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

5A/2A Step-Down, PWM,

Switch-Mode DC-DC Regulators

______________________________________________________________Pin Description

MAX724/MAX726

PIN

Feedback Input is the error amplifier's inverting input, and controls output voltage by adjusting switch duty cycle.

FB1

Input bias current is typically 0.5µA when the error amplifier is balanced (I
by reducing the oscillator frequency when the output voltage is low. (See the

Error-Amplifier Output. A series RC network connected to this pin compensates the MAX724/MAX726. Output

V

2

4

5

swing is limited to about 5.8V in the positive direction and -0.7V in the negative direction. V

C

nize the MAX724/MAX726 to an external clock. (See the
Ground requires a short low-noise connection to ensure good load regulation. The internal reference is referred

GND3

to GND, so errors at this pin are multiplied by the error amplifier. See the
grounding details.

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

V

SW

(MAX726).
VINsupplies power to the MAX724/MAX726's internal circuitry and also connects to the collector. VINmust be

V

IN

bypassed with a low-ESR capacitor, typically 200µF or 220µF.

_________________Detailed Description

The MAX724/MAX726 are complete, single-chip, pulse­width modulation (PWM), step-down DC-DC converters
(Figure 1). All oscillator (100kHz), control, and current­limit 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 approxi­mately 93% at the MAX724/MAX726's 100kHz switch­ing frequency.

Both the input (FB) and output (V
amplifier are brought out to simplify compensation.
Most applications require only a single series RC
network connected from VCto ground. The error
amplifier is a transconductance amplifier with a gMof
approximately 5000µmho. When slewing, VCcan
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 com­parator. If the current-limit threshold is exceeded, the
switch cycle terminates within about 600ns. The cur­rent-limit threshold is internally set to approximately

) of the error

C

FUNCTIONNAME

= 0V). FB also aids current limiting

OUT

Applications Information

can also synchro-

Applications Information

Applications Information

6.5A (2.6A for MAX726). V
driven by the PWM controller circuitry. VSWcan swing

C

section).

is a power NPN, internally

SW

section.)

section for

35V below ground and is rated for 5A (2A for MAX726).

Basic Step-Down Application

Figure 2 shows the MAX724/MAX726 in a basic step­down DC-DC converter. Typical MAX724 waveforms
are shown in Figure 3 for VIN= 20V, V
50µH, and I
forms are shown. One set shows high load current (3A)

= 3A and 0.16A. Two sets of wave-

OUT

OUT

= 5V, L =

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
following equation:

(V

I

DCM

where V
voltage drop across the switch, and f

OUT

=

is the diode forward voltage drop, VSWis the

D

most applications, the distinction between CCM and

) that can be derived with the

DCM

+ VD) [(VIN- VSW) - (V

2 (V

- VSW) f

IN

OSC

L

OSC

OUT+VD

)]

= 100kHz. In

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

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

In DCM, ringing occurs at VSWin the latter part of the
switch off-cycle. This is due to the inductor resonating
with the parallel capacitance of the catch diode and the
VSWnode. This ringing is harmless and does not
appear at the output. Furthermore, attempts to damp
this ringing by adding circuitry will reduce efficiency
and are not advised. No off-state ringing occurs in
CCM because the diode always conducts during the
switch-off time and consequently damps any reso­nance at VSW.

MAX724/MAX726

FB

V

C

2.21V
REF

ERROR

AMPLIFIER

INTERNAL

BIAS

MAX724

Figure 1. MAX724 Block Diagram

INPUT

8V TO 40V

220µF

V

IN

MAX724
MAX726

V

C

R3

0.01µF

2.7k
C2

GND

Figure 2. Basic Step-Down Converter

100kHz

OSCILLATOR

GND

V

SW

FB

CURRENT-LIMIT

PWM

LOGIC

CONTROL

L

50µH (MAX724)

100µH (MAX726)

D
MBR745

COMPARATOR

SWITCH

5V at 5A (MAX724)
5V at 2A (MAX726)

R1

2.8k

R2

2.2k

OUTPUT

V

V

SW

C1
470µF

IN

_______________Component Selection

Table 1 lists component suppliers for inductors, capaci­tors, and diodes appropriate for use with the
MAX724/MAX726. Be sure to observe specified ratings
for all components.

Table 1. Component Suppliers

Surface-Mount Components (for designs typically below 2A)
Inductors: Sumida Electric - CDR125 Series

Capacitors: Matsuo - 267 series

Diodes: Motorola - MBRS series

Through-Hole Components

Inductors: Sumida - RCH-110 series

Capacitors: Nichicon - PL series low-ESR electrolytics

Diodes: General Purpose - 1N5820-1N5825

USA: Phone [phone]
Japan: Phone [phone]

Coiltronics - CTX series

Sprague - 595D series

Nihon - NSQ series

Cadell-Burns - 7070, 7300, 6860, and 7200 series

Renco - various series

Coiltronics - various series

United Chemi-Con - LXF series

Sanyo - OS-CON low-ESR organic semiconductor

Motorola - MBR and MBRD series

FAX [phone]

USA: Phone [phone]

FAX [phone]

USA: Phone [phone]

FAX [phone]

Japan: Phone [phone]
USA: Phone [phone]

FAX [phone]

USA: Phone [phone]

FAX [phone]

USA: Phone [phone]

FAX [phone]

Japan: Phone [phone]

FAX [phone]

(see above for phone number)
USA: Phone [phone]

FAX [phone]

USA: Phone [phone]

FAX [phone]

(see above for phone number)

USA: Phone [phone]

FAX [phone]

Japan: Phone [phone]

FAX [phone]

USA: Phone [phone]

FAX [phone]

USA: Phone [phone]

FAX [phone]

Japan: Phone 81-7-2070-6306

FAX 81-7-2070-1174

(see above for phone number)

6 _______________________________________________________________________________________

Switch-Mode DC-DC Regulators

Inductor Selection

Although most MAX724 designs perform satisfactorily
with 50µH inductors (100µH for the MAX726), the
MAX724/MAX726 are able to operate with values rang­ing from 5µH to 200µH. In some cases, inductors other
than 50µH may be desired to minimize size (lower
inductance), or reduce ripple (higher inductance). In
any case, inductor current must at least be rated for the
desired output current.

In high-current applications, pay particular attention to
both the RMS and peak inductor ratings. The induc­tor's peak current is limited by core saturation.
Exceeding the saturation limit actually reduces the
coil's inductance and energy storage ability, and
increases power loss. Inductor RMS current ratings
depend on heating effects in the coil windings.

The following equation calculates maximum output cur­rent as a function of inductance and input conditions:

V

I

= ISW-

OUT

where I
MAX724), VINis the maximum input voltage, V

is the maximum switch current (5.5A for

SW

output voltage, and f

OUT(VIN

is the switching frequency.

OSC

For the MAX724 example in Figure 2, with L = 50µH

= 25V,

and V

IN

I

OUT

= 5.5A -

5V (25V - 5V)

2 (105Hz) 25V (50 x 10-6H)

Note that increasing or decreasing inductor value pro­vides only small changes in maximum output current
(100µH = 5.3A, 20µH = 4.5A). The equation shows that
output current is mostly a function of the
MAX724/MAX726 current-limit value. Again, a 50µH
inductor works well in most applications and provides
5A with a wide range of input voltages.

D1 provides a path for inductor current when VSWturns
off. Under normal load conditions, the average diode
current may only be a fraction of load current; but dur­ing short-circuit or current-limit, diode current is higher.
Conservative design dictates that the diode average
current rating be 2 times the desired output current. If
operation with extended short-circuit or overload time is
expected, then the diode current rating must exceed
the current limit (6.5A = MAX724, 2.6A = MAX726), and
heat sinking may be necessary.

Under normal operating conditions (not shorted), power
dissipated in the diode P

is calculated by:

D

2 f

OSCVIN

- V

)

OUT

L

Catch Diode

OUT

is the

= 5.1A

5A/2A Step-Down, PWM,

PD= I

(VIN- V

OUT

where VDis forward drop of the diode at a current
equal to I
provide the best performance and are recommended

. In nearly all circuits, Schottky diodes

OUT

due to their fast switching times and low forward voltage
drop. Standard power rectifiers such as the 1N4000
series are too slow for DC-DC conversion circuits and are
not recommended.

Output Filter Capacitor

For most MAX724/MAX726 applications, a high-quality,
low-ESR, 470µF or 500µF output filter capacitor will suf­fice. To reduce ripple, minimize capacitor lead length
and connect the capacitor directly to the GND pin.
Capacitor suppliers are listed in Table 1. Output ripple
is a function of inductor value and output capacitor
effective series resistance (ESR). In continuous-con­duction mode:

ESR (V

V

CR(p-p)

=

OUT

It is interesting to note that input voltage (VIN), and not
load current, affects output ripple in CCM. This is
because only the DC, and not the peak-to-peak, induc­tor current changes with load (see Figure 3).

In discontinuous-conduction mode, the equation is dif­ferent because the peak-to-peak inductor current does
depend on load:

= ESR

√

V

DR(p-p)

2 I

OUTVOUT(VIN

where output ripple is proportional to the square root of
load current. Refer to the earlier equation for I
determine where DCM occurs and hence when the
DCM ripple equation should be used.

Input Bypass Capacitor

An input capacitor (200µF or 220µF) is required for step­down converters because the input current, rather than
being continuous (like output current), is a square wave.
For this reason the capacitor must have low ESR and a
ripple-current rating sufficiently large so that its ESR and
the AC input current do not conspire to overheat the
capacitor. In CCM, the capacitor's RMS ripple current is:

= I

OUT

√

I

R(RMS)

V

The power dissipated in the input capacitor is then PC:

PC= I

R(RMS)

OUT

V

IN

) (1 - V

L f

OSC

L f

OUT(VIN

V

2

(ESR)

) V

D

OUT/VIN

OSCVIN

- V

OUT

2

IN

- V

)

OUT

)

)

to

DCM

MAX724/MAX726

_______________________________________________________________________________________ 7

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

CONTINUOUS-CURRENT MODE (I

V

D

0

-0.5

IP = 3.4A

MAX724/MAX726

I

SW

0

IP = 3.4A

I

= I

= 3A

AVG

OUT

0

IP = 3.4A

OUT

= 3A)

I

L

VSW VOLTAGE (TO GND)
(ALSO DIODE VOLTAGE)
5V/div

SWITCH CURRENT
1A/div

INDUCTOR CURRENT
1A/div

DISCONTINUOUS-CURRENT MODE 

(I

= 0.16A)

OUT

IP = 0.5A

I

D

0

I

= 2.1A

AVG

DIODE CURRENT
1A/div

Figure 3. MAX724 Step-Down Converter Waveforms with VIN= 20V, L = 50µH (all waveforms 2µs/div)

8 _______________________________________________________________________________________

Switch-Mode DC-DC Regulators

Be sure that the selected capacitor can handle the ripple
current over the required temperature range. Also locate
the input capacitor very close to the MAX724/MAX726 and
use minimum length leads (surface-mount or radial
through-hole types). In most applications, ESR is more
important than actual capacitance value since electrolytic
capacitors are mostly resistive at the MAX724/MAX726's
100kHz switching frequency.

__________Applications Information

Setting Output Voltage

R1 and R2 set output voltage as follows:

V

R2

OUT

R1 =

2.21V is the reference voltage, so setting R2 to 2.21kΩ
(standard 1% resistor value) results in 1mA flowing
through R1 and R2 and simplifies the above equation.
Other values will also work for R2, but should not
exceed 4kΩ.

Synchronizing the Oscillator

The MAX724/MAX726 can be synchronized to an exter­nal 110kHz to 160kHz source by pulsing the VCpin to
ground at the desired clock rate. This is conveniently
done with the collector of an external grounded-emitter
NPN transistor. VCshould be pulled low for 300ns.
Doing this may have some impact on output regulation,
but the effect should be minimal for compensation
resistor values between 1kΩ and 4kΩ.

The MAX724/MAX726 draw about 7.5mA operating cur­rent, which is largely independent of input voltage or
load current. They draw an additional 5mA during
switch on-time. Power dissipated in the internal V
transistor is proportional to load current and depends
on both conduction losses (product of switch on-volt­age and switch current) and dynamic switching losses
(due to switch rise and fall times). Total MAX724 power
dissipation can be calculated as follows:

P = VIN[7.5mA + 5mA (DC) + 2 I

. . . DC [I

(1.8V) + 0.1Ω (I

OUT

DC = Duty Cycle =

= Overlap Time = 50ns + (3ns/A) I

t

SW

where tSWis "overlap" time. Switch dissipation is
momentarily high during overlap time because both cur-

-R2

2.21V

Power Dissipation

OUTtSWfOSC

)2]

OUT

V

+ 0.5V

OUT

- 2V

V

IN

OUT

SW

] + . . .

5A/2A Step-Down, PWM,

rent and voltage appear across the switch at the same
time. t
the MAX724.

Power dissipation in the MAX726 can be estimated in
exactly the same way as the MAX724, except that 1.1V
(and not 1.8V) is a more reasonable value for the nomi­nal voltage drop across the on-board power switch.

GND demands a short low-noise connection to ensure
good load regulation. Since the internal reference is
referred to GND, errors in the GND pin voltage get mul­tiplied by the error amplifier and appear at the output.
If the MAX724/MAX726 GND pin is separated from the
negative side of the load, then high load return current
can generate significant error across a seemingly small
ground resistance. Single-point grounding is the most
effective way to eliminate these errors. A recommend­ed ground arrangement is shown in Figure 4.

The VSWcurrent is internally limited to about 6.5A in the
MAX724 and 2.6A in the MAX726. In addition, another
feature of the MAX724/MAX726's overload protection
scheme is that the oscillator frequency is reduced
when the output voltage falls below approximately half
its regulated value. This is the case during short-circuit
and heavy overload conditions.

Since the minimum on-time for the switch is about

0.6µs, frequency reduction during overload ensures
that switch duty cycle can fall to a low enough value to
maintain control of output current. At the normal
100kHz switching frequency, an on-time as short as

HIGH CURRENT
RETURN PATH

Figure 4. Recommended Ground Connection

is approximately: [50ns + (3ns/A) (I

SW

Ground Connections

Overload Protection

MAX724
MAX726

FB

GND

R1

R2

)] for

OUT

NEGATIVE OUTPUT
NODE WHERE LOAD
REGULATION WILL
BE MEASURED

MAX724/MAX726

_______________________________________________________________________________________ 9

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

A series RC network connected from VCto ground
compensates the MAX724/MAX726. Compensation
RCvalues are shown in the applications circuits. R
and CCshape error-amplifier gain as follows: At DC,
RCand CChave no effect, so the error-amplifier's
gain is the product of its transconductance (approxi­mately 5000µmhos) and an internal 400kΩ load
impedance (r
approximately 2000µmhos. RCand CCthen add a

INT

low-frequency pole and a high-frequency zero, as
shown in Figure 5.

GAIN

A

V(DC)

90° PHASE SHIFT

= gM(400kΩ) ≈ 2000

f

= 1/[2π(400kΩ)]C

POLE

-A

= gM / (2π f CC)

V(MID)

C

f

= 1 / (2π RC CC)

ZERO

Compensation Network

) at VC. So at DC, A

V(DC)

= gM(r

INT

C

) =

A

= gMR

V(HI)

C

MAX724/MAX726

Figure 5. Error-Amplifier Gain as Set by RCand CCat VCPin

FEEDBACK RESISTOR

MAIN FILTER CAP

Figure 6. Optional LC Output Filter

FREQUENCY

L

F

C

F

0.2µs would be needed to provide a narrow enough
duty cycle that could control current when the output is
shorted. Since 0.6µs is too long (at 100kHz), the f
is lowered to 20kHz once FB (and hence the output)
drops below about 1.3V (see Frequency vs. VFBVoltage
graph in the

Typical Operating Characteristics

way, the MAX724/MAX726's 0.6µs minimum tONallows
a sufficiently small duty cycle (at the reduced f
that current can still be limited.

TO LOAD

OSC

). This

) so

OSC

Output Overshoot

The MAX724/MAX726 error-amplifier design minimizes
overshoot, but precautions against overshoot should
still be exercised in sensitive applications. Worst-case
overshoot typically occurs when recovering from an
output short because VCslews down from its highest
voltage. This can be checked by simply shorting and
releasing the output.

Reduce objectional overshoot by increasing the com­pensation resistor (to 3kΩ or 4kΩ) at V
the error-amplifier output, VC, to move more rapidly in

. This allows

C

the negative direction. In some cases, loop stability
may suffer with a high-value compensation resistor. An
option, then, is to add output filter capacitance, which
reduces short-circuit recovery overshoot by limiting out­put rise time. Lowering the compensation capacitor to
below 0.05µF may also help by allowing VCto slew fur­ther before the output rises too far.

Optional Output Filters

Though not shown in the application circuits in Figures
2, 7, and 8, additional filtering can easily be added to
reduce output ripple to levels below 2%. It is more
effective to add an LC type filter rather than additional
output capacitance alone. A small-value inductor (2µH
to 10µH) and between 47µF and 220µF of filter capaci­tance should suffice (Figure 6). Although the inductor
does not need to be of high quality (it is not switching),
it must still be rated for the full load current.

When an LC filter is added, do not move the connection
of the feedback resistor to the LC output. It should be left
connected to the main output filter capacitor (C1 in Figure

2). If the feedback connection is moved to the LC filter
point, the added phase shift may impact stability.

10 ______________________________________________________________________________________

Switch-Mode DC-DC Regulators

___________________Typical Applications

Positive-to-Negative DC-DC Inverter

The MAX724/MAX726 can convert positive input volt­ages 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 con­nected to the negative output, which allows the feed­back 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 volt­ages 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 cur­rent rating of both D1 and L1 may be reduced. In addi­tion, 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-volt­age calculation because they are effectively connected
in parallel with R3. For larger negative outputs,
increase R1, R2, and R3 proportionally while maintain-

5A/2A Step-Down, PWM,

ing the following relationships. If V
2V

, then R1, R2, and C4 can be omitted and only R3

OUT

and R4 set the output voltage.

R4 = 1.82kΩ
R3 = |V
R1 = 1.86 (R3)

| - 2.37 (in kΩ)

OUT

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 volt­age as in a conventional connection, with R1 selected
from:

V

OUT

R1 =

2.21

L1 must be a low value to maintain stability, but if V
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 sup­ply. For this reason, this configuration is not short-cir­cuit protected.

does not fall below

IN

R2

- R2

is

IN

MAX724/MAX726

VIN

+4.5V TO +35V

V

220µH

C1

50V

L1

50µH

IN

V

SW

5A

R1

5.1k

MAX724

R2
10k

D1

C4

0.01µF

C2 - NICHICON UPL1A102MRH6
L1 - COILTRONICS CTX25-5-52

V

C

C3

0.1µF

D1 - MOTOROLA MBR745
C1 - NICHICON UPL1C221MRH6

ALL RESISTORS HAVE 1% TOLERANCE

FB

GND

Figure 7. Positive-to-Negative DC-DC Inverter

______________________________________________________________________________________ 11

R3

2.74k

R4

1.82k

C2
1000µF
10V

-5V
1A

V

IN

1000pF

FB

MAX724

C3

100µF

25V



-V

IN

-4.5V TO -15V

GND

0.01µF

V

SW

V

C

C2

1µF

L1

25µH

R3

750Ω

Figure 8. Negative Step-Up DC-DC Converter

R2

2.21k

D1

MBR735

R1

12.7k

C1

1000µF

25V

V

-15V



OUT

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

________________________________________________________Package Information

INCHES MILLIMETERS

E

Q

H1

D

MAX724/MAX726

B

Q

H1

D

e

E

A

F

φP

L2

J1

L

L1

C1

J2

J3

A

F

φP

J1

DIM



A
B

C1

D
E
e
F

H1

J1
J2
J3

L
L1
L2

φP

Q

MIN

0.140

0.015

0.014

0.560

0.380

0.045

0.230

0.080

0.170

0.327

0.170

0.260

0.700

0.139

0.100

MAX

MIN

0.190

3.56

0.040

0.38

0.022

0.41

0.650

14.23

0.420

9.66





0.055

1.14

0.270

5.85

0.115

2.04

0.185

4.32

0.335

8.31

0.200

4.32

0.340

6.60

0.720

17.78

0.161

3.54

0.120

2.54

5-PIN TO-220

(STAGGERED LEAD)

PACKAGE

INCHES MILLIMETERS

DIM



A

B

C1

D

E

e

F

H1

J1

L

φP

Q

MIN

0.140

0.015

0.014

0.560

0.380


0.045

0.230

0.080

0.500

0.139

0.100

MAX

0.190

0.040

0.022

0.650

0.420

0.055

0.270

0.115

0.580

0.161

0.120

MIN

3.56

0.38

0.41

14.23

9.66



1.14

5.85

2.04

12.70

3.54

2.54

1.70 BSC0.067 BSC



1.70 BSC0.067 BSC



MAX

4.82

1.01

0.50

16.51

10.66


1.39

6.85

2.92

4.70

8.51

5.08

8.64

18.29

4.08

3.04

21-005-

MAX

4.82

1.01

0.50

16.51

10.66



1.39

6.85

2.92

14.73

4.08

3.04

21-4737-

L

5-PIN TO-220

(STRAIGHT LEAD)

B

Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are

Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.

implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.

12

__________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 [phone]

12

__________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 [phone]

© 1995 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

© 1995 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

e

C1

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PACKAGE