Rainbow Electronics MAX726 User Manual

19-0107; Rev 3; 9/95

Switch-Mode DC-DC Regulators

_______________General Description

The MAX724/MAX726 are monolithic, bipolar, pulse­width modulation (PWM), switch-mode DC-DC regula­tors optimized for step-down applications. The
MAX724 is rated at 5A, and the MAX726 at 2A. Few
external components are needed for standard opera­tion because the power switch, oscillator, and control
circuitry are all on-chip. Employing a classic buck
topology, these regulators perform high-current step­down functions, but can also be configured as invert­ers, 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 step­down configuration. In inverting and boost configura­tions, 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

5A/2A Step-Down, PWM,

___________________________Features

♦ Input Range: Up to 40V
♦ 5A On-Chip Power Switch (MAX724)

2A On-Chip Power Switch (MAX726)

♦ Adjustable Output: 2.5V to 35V
♦ 100kHz Switching Frequency
♦ Excellent Dynamic Characteristics
♦ Few External Components
♦ 8.5mA Quiescent Current
♦ TO-220 Package

______________Ordering Information

PART TEMP. RANGE PIN-PACKAGE

MAX724CCK

MAX724ECK -40°C to+85°C 5 TO-220
MAX726CCK
MAX726ECK -40°C to+85°C 5 TO-220

0°C to+70°C 5 TO-220

0°C to+70°C 5 TO-220

MAX724/MAX726

__________Typical Operating Circuit

INPUT

8V TO 40V

220µF

2.7k

V

IN

V

C

0.01µF

5A STEP-DOWN CONVERTER

V

SW

MAX724

FB

GND

________________________________________________________________

50µH

MBR745

2.21k

2.8k

OUTPUT
5V AT 5A

470µF

__________________Pin Configuration

FRONT VIEW

MAX724
MAX726

5-PIN TO-220

CASE IS CONNECTED TO GROUND. 
STANDARD PACKAGE HAS STAGGERED LEADS.
CONTACT FACTORY FOR STRAIGHT LEADS.

Maxim Integrated Products

Call toll free 1-800-998-8800 for free samples or literature.

V

GND

FB

IN

V

SW

V

C

1

5
4
3
2

1

5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators

ABSOLUTE MAXIMUM RATINGS

Input Voltage..................................................45V

Switch Voltage with Respect to Input Voltage................50V

Switch Voltage with Respect to Ground Pin (V

(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

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.

Negative)

SW

ELECTRICAL CHARACTERISTICS

(VIN= 25V, Tj= T

MAX724/MAX726

Input Supply Voltage Range 8.0 40.0 V

Switch-On Voltage (Note 2)

Switch-Off Leakage

Minimum Supply Voltage

Switch-Current Limit (Note 5) A

Switching Frequency kHz

to T

MIN

PARAMETER CONDITIONS MIN TYP MAX UNITS

, unless otherwise noted.)

MAX

MAX724

MAX726

MAX724

MAX726

VFB= 2.5V, VIN≤ 40V 8.5 11Supply Current (Note 3) mA
Normal Mode 7.3 8.0

Start-Up Mode (Note 4)

MAX724 5.5 6.5 8.5
MAX726 2.0 2.6 3.2

VFB= grounded through 2kΩ (Note 5)
8V ≤ VIN≤ 40V

ISW= 1A

ISW= 5A

ISW= 0.5A
ISW= 2A
VIN≤ 25V, VSW= 0V
VIN= 40V, VSW= 0V
VIN≤ 25V, VSW= 0V
VIN= 40V, VSW= 0V

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

Tj≥ 0°C
Tj< 0°C
Tj≥ 0°C
Tj< 0°C

Tj= +25°C
Tj= +25°C
Tj= +25°C
Tj= +25°C

Tj≥ 0°C
Tj< 0°C

Tj= +25°C
Tj≤ +125°C
Tj= +25°C

85 90Maximum Duty Cycle %
90 100 110
85 120

1.85

2.10

2.30

2.50

1.2

1.7

5 300

10 500

150
250

3.5 4.8

3.5 5.0

20

0.03 0.1Switching Frequency Line Regulation %/V

V

µA

V

V

2 _______________________________________________________________________________________

5A/2A Step-Down, PWM,

Switch-Mode DC-DC Regulators

ELECTRICAL CHARACTERISTICS (continued)

(VIN= 25V, Tj= T

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)

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

zero, approximating the zero load condition.

Note 4: For proper regulation, total voltage from V
Note 5: To avoid extremely short switch-on times, the switch frequency is internally scaled down when V

current limit is tested with V

Note 6: Guaranteed, not production tested.

to T

MIN

, unless otherwise noted.)

MAX

PARAMETER CONDITIONS MIN TYP MAX UNITS

1V ≤ VC≤ 4V Tj= +25°C

Tj= +25°C

VFB= 2V
VFB= 2.5V

Tj= +25°C
Tj= +25°C

VFB= VREF
VC= 2V

VREF (nominal) = 2.21V

Tj= +25°C

All conditions of input voltage, output voltage,
temperature and load current

8V ≤ VIN≤ 40V

Tj= +25°C
Tj= T

MIN

to T

MAX

2000 V/V

3000 5000 9000 µmho

100 140 225 µA

0.6 1.0 1.7 mA

0.5 2 µA

2.155 2.210 2.265 V
±0.5 ±1.5

±1.0 ±2.5

0.005 0.02 %/V

1.5 V

-4 mV/°C
MAX724 2.5
MAX726 4.0

pin is forced to its low clamp level and the switch duty cycle is forced to

C

to GND must be ≥ 8V after start-up.

IN

adjusted to give a 1µs minimum switch-on time.

FB

is less than 1.3V. Switch-

FB

MAX724/MAX726

%

°C/W

__________________________________________Typical Operating Characteristics

STEP-DOWN CONVERTER EFFICIENCY

110

100

90

80

EFFICIENCY (%)

70

60

50

06

MAX724

vs. OUTPUT CURRENT

CIRCUIT OF FIGURE 2

V

= 12V, VIN = 20V

OUT

V

= 5V, VIN = 15V

OUT

135

2

OUTPUT CURRENT (A)

4

16
14

12

10

8
6

SUPPLY CURRENT (mA)

4
2
0

-40 100

_______________________________________________________________________________________ 3

SUPPLY CURRENT

vs. JUNCTION TEMPERATURE

CIRCUIT OF FIGURE 2

VIN = 25V, V
I

OUT

-25 25 75

0

JUNCTION TEMPERATURE (°C)

= 1mA

= 5V

OUT

50

QUIESCENT SUPPLY CURRENT (mA)

125

QUIESCENT SUPPLY CURRENT

20
18
16

14
12
10

8
6
4
2
0

vs. INPUT VOLTAGE

DEVICE NOT SWITCHING

VC = 1V

0

10 30

V

IN

20

INPUT VOLTAGE (V)

40

5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators

____________________________Typical Operating Characteristics (continued)

REFERENCE VOLTAGE

vs. JUNCTION TEMPERATURE

2.25

2.24

2.23

2.22

2.21

2.20

2.19

REFERENCE VOLTAGE (V)

2.18

MAX724/MAX726

2.17

-25 25 75

-40 100
JUNCTION TEMPERATURE (°C)

8000

7000

6000

5000

4000

3000
2000

TRANSCONDUCTANCE (µmho)

1000

0

50

0

ERROR-AMPLIFIER PHASE AND g

PHASE

1k



g

M

10k 1M

100k

FREQUENCY (Hz)

125

SWITCHING FREQUENCY

vs. JUNCTION TEMPERATURE

120
115

110
105
100

95
90

SWITCHING FREQUENCY (kHz)

85
80

-25 25 75

0

-40 100
JUNCTION TEMPERATURE (°C)

M

200
150
100
50
0

-50

PHASE (degrees)

-100

-150

-200

10M

50

SWITCHING FREQUENCY (kHz)

SWITCH-ON VOLTAGE (V)

125

SWITCHING FREQUENCY

vs. FEEDBACK PIN VOLTAGE

160
140
120

100

-40°C

80
60

40
20

0

0

0.5 1.5

SWITCH-ON VOLTAGE

3.0

2.5

2.0

1.5

1.0

0.5

1.0

vs. SWITCH CURRENT

Tj = +25°C

MAX724

MAX726

06

135

2

SWITCH CURRENT (A)

+125°C

+25°C

FB VOLTAGE (V)

2.0

4

2.5 3.0

FB CURRENT (µA)

500
400

300
200
100

0

-100

-200

-300

-400

-500

FEEDBACK PIN CURRENT

vs. FB VOLTAGE

START OF FREQUENCY
SHIFTING

0

13

2

FB VOLTAGE (V)

4

56

OUTPUT CURRENT LIMIT (A)

78910

OUTPUT CURRENT LIMIT

8

7

5
4
3
2
1
0

-40

-25 2560
JUNCTION TEMPERATURE (°C)

vs. TEMPERATURE

MAX724

MAX726

50

75

4 _______________________________________________________________________________________

100 125