max724-6.pdf

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

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

MAX724 CCK

0°C to +70°C

5 TO-220

MAX724ECK

-40°C to +85°C

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

MAX726 CCK

0°C to +70°C

5 TO-220

MAX726ECK

-40°C to +85°C

5 TO-220

__________Typical Operating Circuit

__________________Pin Configuration

FRONT VIEW

INPUT

8V TO 40V

OUTPUT

5V AT 5A

V IN

V SW

220

V SW

MAX724

MAX726

2.8k

MBR745

MAX724

GND

V C

470 m F

2.7k

2.21k

GND

5-PIN TO-220

0.01 m F

5A STEP-DOWN CONVERTER

CASE IS CONNECTED TO GROUND.

STANDARD PACKAGE HAS STAGGERED LEADS.

CONTACT FACTORY FOR STRAIGHT LEADS.

________________________________________________________________ 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

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

CONDITIONS

MIN TYP MAX UNITS

Input Supply Voltage Range

8.0

40.0

I SW = 1A

T j ³

0°C

1.85

T j < 0°C

2.10

MAX724

0°C

T j < 0°C

2.30

Switch-On Voltage (Note 2)

I SW = 5A

2.50

MAX726

I SW = 0.5A

I SW = 2A

V IN ² 25V, V SW = 0V

V IN = 40V, V SW = 0V

V IN ²

1.2

1.7

T j = +25°C

300

MAX724

T j = +25°C

500

Switch-Off Leakage

µA

25V, V SW = 0V

V IN = 40V, V SW = 0V

T j = +25°C

150

MAX726

T j = +25°C

250

Supply Current (Note 3)

V FB = 2.5V, V IN ²

40V

8.5

Normal Mode

7.3

8.0

Minimum Supply Voltage

Start-Up Mode (Note 4)

T j ³ 0°C

3.5

4.8

T j < 0°C

3.5

5.0

Switch-Current Limit (Note 5)

MAX724

5.5

6.5

8.5

MAX726

2.0

2.6

3.2

Maximum Duty Cycle

T j = +25°C

100 110

Switching Frequency

T j ²

+125°C

120

kHz

V FB = grounded through 2k

(Note 5)

T j = +25°C

Switching Frequency Line Regulation

8V ²

V IN ²

40V

0.03 0.1

%/V

2 _______________________________________________________________________________________

T j ³

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

CONDITIONS

MIN TYP MAX UNITS

Error-Amplifier Voltage Gain

1V ²

V C ²

T j = +25°C

2000

V/V

Error-Amplifier Transconductance

T j = +25°C

3000 5000 9000 µmho

Error-Amplifier Source Current

V FB = 2V

T j = +25°C

100 140 225

µA

Error-Amplifier Sink Current

V FB = 2.5V

T j = +25°C

0.6

1.0

1.7

Feedback Pin Bias Current

V FB = VREF

0.5

µA

Reference Voltage

V C = 2V

2.155 2.210 2.265

VREF (nominal) = 2.21V

T j = +25°C

±0.5 ±1.5

Reference Voltage Tolerance

All conditions of input voltage, output voltage,

temperature and load current

±1.0 ±2.5

Reference Voltage Line Regulation

V IN ²

40V

0.005 0.02

%/V

VC Voltage at 0% Duty Cycle

T j = +25°C

1.5

T j = T MIN to T MAX

-4

mV/°C

Thermal Resistance,

Junction to Case (Note 6)

MAX724

2.5

°C/W

MAX726

4.0

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

__________________________________________Typical Operating Characteristics

MAX724

STEP-DOWN CONVERTER EFFICIENCY

vs. OUTPUT CURRENT

SUPPLY CURRENT

vs. JUNCTION TEMPERATURE

QUIESCENT SUPPLY CURRENT

vs. INPUT VOLTAGE

110

CIRCUIT OF FIGURE 2

DEVICE NOT SWITCHING

100

VOUT = 12V, VIN = 20V

V IN = 25V, V OUT = 5V

IOUT = 1mA

V C = 1V

VOUT = 5V, VIN = 15V

-40

-25

100

125

OUTPUT CURRENT (A)

JUNCTION TEMPERATURE (°C)

VIN INPUT VOLTAGE (V)

_______________________________________________________________________________________ 3

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.

5A/2A Step-Down, PWM,

Switch-Mode DC-DC Regulators

____________________________Typical Operating Characteristics (continued)

REFERENCE VOLTAGE

vs. JUNCTION TEMPERATURE

SWITCHING FREQUENCY

vs. JUNCTION TEMPERATURE

SWITCH-ON VOLTAGE

vs. SWITCH CURRENT

2.25

120

3.0

2.24

115

T j = +25°C

2.5

2.23

110

2.22

105

2.0

MAX724

2.21

100

2.20

1.5

2.19

1.0

MAX726

2.18

2.17

0.5

-40

-25

100

125

-40

-25

100

125

JUNCTION TEMPERATURE (°C)

SWITCH CURRENT (A)

ERROR-AMPLIFIER PHASE AND g M

SWITCHING FREQUENCY

vs. FEEDBACK PIN VOLTAGE

8000

200

160

7000

PHASE

150

140

+125°C

6000

100

120

5000

4000

g M

100

-40°C

+25°C

3000

-50

2000

1000

-100

-150

-200

10k

100k

10M

0.5

1.0

1.5

2.0

2.5

3.0

FREQUENCY (Hz)

FB VOLTAGE (V)

FEEDBACK PIN CURRENT

vs. FB VOLTAGE

OUTPUT CURRENT LIMIT

vs. TEMPERATURE

500

400

300

MAX724

200

100

START OF FREQUENCY

SHIFTING

-100

-200

MAX726

-300

-400

-500

7890

-40

-25

100 125

FB VOLTAGE (V)

JUNCTION TEMPERATURE (°C)

4 _______________________________________________________________________________________

5A/2A Step-Down, PWM,

Switch-Mode DC-DC Regulators

______________________________________________________________Pin Description

NAME

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 Informationsection.)

V C

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 Informationsection).

GND

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 Informationsection for

grounding details.

V SW

Internal Power Switch Output. The S witch output can swing 35V below ground and is rated for 5A (MAX724), 2A

(MAX726).

V IN

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.

_________________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 5000 m mho. When slewing, V C can

source about 140 m A, 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

H, 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.

_______________________________________________________________________________________ 5

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 =

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