Lead Temperature (soldering, 10sec). . . . . . . . . . . . . . . . . . . . +300°C
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(V
IN
= 25V, T
j
= T
MIN
to T
MAX
, unless otherwise noted.)
PARAMETER
Input Supply Voltage Range
I
SW
= 1A
MAX724
Switch-On Voltage (Note 2)
I
SW
= 5A
MAX726
MAX724
Switch-Off Leakage
MAX726
Supply Current (Note 3)
Minimum Supply Voltage
I
SW
= 0.5A
I
SW
= 2A
V
IN
≤
25V, V
SW
= 0V
V
IN
= 40V, V
SW
= 0V
V
IN
≤
25V, V
SW
= 0V
V
IN
= 40V, V
SW
= 0V
T
j
= +25°C
T
j
= +25°C
T
j
= +25°C
T
j
= +25°C
8.5
7.3
T
j
≥
0°C
T
j
< 0°C
5.5
2.0
85
T
j
= +25°C
Switching Frequency
V
FB
= grounded through 2kΩ (Note 5)
Switching Frequency Line Regulation
8V
≤
V
IN
≤
40V
T
j
≤
+125°C
T
j
= +25°C
90
85
20
0.03
0.1
%/V
3.5
3.5
6.5
2.6
90
100
110
120
kHz
5
10
T
j
≥
0°C
T
j
< 0°C
T
j
≥
0°C
T
j
< 0°C
CONDITIONS
MIN
8.0
TYP MAX
40.0
1.85
2.10
2.30
2.50
1.2
1.7
300
500
150
250
11
8.0
4.8
5.0
8.5
3.2
mA
V
V
A
%
µA
V
UNITS
V
V
FB
= 2.5V, V
IN
≤
40V
Normal Mode
Start-Up Mode (Note 4)
MAX724
MAX726
Switch-Current Limit (Note 5)
Maximum Duty Cycle
2
_______________________________________________________________________________________
5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
ELECTRICAL CHARACTERISTICS (continued)
(V
IN
= 25V, T
j
= T
MIN
to T
MAX
, unless otherwise noted.)
PARAMETER
Error-Amplifier Voltage Gain
Error-Amplifier Transconductance
Error-Amplifier Source Current
Error-Amplifier Sink Current
Feedback Pin Bias Current
Reference Voltage
Reference Voltage Tolerance
Reference Voltage Line Regulation
VC Voltage at 0% Duty Cycle
Thermal Resistance,
Junction to Case (Note 6)
MAX724
MAX726
V
FB
= 2V
V
FB
= 2.5V
V
FB
= VREF
V
C
= 2V
VREF (nominal) = 2.21V
T
j
= +25°C
All conditions of input voltage, output voltage,
temperature and load current
8V
≤
V
IN
≤
40V
T
j
= +25°C
T
j
= T
MIN
to T
MAX
1V
≤
V
C
≤
4V
CONDITIONS
T
j
= +25°C
T
j
= +25°C
T
j
= +25°C
T
j
= +25°C
100
0.6
MIN
TYP MAX
2000
3000 5000 9000
140
1.0
0.5
225
1.7
2
UNITS
V/V
µmho
µA
mA
µA
V
%
%/V
V
mV/°C
2.5
4.0
°C/W
MAX724/MAX726
2.155 2.210 2.265
±0.5 ±1.5
±1.0
±2.5
0.005 0.02
1.5
-4
Note 1:
Do not exceed switch-to-input voltage limitation.
Note 2:
For switch currents between 1A and 5A (2A for MAX726), maximum switch-on voltage can be calculated via linear
interpolation.
Note 3:
By setting the feedback pin (FB) to 2.5V, the V
C
pin is forced to its low clamp level and the switch duty cycle is forced to
zero, approximating the zero load condition.
Note 4:
For proper regulation, total voltage from V
IN
to GND must be
≥
8V after start-up.
Note 5:
To avoid extremely short switch-on times, the switch frequency is internally scaled down when V
FB
is less than 1.3V. Switch-
current limit is tested with V
FB
adjusted to give a 1µs minimum switch-on time.
Note 6:
Guaranteed, not production tested.
__________________________________________Typical Operating Characteristics
MAX724
STEP-DOWN CONVERTER EFFICIENCY
vs. OUTPUT CURRENT
110
CIRCUIT OF FIGURE 2
100
SUPPLY CURRENT (mA)
EFFICIENCY (%)
90
80
70
60
50
0
1
2
3
4
5
6
OUTPUT CURRENT (A)
V
OUT
= 5V, V
IN
= 15V
V
OUT
= 12V, V
IN
= 20V
16
QUIESCENT SUPPLY CURRENT (mA)
14
12
10
8
6
4
2
0
-40 -25
0
25
50
75
100
125
JUNCTION TEMPERATURE (°C)
CIRCUIT OF FIGURE 2
V
IN
= 25V, V
OUT
= 5V
I
OUT
= 1mA
SUPPLY CURRENT
vs. JUNCTION TEMPERATURE
20
18
16
14
12
10
8
6
4
2
0
0
QUIESCENT SUPPLY CURRENT
vs. INPUT VOLTAGE
DEVICE NOT SWITCHING
V
C
= 1V
10
20
30
40
V
IN
INPUT VOLTAGE (V)
_______________________________________________________________________________________
3
5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
MAX724/MAX726
____________________________Typical Operating Characteristics (continued)
REFERENCE VOLTAGE
vs. JUNCTION TEMPERATURE
2.25
SWITCHING FREQUENCY (kHz)
2.24
REFERENCE VOLTAGE (V)
2.23
2.22
2.21
2.20
2.19
2.18
2.17
-40 -25
0
25
50
75
100
125
JUNCTION TEMPERATURE (°C)
120
115
110
105
100
95
90
85
80
SWITCHING FREQUENCY
vs. JUNCTION TEMPERATURE
3.0
SWITCH-ON VOLTAGE
vs. SWITCH CURRENT
T
j
= +25°C
SWITCH-ON VOLTAGE (V)
2.5
MAX724
2.0
1.5
1.0
MAX726
0.5
-40 -25
0
25
50
75
100
125
0
1
2
3
4
5
6
JUNCTION TEMPERATURE (°C)
SWITCH CURRENT (A)
ERROR-AMPLIFIER PHASE AND g
M
8000
TRANSCONDUCTANCE (µmho)
7000
PHASE
6000
5000
4000
3000
2000
1000
0
1k
10k
100k
FREQUENCY (Hz)
1M
10M
g
M
200
SWITCHING FREQUENCY (kHz)
150
100
50
0
-50
-100
-150
-200
PHASE (degrees)
160
140
120
100
80
60
40
20
0
0
SWITCHING FREQUENCY
vs. FEEDBACK PIN VOLTAGE
+125°C
-40°C
+25°C
0.5
1.0
1.5
2.0
2.5
3.0
FB VOLTAGE (V)
FEEDBACK PIN CURRENT
vs. FB VOLTAGE
500
400
300
FB CURRENT (µA)
200
100
0
-100
-200
-300
-400
-500
0
1
2
3
4
5
6
7
8
9 10
FB VOLTAGE (V)
START OF FREQUENCY
SHIFTING
OUTPUT CURRENT LIMIT (A)
8
7
6
5
4
3
2
1
0
-40 -25
OUTPUT CURRENT LIMIT
vs. TEMPERATURE
MAX724
MAX726
0
25
50
75
100
125
JUNCTION TEMPERATURE (°C)
4
_______________________________________________________________________________________
5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
______________________________________________________________Pin Description
PIN
1
NAME
FB
FUNCTION
Feedback Input is the error amplifier's inverting input, and controls output voltage by adjusting switch duty cycle.
Input bias current is typically 0.5µA when the error amplifier is balanced (I
OUT
= 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
swing is limited to about 5.8V in the positive direction and -0.7V in the negative direction. V
C
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
to GND, so errors at this pin are multiplied by the error amplifier. See the
Applications Information
section for
grounding details.
Internal Power Switch Output. The
S
witch output can swing 35V below ground and is rated for 5A (MAX724), 2A
(MAX726).
V
IN
supplies power to the MAX724/MAX726's internal circuitry and also connects to the collector. V
IN
must be
bypassed with a low-ESR capacitor, typically 200µF or 220µF.
MAX724/MAX726
2
V
C
3
GND
4
5
V
SW
V
IN
_________________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 (V
SW
) 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 (V
C
) of the error
amplifier are brought out to simplify compensation.
Most applications require only a single series RC
network connected from V
C
to ground. The error
amplifier is a transconductance amplifier with a g
M
of
approximately 5000µmho. When slewing, V
C
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). V
SW
is a power NPN, internally
driven by the PWM controller circuitry. V
SW
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 V
IN
= 20V, V
OUT
= 5V, L =
50µH, and I
OUT
= 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 (I
DCM
) that can be derived with the
following equation:
I
DCM
= (V
OUT
+ V
D
) [(V
IN
- V
SW
) - (V
OUT
+ V
D
)]
2 (V
IN
- V
SW
) f
OSC
L
where V
D
is the diode forward voltage drop, V
SW
is the
voltage drop across the switch, and f
OSC
= 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.
_______________________________________________________________________________________
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