Not Recommended for New Designs
This product was manufactured for Maxim by an outside wafer foundry
using a process that is no longer available. It is not recommended for
new designs. The data sheet remains available for existing users.
A Maxim replacement or an industry second-source may be available.
Please see the QuickView data sheet for this part or contact technical
support for assistance.
For further information, contact Maxim’s Applications Tech Support.
19-0107; Rev 3; 9/95
5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
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
___________________________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
0°C to +70°C
5 TO-220
MAX724ECK
-40°C to +85°C
5 TO-220
MAX726CCK
0°C to +70°C
5 TO-220
MAX726ECK
-40°C to +85°C
5 TO-220
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
50µH
VSW
VIN
220µF
FB
VC
MAX724
MAX726
2.8k
MBR745
MAX724
OUTPUT
5V AT 5A
470µF
2.21k
2.7k
GND
0.01µF
5A STEP-DOWN CONVERTER
5
4
VIN
3
2
GND
1
FB
VSW
VC
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.
1
MAX724/MAX726
_______________General Description
MAX724/MAX726
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 (VSW 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
(VIN = 25V, Tj = TMIN to TMAX, unless otherwise noted.)
PARAMETER
CONDITIONS
Input Supply Voltage Range
ISW = 1A
MAX724
ISW = 5A
Switch-On Voltage (Note 2)
MAX726
MAX724
Switch-Off Leakage
MAX726
Supply Current (Note 3)
Minimum Supply Voltage
Switch-Current Limit (Note 5)
40.0
1.85
Tj < 0°C
2.10
Tj ≥ 0°C
2.30
Tj < 0°C
2.50
ISW = 0.5A
UNITS
V
V
1.2
ISW = 2A
1.7
VIN ≤ 25V, VSW = 0V
Tj = +25°C
5
VIN = 40V, VSW = 0V
Tj = +25°C
10
VIN ≤ 25V, VSW = 0V
Tj = +25°C
VIN = 40V, VSW = 0V
Tj = +25°C
300
500
150
µA
250
VFB = 2.5V, VIN ≤ 40V
8.5
11
mA
Normal Mode
7.3
8.0
V
Tj ≥ 0°C
3.5
4.8
Tj < 0°C
3.5
5.0
Start-Up Mode (Note 4)
MAX724
5.5
6.5
8.5
MAX726
2.0
2.6
3.2
Switching Frequency
VFB = grounded through 2kΩ (Note 5)
2
TYP MAX
8.0
Tj ≥ 0°C
Maximum Duty Cycle
Switching Frequency Line Regulation
MIN
8V ≤ VIN ≤ 40V
85
90
Tj = +25°C
90
100
Tj ≤ +125°C
85
Tj = +25°C
V
A
%
110
120
kHz
0.1
%/V
20
0.03
_______________________________________________________________________________________
5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
(VIN = 25V, Tj = TMIN to TMAX, unless otherwise noted.)
PARAMETER
CONDITIONS
1V ≤ VC ≤ 4V
Error-Amplifier Voltage Gain
Error-Amplifier Transconductance
MIN
TYP MAX
UNITS
Tj = +25°C
2000
V/V
Tj = +25°C
3000 5000 9000
µmho
Error-Amplifier Source Current
VFB = 2V
Tj = +25°C
100
140
225
µA
Error-Amplifier Sink Current
VFB = 2.5V
Tj = +25°C
0.6
1.0
1.7
mA
Feedback Pin Bias Current
VFB = VREF
VC = 2V
0.5
2
µA
Reference Voltage
2.155 2.210 2.265
±0.5 ±1.5
Tj = +25°C
VREF (nominal) = 2.21V
Reference Voltage Tolerance
All conditions of input voltage, output voltage,
temperature and load current
±1.0
Reference Voltage Line Regulation
8V ≤ VIN ≤ 40V
0.005 0.02
VC Voltage at 0% Duty Cycle
Thermal Resistance,
Junction to Case (Note 6)
V
%
±2.5
%/V
Tj = +25°C
1.5
V
Tj = TMIN to TMAX
-4
mV/°C
MAX724
2.5
MAX726
4.0
°C/W
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 VC 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 VIN 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 VFB is less than 1.3V. Switchcurrent limit is tested with VFB 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
20
CIRCUIT OF FIGURE 2
14
SUPPLY CURRENT (mA)
100
VOUT = 12V, VIN = 20V
90
80
70
VOUT = 5V, VIN = 15V
60
QUIESCENT SUPPLY CURRENT (mA)
16
110
EFFICIENCY (%)
QUIESCENT SUPPLY CURRENT
vs. INPUT VOLTAGE
SUPPLY CURRENT
vs. JUNCTION TEMPERATURE
CIRCUIT OF FIGURE 2
12
VIN = 25V, VOUT = 5V
IOUT = 1mA
10
8
6
4
2
0
1
2
3
4
OUTPUT CURRENT (A)
5
6
DEVICE NOT SWITCHING
16
VC = 1V
14
12
10
8
6
4
2
0
0
50
18
-40 -25
0
25
50
75
100
JUNCTION TEMPERATURE (°C)
125
0
10
20
30
40
VIN INPUT VOLTAGE (V)
_______________________________________________________________________________________
3
MAX724/MAX726
ELECTRICAL CHARACTERISTICS (continued)
5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
MAX724/MAX726
____________________________Typical Operating Characteristics (continued)
SWITCHING FREQUENCY
vs. JUNCTION TEMPERATURE
120
115
2.23
2.22
2.21
2.20
2.19
SWITCH-ON VOLTAGE
vs. SWITCH CURRENT
3.0
Tj = +25°C
SWITCH-ON VOLTAGE (V)
2.25
2.24
SWITCHING FREQUENCY (kHz)
REFERENCE VOLTAGE (V)
REFERENCE VOLTAGE
vs. JUNCTION TEMPERATURE
110
105
100
95
90
MAX724
2.0
1.5
1.0
MAX726
85
2.18
80
2.17
-40 -25
0
25
50
75
0.5
-40 -25
125
100
0
25
50
75
125
100
0
6000
160
150
140
100
5000
50
gM
0
SWITCHING FREQUENCY (kHz)
PHASE
-50
1000
-150
20
0
-200
0
100k
1M
10M
+25°C
-40°C
80
-100
60
40
0
0.5
1.0
1.5
2.0
2.5
FREQUENCY (Hz)
FB VOLTAGE (V)
FEEDBACK PIN CURRENT
vs. FB VOLTAGE
OUTPUT CURRENT LIMIT
vs. TEMPERATURE
500
3.0
8
300
OUTPUT CURRENT LIMIT (A)
400
FB CURRENT (µA)
100
3000
10k
+125°C
120
2000
1k
4
3
SWITCHING FREQUENCY
vs. FEEDBACK PIN VOLTAGE
200
PHASE (degrees)
TRANSCONDUCTANCE (µmho)
7000
2
SWITCH CURRENT (A)
ERROR-AMPLIFIER PHASE AND gM
8000
4000
1
JUNCTION TEMPERATURE (°C)
JUNCTION TEMPERATURE (°C)
START OF FREQUENCY
SHIFTING
200
100
0
-100
-200
-300
7
MAX724
6
5
4
MAX726
3
2
1
-400
-500
0
0
1
2
3
4
5
6
FB VOLTAGE (V)
4
2.5
7
8
9 10
-40 -25
0
25
50
75
100
JUNCTION TEMPERATURE (°C)
_______________________________________________________________________________________
125
5
6
5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
PIN
NAME
FUNCTION
1
FB
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 (IOUT = 0V). FB also aids current limiting
by reducing the oscillator frequency when the output voltage is low. (See the Applications Information section.)
2
VC
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. VC can also synchronize the MAX724/MAX726 to an external clock. (See the Applications Information section).
3
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 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, 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 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 comparator. If the current-limit threshold is exceeded, the
switch cycle terminates within about 600ns. The current-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 stepdown 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 waveforms 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
MAX724/MAX726
______________________________________________________________Pin Description
MAX724/MAX726
5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
In DCM, ringing occurs at VSW in 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
V SW node. 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.
_______________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
USA: Phone (708) 956-0666
Japan: Phone 81-3607-5111
FAX 81-3607-5144
Coiltronics - CTX series
USA: Phone (305) 781-8900
FAX (305) 782-4163
Capacitors:
Matsuo - 267 series
USA: Phone (714) 969-2491
FAX (714) 960-6492
Japan: Phone 81-6337-6450
Sprague - 595D series
USA: Phone (603) 224-1961
FAX (603) 224-1430
Diodes:
Motorola - MBRS series
USA: Phone (602) 244-5303
FAX (602) 244-4015
Nihon - NSQ series
USA: Phone (805) 867-2555
FAX (805) 867-2556
Japan: Phone 81-3-3494-7411
FAX 81-3-3494-7414
VIN
2.21V
REF
INTERNAL
BIAS
ERROR
AMPLIFIER
100kHz
OSCILLATOR
CURRENT-LIMIT
COMPARATOR
PWM
LOGIC
CONTROL
FB
VC
SWITCH
MAX724
Through-Hole Components
GND
VSW
Inductors:
Sumida - RCH-110 series
(see above for phone number)
Cadell-Burns - 7070, 7300, 6860, and 7200 series
USA: Phone (516) 746-2310
FAX (516) 742-2416
Renco - various series
USA: Phone (516) 586-5566
FAX (516) 586-5562
Coiltronics - various series
(see above for phone number)
Capacitors:
Nichicon - PL series low-ESR electrolytics
USA: Phone (708) 843-7500
FAX (708) 843-2798
Japan: Phone 81-7-5231-8461
FAX 81-7-5256-4158
United Chemi-Con - LXF series
USA: Phone (714) 255-9500
FAX (714) 255-9400
Sanyo - OS-CON low-ESR organic semiconductor
USA: Phone (619) 661-6835
FAX (619) 661-1055
Japan: Phone 81-7-2070-6306
FAX 81-7-2070-1174
Diodes:
General Purpose - 1N5820-1N5825
Motorola - MBR and MBRD series
(see above for phone number)
Figure 1. MAX724 Block Diagram
L
50µH (MAX724)
100µH (MAX726)
INPUT
8V TO 40V
VIN
VSW
220µF
VC
R3
2.7k
MAX724
MAX726
D
MBR745
OUTPUT
5V at 5A (MAX724)
5V at 2A (MAX726)
R1
2.8k
C1
470µF
FB
R2
2.2k
GND
C2
0.01µF
Figure 2. Basic Step-Down Converter
6
_______________________________________________________________________________________
5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
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:
IOUT = ISW -
VOUT (VIN - VOUT)
2 fOSC VINL
For the MAX724 example in Figure 2, with L = 50µH
and VIN = 25V,
5V (25V - 5V)
2 (105Hz) 25V (50 x 10-6H)
= 5.1A
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.
Under normal operating conditions (not shorted), power
dissipated in the diode PD is calculated by:
VIN
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 (VOUT) (1 - VOUT/VIN)
L fOSC
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:
VDR(p-p) = ESR
√2 I
OUT
VOUT (VIN - VOUT)
L fOSC VIN
where output ripple is proportional to the square root of
load current. Refer to the earlier equation for IDCM to
determine where DCM occurs and hence when the
DCM ripple equation should be used.
Input Bypass Capacitor
Catch Diode
D1 provides a path for inductor current when VSW turns
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.
(VIN - VOUT) VD
where V D is forward drop of the diode at a current
equal to IOUT. In nearly all circuits, Schottky diodes
provide the best performance and are recommended
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.
VCR(p-p) =
where I SW is the maximum switch current (5.5A for
MAX724), VIN is the maximum input voltage, VOUT is the
output voltage, and fOSC is the switching frequency.
IOUT = 5.5A -
PD = IOUT
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:
IR(RMS) = IOUT
√V
OUT
(VIN - VOUT)
VIN2
The power dissipated in the input capacitor is then PC:
PC = IR(RMS)2 (ESR)
_______________________________________________________________________________________
7
MAX724/MAX726
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.
MAX724/MAX726
5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
DISCONTINUOUS-CURRENT MODE
(IOUT = 0.16A)
CONTINUOUS-CURRENT MODE (IOUT = 3A)
VD
0
VSW VOLTAGE (TO GND)
(ALSO DIODE VOLTAGE)
5V/div
-0.5
IP = 3.4A
ISW
IP = 0.5A
0
SWITCH CURRENT
1A/div
IP = 3.4A
IL
IAVG = IOUT = 3A
0
INDUCTOR CURRENT
1A/div
IP = 3.4A
ID
0
IAVG = 2.1A
DIODE CURRENT
1A/div
Figure 3. MAX724 Step-Down Converter Waveforms with VIN = 20V, L = 50µH (all waveforms 2µs/div)
8
_______________________________________________________________________________________
5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
__________Applications Information
Setting Output Voltage
R1 and R2 set output voltage as follows:
R1 =
VOUT R2
2.21V
-R2
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 VC pin to
ground at the desired clock rate. This is conveniently
done with the collector of an external grounded-emitter
NPN transistor. VC should 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.
Power Dissipation
rent and voltage appear across the switch at the same
time. tSW is approximately: [50ns + (3ns/A) (IOUT)] for
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.
Ground Connections
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.
Overload Protection
The VSW current 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
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 VSW
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:
R1
MAX724
MAX726 FB
P = VIN [7.5mA + 5mA (DC) + 2 IOUT tSW fOSC] + . . .
GND
. . . DC [IOUT (1.8V) + 0.1Ω (IOUT)2]
DC = Duty Cycle =
R2
NEGATIVE OUTPUT
NODE WHERE LOAD
REGULATION WILL
BE MEASURED
VOUT + 0.5V
VIN - 2V
tSW = Overlap Time = 50ns + (3ns/A) IOUT
where t SW is "overlap" time. Switch dissipation is
momentarily high during overlap time because both cur-
HIGH CURRENT
RETURN PATH
Figure 4. Recommended Ground Connection
_______________________________________________________________________________________
9
MAX724/MAX726
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.
5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
MAX724/MAX726
Compensation Network
AV(DC) = gM(400kΩ) ≈ 2000
fPOLE = 1/[2π(400kΩ)]CC
GAIN
-AV(MID) = gM / (2π f CC)
90° PHASE SHIFT
fZERO = 1 / (2π RC CC)
A series RC network connected from VC to ground
compensates the MAX724/MAX726. Compensation
RC values are shown in the applications circuits. R C
and CC shape error-amplifier gain as follows: At DC,
R C and C C have no effect, so the error-amplifier's
gain is the product of its transconductance (approximately 5000µmhos) and an internal 400kΩ load
impedance (rINT) at VC. So at DC, AV(DC) = gM(rINT) =
approximately 2000µmhos. R C and C C then add a
low-frequency pole and a high-frequency zero, as
shown in Figure 5.
Output Overshoot
AV(HI) = gMRC
FREQUENCY
Figure 5. Error-Amplifier Gain as Set by RC and CC at VC Pin
FEEDBACK RESISTOR
LF
TO LOAD
MAIN FILTER CAP
CF
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 VC slews 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 VC. This allows
the error-amplifier output, VC, to move more rapidly in
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 VC to slew further before the output rises too far.
Optional Output Filters
Figure 6. Optional LC Output Filter
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 fOSC
is lowered to 20kHz once FB (and hence the output)
drops below about 1.3V (see Frequency vs. VFB Voltage
graph in the Typical Operating Characteristics). This
way, the MAX724/MAX726's 0.6µs minimum tON allows
a sufficiently small duty cycle (at the reduced fOSC) so
that current can still be limited.
10
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.
______________________________________________________________________________________
5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
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-
VIN
+4.5V TO +35V
C1
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
R3
R1
R2
=
=
=
=
1.82kΩ
|VOUT| - 2.37 (in kΩ)
1.86 (R3)
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:
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 supply. For this reason, this configuration is not short-circuit protected.
220µH
50V
VIN
L1
50µH
5A
VIN
VSW
R1
5.1k
1000pF
R1
12.7k
FB
R3
2.74k
MAX724
C2
1000µF
10V
MAX724
R2
10k
C3
100µF
25V
R2
2.21k
VSW
GND
VC
C1
1000µF
25V
FB
VC
C3
0.1µF
GND
D1
C4
0.01µF
D1 - MOTOROLA MBR745
C2 - NICHICON UPL1A102MRH6
C1 - NICHICON UPL1C221MRH6
L1 - COILTRONICS CTX25-5-52
ALL RESISTORS HAVE 1% TOLERANCE
Figure 7. Positive-to-Negative DC-DC Inverter
C2
1µF
R4
1.82k
0.01µF
R3
750Ω
-5V
1A
L1
25µH
D1
MBR735
-VIN
-4.5V TO -15V
VOUT
-15V
Figure 8. Negative Step-Up DC-DC Converter
______________________________________________________________________________________
11
MAX724/MAX726
___________________Typical Applications
MAX724/MAX726
5A/2A Step-Down, PWM,
Switch-Mode DC-DC Regulators
________________________________________________________Package Information
DIM
A
E
F
φP
Q
H1
D
L2
J1
A
B
C1
D
E
e
F
H1
J1
J2
J3
L
L1
L2
φP
Q
INCHES
MAX
MIN
0.190
0.140
0.040
0.015
0.022
0.014
0.650
0.560
0.420
0.380
0.067 BSC
0.055
0.045
0.270
0.230
0.115
0.080
0.185
0.170
0.335
0.327
0.200
0.170
0.340
0.260
0.720
0.700
0.161
0.139
0.120
0.100
MILLIMETERS
MIN
MAX
3.56
4.82
0.38
1.01
0.41
0.50
14.23
16.51
9.66
10.66
1.70 BSC
1.14
1.39
5.85
6.85
2.04
2.92
4.32
4.70
8.31
8.51
4.32
5.08
6.60
8.64
17.78
18.29
3.54
4.08
2.54
3.04
21-005-
L
L1
5-PIN TO-220
(STAGGERED LEAD)
PACKAGE
C1
B
J2
e
J3
DIM
A
E
F
φP
Q
H1
D
J1
A
B
C1
D
E
e
F
H1
J1
L
φP
Q
INCHES
MAX
MIN
0.190
0.140
0.040
0.015
0.022
0.014
0.650
0.560
0.420
0.380
0.067 BSC
0.055
0.045
0.270
0.230
0.115
0.080
0.580
0.500
0.161
0.139
0.120
0.100
MILLIMETERS
MIN
MAX
3.56
4.82
0.38
1.01
0.41
0.50
14.23
16.51
9.66
10.66
1.70 BSC
1.14
1.39
5.85
6.85
2.04
2.92
12.70
14.73
3.54
4.08
2.54
3.04
21-4737-
L
B
C1
e
5-PIN TO-220
(STRAIGHT LEAD)
PACKAGE
CONTACT FACTORY FOR AVAILABILITY
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.
12 __________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 (408) 737-7600
© 1995 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
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