View MAX726CCK_127166.PDF datasheet online --- IC-ON-LINE

Datasheet File OCR Text:

19-0107; Rev 3; 9/95
5A/2A Step-Down, PWM, 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).
___________________________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 0C to +70C -40C to +85C 0C to +70C -40C to +85C 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 VIN 220F VSW
50H MBR745 FB 2.21k GND 0.01F
OUTPUT 5V AT 5A 2.8k 470F
MAX724
VC 2.7k
MAX724 MAX726
5 4 3 2 1
VIN VSW GND VC 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 (VSW Negative) (Note 1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35V Feedback Pin Voltage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V, +10V Operating Temperature Ranges MAX72_CCK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0C to +70C MAX72_ECK. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -40C to +85C Junction Temperature Ranges MAX72_CCK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0C to +125C MAX72_ECK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - 40C to +125C Storage Temperature Range . . . . . . . . . . . . . . . . . . . -65C to +160C Lead Temperature (soldering, 10sec). . . . . . . . . . . . . . . . . . . . +300C
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 Input Supply Voltage Range ISW = 1A MAX724 Switch-On Voltage (Note 2) ISW = 5A MAX726 MAX724 Switch-Off Leakage MAX726 Supply Current (Note 3) Minimum Supply Voltage ISW = 0.5A ISW = 2A VIN 25V, VSW = 0V VIN = 40V, VSW = 0V VIN 25V, VSW = 0V VIN = 40V, VSW = 0V Tj = +25C Tj = +25C Tj = +25C Tj = +25C 8.5 7.3 Tj 0C Tj < 0C 5.5 2.0 85 Tj = +25C Switching Frequency VFB = grounded through 2k (Note 5) Switching Frequency Line Regulation 8V VIN 40V Tj +125C Tj = +25C 90 85 20 0.03 0.1 %/V 3.5 3.5 6.5 2.6 90 100 110 120 kHz 5 10 Tj 0C Tj < 0C Tj 0C Tj < 0C 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 % A V UNITS V
VFB = 2.5V, VIN 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)
(VIN = 25V, Tj = TMIN to TMAX, 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 VFB = 2V VFB = 2.5V VFB = VREF VC = 2V VREF (nominal) = 2.21V Tj = +25C All conditions of input voltage, output voltage, temperature and load current 8V VIN 40V Tj = +25C Tj = TMIN to TMAX 1V VC 4V CONDITIONS Tj = +25C Tj = +25C Tj = +25C Tj = +25C 100 0.6 MIN TYP MAX 2000 3000 5000 9000 140 1.0 0.5 225 1.7 2 UNITS V/V mho A mA A 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 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 1s 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) VOUT = 5V, VIN = 15V VOUT = 12V, VIN = 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 VIN = 25V, VOUT = 5V IOUT = 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 VC = 1V
10
20
30
40
VIN 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
Tj = +25C 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 gM
8000 TRANSCONDUCTANCE (mho) 7000 PHASE 6000 5000 4000 3000 2000 1000 0 1k 10k 100k FREQUENCY (Hz) 1M 10M gM 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
+125C
-40C
+25C
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 (A) 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 0.5A 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.) 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). 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 Switch output can swing 35V below ground and is rated for 5A (MAX724), 2A (MAX726). 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 200F or 220F.
MAX724/MAX726
2
VC
3
GND
4 5
VSW VIN
_________________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 5000mho. When slewing, V C can source about 140A, 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 = 50H, 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
5A/2A Step-Down, PWM, Switch-Mode DC-DC Regulators MAX724/MAX726
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 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 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 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) 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 General Purpose - 1N5820-1N5825 Motorola - MBR and MBRD series (see above for phone number)
VIN
Capacitors:
2.21V REF INTERNAL BIAS 100kHz OSCILLATOR CURRENT-LIMIT COMPARATOR
ERROR AMPLIFIER
FB VC
PWM LOGIC CONTROL
Diodes:
SWITCH
MAX724
Through-Hole Components
GND VSW
Inductors:
Figure 1. MAX724 Block Diagram
INPUT 8V TO 40V VIN 220F VSW
L 50H (MAX724) 100H (MAX726) D MBR745 FB
OUTPUT 5V at 5A (MAX724) 5V at 2A (MAX726) R1 2.8k C1 470F R2 2.2k
Capacitors:
VC R3 2.7k C2 0.01F
MAX724 MAX726
GND
Diodes:
Figure 2. Basic Step-Down Converter
6
_______________________________________________________________________________________
5A/2A Step-Down, PWM, Switch-Mode DC-DC Regulators
Inductor Selection
Although most MAX724 designs perform satisfactorily with 50H inductors (100H for the MAX726), the MAX724/MAX726 are able to operate with values ranging from 5H to 200H. In some cases, inductors other than 50H 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: IOUT = ISW VOUT (VIN - VOUT) 2 fOSC VINL PD = IOUT (VIN - VOUT) VD VIN
MAX724/MAX726
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.
Output Filter Capacitor
For most MAX724/MAX726 applications, a high-quality, low-ESR, 470F or 500F 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: VCR(p-p) = ESR (VOUT) (1 - VOUT/VIN) L fOSC
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. For the MAX724 example in Figure 2, with L = 50H and VIN = 25V, 5V (25V - 5V) IOUT = 5.5A 2 (105Hz) 25V (50 x 10-6H) = 5.1A
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
Note that increasing or decreasing inductor value provides only small changes in maximum output current (100H = 5.3A, 20H = 4.5A). The equation shows that output current is mostly a function of the MAX724/MAX726 current-limit value. Again, a 50H inductor works well in most applications and provides 5A with a wide range of input voltages.
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.
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. Under normal operating conditions (not shorted), power dissipated in the diode PD is calculated by:
Input Bypass Capacitor
An input capacitor (200F or 220F) 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
_______________________________________________________________________________________
5A/2A Step-Down, PWM, Switch-Mode DC-DC Regulators MAX724/MAX726
CONTINUOUS-CURRENT MODE (IOUT = 3A) DISCONTINUOUS-CURRENT MODE (IOUT = 0.16A)
VD
0 -0.5 VSW VOLTAGE (TO GND) (ALSO DIODE VOLTAGE) 5V/div 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 = 50H (all waveforms 2s/div)
8
_______________________________________________________________________________________
5A/2A Step-Down, PWM, 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. 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.
MAX724/MAX726
__________Applications Information
Setting Output Voltage
R1 and R2 set output voltage as follows: R1 = VOUT R2 2.21V -R2
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.
2.21V is the reference voltage, so setting R2 to 2.21k (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 4k.
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 0.6s, 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
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 1k and 4k.
Power Dissipation
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: P = VIN [7.5mA + 5mA (DC) + 2 IOUT tSW fOSC] + . . . . . . DC [IOUT (1.8V) + 0.1 (IOUT)2] DC = Duty Cycle = VOUT + 0.5V VIN - 2V
R1
MAX724 MAX726 FB
GND R2 NEGATIVE OUTPUT NODE WHERE LOAD REGULATION WILL BE MEASURED HIGH CURRENT RETURN PATH
tSW = Overlap Time = 50ns + (3ns/A) IOUT where t SW is "overlap" time. Switch dissipation is momentarily high during overlap time because both cur-
Figure 4. Recommended Ground Connection
9
_______________________________________________________________________________________
5A/2A Step-Down, PWM, Switch-Mode DC-DC Regulators MAX724/MAX726
Compensation Network
AV(DC) = gM(400k) 2000 fPOLE = 1/[2(400k)]CC
-AV(MID) = gM / (2 f CC) GAIN 90 PHASE SHIFT fZERO = 1 / (2 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 5000mhos) and an internal 400k load impedance (rINT) at VC. So at DC, AV(DC) = gM(rINT) = approximately 2000mhos. 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
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 3k or 4k) 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 0.05F may also help by allowing VC to slew further before the output rises too far.
FEEDBACK RESISTOR
LF TO LOAD CF
MAIN FILTER CAP
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 (2H to 10H) and between 47F and 220F 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.
Figure 6. Optional LC Output Filter
0.2s would be needed to provide a narrow enough duty cycle that could control current when the output is shorted. Since 0.6s 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 0.6s minimum tON allows a sufficiently small duty cycle (at the reduced fOSC) so that current can still be limited.
10
______________________________________________________________________________________
5A/2A Step-Down, PWM, 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 maintaining 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 = = = = 1.82k |VOUT| - 2.37 (in k) 1.86 (R3) 3.65 (R3)
MAX724/MAX726
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 50H. 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.
VIN +4.5V TO +35V
C1
220H 50V L1 50H 5A VIN FB R3 2.74k C2 1000F 10V C3 100F 25V GND 1000pF R1 12.7k
VIN VSW
R1 5.1k
MAX724
R2 2.21k C1 1000F 25V
MAX724
R2 10k FB VC C3 0.1F GND D1 C4 0.01F R4 1.82k
VSW VC C2 1F 0.01F
D1 - MOTOROLA MBR745 C2 - NICHICON UPL1A102MRH6 C1 - NICHICON UPL1C221MRH6 L1 - COILTRONICS CTX25-5-52 ALL RESISTORS HAVE 1% TOLERANCE
-5V 1A -VIN -4.5V TO -15V
R3 750
L1 25H
D1 MBR735
VOUT -15V
Figure 7. Positive-to-Negative DC-DC Inverter
Figure 8. Negative Step-Up DC-DC Converter
______________________________________________________________________________________
11
5A/2A Step-Down, PWM, Switch-Mode DC-DC Regulators MAX724/MAX726
________________________________________________________Package Information
A E Q H1 P F
DIM A B C1 D E e F H1 J1 J2 J3 L L1 L2 P 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-
D L2
J1
L L1 C1 B e J2 J3
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
5-PIN TO-220 (STAGGERED LEAD) PACKAGE
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-
A E Q H1 P F
DIM A B C1 D E e F H1 J1 L P Q
D
J1
L
B
e
C1
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 (c) 1995 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

▲Up To Search▲

Price & Availability of MAX726CCK

	To Download MAX726CCK Datasheet File
If you can't view the Datasheet, Please click here to try to view without PDF Reader .