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APPLICATION NOTE
DESIGNING MULTIPLE-OUTPUT POWER SUPPLIES WITH THE L296 AND L4960
Multiple output supplies can be realized simply and economically using the SGS THOMSON Microelectronics L296 and L4960 high power switching regulators. This note describes several practical circuits of this type.
Most of the switching regulators produced today have multiple outputs.The output voltages most frequently used - at least for powers up to 50W - are + 5V - 5V, + 12V and - 12V. In these supplies the 5 V output is normally the output which delivers the highest current and requires the highest precision. For the other voltages - particularly the negative outputs - less precision ( 5 % 7 %) is usually sufficient. Often, however, for high current 12V outputs better stabilization and greater precision (typically
AN245/1288
4 % - the outputtolerance of an L7800 series linear regulator) are required. Multiple output supplies which satisfy theserequirements can be realized using the SGS THOMSON L296and L4960 high powerswitching regulator ICs, Several practical supply designs are described below to illustrate how these components are used to build compact and inexpensive multi-output supplies.
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APPLICATION NOTE
DUAL OUTPUT 15W SUPPLY Vo1 = 5V/3A, Vo2 = 12V/150mA A single L296 is used in this application to produce two outputs.The application circuit, figure 1, illustrates how the second output (12V) is obtained by adding a second winding to the output inductor. Energy is transferred to the secondaryduring the recirculation period when the internal power device of the L296 is OFF. Since the 12V output is not separated from the 5V outputfewerturns are necessary for the secondwin-
ding, therefore less copper is needed and load regulation is improved. In applications of this type it is a good rule to ensure that the power drain on the auxiliary output is no more than 20-25% of the power delivered by the main output. Table 1 shows the performance obtained with this dual output supply. This circuit operates at a switching frequency of 50KHz.
Figure 1 : Dual Output DC-DC Converter (5V/3A, 12V/150mA).
Transformer : magnetics 58930, N1 = 30 turns, N2 = 40 turns.
Table 1.
Parameter Output Voltage IO1 = 3A Output Ripple Line Regulation IO1 = 3A Line Regulation IO1 = 700mA Load Regulation IO1 = 700mA 3A Load Regulation IO1 = 700mA Load Regulation IO1 = 3A Efficiency 20V Vi 40V IO2 = 150mA 20V Vi 40V IO2 = 100mA Vi = 30V IO2 = 150mA Vi = 30V IO2 = 100 150mA Vi = 30V IO2 = 100 150mA Vi = 30V VO1 = 5.120V IO1 = 3A VO2 = 12.089V IO2 = 150mA Vi = 30V IO2 = 150mA VO1 5.120 70 15 15 10 0 0 VO2 12.089 40 30 10 130 40 40 Unit V mV mV mV mV mV mV
75
%
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APPLICATION NOTE
DUAL OUTPUT 7.5W SUPPLY Vo1 = 5V/1.5A, Vo2 = 12V/100mA The same technique - adding a secondary winding - can also be used to produce an economical and simple dual output supply with the L4960, a device
containing the same control loop blocks as the L296 and a 2A output stage (fig. 2). Though this circuit costs very little the performance obtained (see table 2) is more than satisfactory. The switching frequency is 50kHz.
Figure 2 : Dual Output DC-DC Converter (5V/1.5A, 12V/100mA).
Transformer : magnetics 58206, N1 = 30 turns, N2 = 40 turns.
Table 2.
Parameter Output Voltage IO1 = 1.5A Output Ripple Line Regulation IO1 = 1.5A Line Regulation IO1 = 500mA Load Regulation IO1 = 0.5A 1.5A Load Regulation IO1 = 500mA Load Regulation IO1 = 1.5A Efficiency 15V Vi 35V IO2 = 100mA 15V Vi 35V IO2 = 50mA Vi = 25V IO2 = 100mA Vi = 25V IO2 = 50mA 100mA Vi = 25V IO2 = 50mA 100mA Vi = 25V VO1 = 1.5V VO2 = 100mA Vi = 25V IO2 = 100mA VO1 5.050 50 7 7 3 0 0 VO2 12.010 30 75 60 100 55 50 Unit V mV mV mV mV mV mV
78
%
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APPLICATION NOTE
TRIPLE OUTPUT 15W SUPPLY Vo1 = 5V/3A, Vo2 = 12V/100mA, Vo3 = -12V/100mA Figure 3 shows how to obtain two auxiliary outputs ( 12V) which are isolated from the 5V output. For this output power range an L296 is used. To ensure good tracking of the 12V and - 12V outputs the secondary outputsin this applicationshould be bifilar wound. This circuit operates at 50KHz and gives the performance indicated in table 3.
Figure 3 : Triple Output DC-DC Converter (5V/3A, 12V/100mA).
Transformer : magnetics 58206, N1 = 30 turns, N2 = 40 turns.
Table 3.
Parameter Output Voltage IO1 = 3A Output Ripple Line Regulation IO1 = 700 mA Line Regulation IO1 = 3A Load Regulation IO1 = 0.7A 3A Load Regulation IO1 = 3A Load Regulation IO1 = 3A IO2 = 50 100mA Efficiency 20V Vi 40V IO2 = IO3 = 100mA 20V Vi 40V IO2 = IO3 = 100mA Vi = 30V IO2 = IO3 =100mA Vi = 30V IO2 = 100mA IO3 = 50 100mA Vi = 30V IO3 = 100mA Vi = 30V IO2 = IO3 = 100mA VO1 5.057 80 15 18 4 0 VO2 12.300 30 60 100 150 125 VO3 - 12.300 30 60 100 150 52 Unit V mV mV mV mV mV
0
50
120
mV
76
%
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APPLICATION NOTE
TRIPLE OUTPUT 7.5W SUPPLY Vo1 = 5V/1.5A, Vo2 = 12V/50mA, Vo3 = - 12/50mA For lower output powers, the L296 in the previous Figure 4 : Triple Output DC-DC Converter (5V/1.5A, 12V/50mA, - 12V/50mA). application may be replaced by an L4960 as shown in figure 4. The performance of this circuit is indicated in table 4.
Table 4.
Parameter Output Voltage IO1 = 1.5A Output Ripple Line Regulation IO1 = 500mA Line Regulation IO1 = 1.5A Load Regulation IO1 = 0.5A 1.5A Load Regulation IO1 = 1.5A IO3 = 20 50mA Load Regulation IO1 = 1.5A IO2 = 20 50mA Efficiency 15V Vi 35V IO2 = IO3 = 50mA 15V Vi 35V IO2 = IO3 = 50mA Vi = 25V IO2 = IO3 = 50mA Vi = 25V IO2 = 50mA Vi = 25V IO3 = 100mA Vi = 25V IO2 = IO3 = 50mA VO1 5.040 60 5 4 5 0 VO2 12.020 30 80 60 120 15 VO3 - 12.020 30 80 60 120 50 Unit V mV mV mV mV mV
0
50
15
mV
70
%
5/11
APPLICATION NOTE
THE L296 AND L4960 HIGH POWER SWITCHING REGULATORS TheSGS THOMSON L296 is a monolithicstepdown switching regulator assembled in the 15-pin Multiwatt package. Operating with supply input voltages up to 46V it provides a regulated 4A output variable from 5.1V to 40V. Internally the device is equipped with current limiter, soft start and reset (or power fail) functions, making it particularly suitable for supplying microprocessors and logic. The precision of the L296's internal reference ( 2%) eliminates the need for external dividers or trinning to obtain a 5V output. The synchronization pin allows synchronous operation of several devices at the same frequency to avoid generating undesirable beat frequencies. The L4960 is a similar device assembled in the 7lead Heptawatt package. Like the L296 it has a maximum input voltage of 46V and it provides a regulated output voltage variable from 5V to 40V with a maximum load current of 2.5A. Current limiting, soft start and thermal protection functions are included. The thermal protection circuit in both the L296 and L4960 has a hysteresis of 30C to allow soft restarting after a fault condition. THE STEP DOWN CONFIGURATION Figure 5 shows the basic structure of a step down switching regulator. The transistor Q is used as a switch and the ON and OFF times are determined by the control circuit. When Q is saturated current flows from the supply, Vi, to the load through the inductor L. Neglecting the saturation voltage of Q, Ve Vi. When Q is OFF, current continues to flow in the inductorL, in the same direction, forcingthe diode into conduction immediately therefore Ve is negative. In these conditions the load current flows through L and D. The average value of the current in the inductor is equal to the load current. In the inductor a triangular current ripple equal to . IL is added to this average current. During the time when Q is ON this ripple is : (Vi - Vo) TON IL = L and when Q is off it is : Vo . TOFF IL = L Equating these expression and assuming that the transistor and diode are ideal we obtain : TON is the conduction time TON Vo = Vi . T of the transistor T is the oscillator period The absolute average current in the supply is therefore : TON Iioc = Io . T Once the working frequency and desired ripple current have been fixed the value of the inductor L is given by : (Vi - Vo) Vo L= Vi f IL and the value of the capacitor C required to give the desired output voltage ripple ( V) is : (Vi - Vo) Vo C= 8 L f 2 fo This capacitor must have a maximum ESR given by : Vo ESRMAX = IL And, finally, the minimum load current, IOMIN, must be : IL (Vi - Vo) Vo IOMIN = = 2 2 Vi fL Figure 5 : Basic STEP-DOWN Configuration.
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APPLICATION NOTE
30W DC-DC CONVERTER Designing power supplies in the 30-40W range is becoming increasingly difficult because it is here that there is the greatest need to maintain performance levels andreduce costs. The application proposed here is very competitive because it exploits new ICs to reduce size, number of componentsand assembly costs. This solution, the DC-DC converter, compares very favourable with off-line switching supplies in terms of cost. DC-DC converters can, in fact, be realized
even by designers with little experience and allows the convenience of working with low voltages, Offline switching supplies are only preferable when the weight and size of the mains transformer in a DC-DC converter would be excessive. In this circuit, figure 6 two devices are used, an L296 andan L4960.The L296 is used, to supply a 5V output with a current of 3A and the auxiliary - 5V/100mA output andthe L4960 is used toprovidethe 12V/1.5Aoutput and the auxiliary - 12V/100mAoutput.
Figure 6 : Multioutput DC-DC Converter with L296 and L4960 (5V / 3A, 12V / 1.5A, -12V/100mA, -5V/100mA).
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APPLICATION NOTE
Table 5 shows the performance obtained with this power supply. Table 5.
Parameter Output Voltage IO1 = 3A IO2 = 100mA Output Ripple Line Regulation IO1 = 1A IO3 = 0.5A Load Regulation IO1 = 1A to 3A IO3 = 0.5 to 1.5A Load Regulation IO1 = 3A IO3 = 1.5A Load Regulation IO1 = 1A IO3 = 0.5A Line Regulation IO1 = 3A IO3 = 1.5A 20V Vi 40V IO2 = 100mA IO4 = 100mA Vi = 30V IO2 = 100mA IO4 = 100mA Vi = 30V IO2 = 50 100mA IO4 = 50 100mA Vi = 30V IO2 = 50 100mA IO4 = 50 100mA 20 Vi 40V IO2 = 100mA IO4 = 100mA 15 45 15 40 0 35 0 90 0 100 0 100 Vi = 30V IO3 = 1.5A IO4 = 100mA VO1 5.080 VO2 - 5010 VO3 11.96 VO4 12.00 Unit V
50 13
30 15
50 10
40 20
mV mV
8
90 3 80
mV mV mV mV mV mV mV mV
This application illustrates how two devices may be synchronized.Note also that the reset circuit is used in this case to monitor the output voltage (see figure 7). Figure 7 : Reset Output Waveforms.
If a power fail function is required in placeof the reset function the figure 6 circuit should be modified as shown in figure 8.
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APPLICATION NOTE
Figure 8.
CALCULATING THE POWER FAIL TIME The 'power fail time' is definedas the time from when the power fail output (pin 14) goes low to the time when the input voltage falls to the minimum level reFigure 9.
quired to maintain the regulated output (see figure 9). From this definition we can evaluate the energy balance.
9/11
APPLICATION NOTE
The energy which the filter capacitor C supplies to the operating device while it discharges is : (1) E = 1/2 C (V12 - V22) The load drains a power of Po = Vo Io. Taking into consideration the average efficiency (derived with the input between V1 and V2), the power to be supplied at the input of the device is : Po Po2 = (2) Equating the expressions (1) and (2) gives : Po . tPF 1/2 C (V12 - V22) = where Vi is the input voltage at which the voltage on pin 12 reaches 5V (through the divider R1/R2) ; V2 is the maximum input voltage below which the device no longer regulates. Rearranging this expression to obtain C : 2 Po tPF C= (V12 - V22) EXAMPLE - Suppose that Vo = 5V, Io = 3A, Tpf = 10ms and Vi = 35V. Fixing V1 = 25V and V2 = 10V we obtain : 2 x 15 x 10 . 10-3 2 Po tPF C= = = 760F (V12 - V22) 0.75 (252 - 102) We obtain choose a capacitor of 1000F. CROWBAR The L296 includes an internal crowbar function ; the only external component needed is an SCR. The intervention threshold of this block is fixed internally at 20% of the nominal value of the internal reference. In the figure 6 circuit the SCR is triggered by an overvoltage on the 5V output (usually the most important output to monitor) and shortcircuits to ground the 5V output and, through the diode which connects the two outputs, the 12V output. Since the internal current limiter in the device is designed to function as shown in figure 10 (that is, with pulsed output current) the SCR turns off in the gap between pulses and is re-activated gain if, when the device restarts softly, the fault condition has not been eliminated. But if the fault no longer exists the SCR remains OFF and the output voltage returns to the normal value. If the designer prefers the supply to remain off after the SCR has been activated the circuit can be modified as shown in figure 11. In this modification, when the SCR is triggered a very high current flows in the fuse, blowing it.
Since the filter capacitor can have a high value and be charged to high voltages the choice of SCR is important. The type used in this circuit - the TYP512 is a plastic packaged SCR able to handle 12 Arms and 300A for 10ms. The maximum forward and reverse voltages are about 50V. If the crowbar circuit is not used it is advisableto connect pin 1 to ground or pin 10. Figure 10 : Load Current in Short Circuit Conditions (Vi = 40V, L = 300H, f = 100KHz).
Current at Pin 2 when the Output is Short Circuited.
Figure 11.
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APPLICATION NOTE
Information furnished is believed to be accurate and reliable. However, SGS-THOMSON Microelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of SGS-THOMSON Microelectronics. Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. SGS-THOMSON Microelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of SGS-THOMSON Microelectronics. (c) 1995 SGS-THOMSON Microelectronics - All Rights Reserved SGS-THOMSON Microelectronics GROUP OF COMPANIES Australia - Brazil - France - Germany - Hong Kong - Italy - Japan - Korea - Malaysia - Malta - Morocco - The Netherlands - Singapore Spain - Sweden - Switzerland - Taiwan - Thaliand - United Kingdom - U.S.A.
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