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| Philips Semiconductors Product specification SMPS control circuit SG3524 DESCRIPTION This monolithic integrated circuit contains all the control circuitry for a regulating power supply inverter or switching regulator. Included in a 16-pin dual-in-line package is the voltage reference, error amplifier, oscillator, pulse-width modulator, pulse steering flip-flop, dual alternating output switches and current-limiting and shut-down circuitry. This device can be used for switching regulators of either polarity, transformer-coupled DC-to-DC converters, transformerless voltage doublers and polarity converters, as well as other power control applications. The SG3524 is designed for commercial applications of 0C to +70C. PIN CONFIGURATION D, F, N Packages INVERT INPUT 1 NON-INV INPUT 2 OSC OUTPUT 3 (+)CL SENSE 4 (-)CL SENSE 5 RT CT 6 7 16 VREF 15 VIN 14 EMITTER B 13 COLLECTOR B 12 COLLECTOR A 11 EMITTER A 10 SHUTDOWN 9 TOP VIEW COMPENSATION FEATURES GROUND 8 * Complete PWM power control circuitry * Single ended or push-pull outputs * Line and load regulation of 0.2% * 1% maximum temperature variation * Total supply current is less than 10mA * Operation beyond 100kHz ORDERING INFORMATION DESCRIPTION 16-Pin Plastic Dual In-Line Package (DIP) 16-Pin Ceramic Dual In-Line Package (CERDIP) 16-Pin Small Outline (SO) Package SL00174 Figure 1. Pin Configuration TEMPERATURE RANGE 0 to +70C 0 to +70C 0 to +70C ORDER CODE SG3524N SG3524F SG3524D DWG # SOT38-4 0582B SOT109-1 BLOCK DIAGRAM VREF 16 VIN 15 REF REG +5V TO ALL INTERNAL CIRCUITRY +5V OSCILLATOR 3 OUTPUT FLIP FLOP 12 NOR 11 CA +5V RT 6 CT 7 (RAMP) OSC +5V + COMPARATOR - +5V ERROR - AMP + 1k 10 SHUTDOWN 10k 9 COMPENSATION +5V + CL - 4 +SENSE NOR 13 CB 14 E B INV INPUT 1 N.I. INPUT 2 GROUND 8 (SUBSTRATE) 5 -SENSE SL00175 Figure 2. Block Diagram 1994 Aug 31 1 853-0891 13721 Philips Semiconductors Product specification SMPS control circuit SG3524 ABSOLUTE MAXIMUM RATINGS SYMBOL VIN IOUT IREF PD Input voltage Output current (each output) Reference output current Oscillator charging current Power dissipation Package limitation Derate above 25C TA TSTG Operating temperature range Storage temperature range 1000 8 0 to +70 -65 to +150 mW mW/C C C PARAMETER RATING 40 100 50 5 UNIT V mA mA mA DC ELECTRICAL CHARACTERISTICS TA=0C to +70C, VIN=20V, and f=20kHz, unless otherwise specified. SYMBOL PARAMETER TEST CONDITIONS LIMITS Min 4.6 VIN=8 to 40V IL=0 to 20mA f=120Hz, TA=25C VREF=0, TA=25C Over operating temperature range TA=25C CT=0.001 F, RT=2k RT and CT constant VIN=8 to 40V, TA=25C Over operating temperature range Pin 3, TA=25C CT=0.01 F, TA=25C VCM=2.5V VCM=2.5V 68 TA=25C TA=25C AV=0dB, TA=25C TA=25C % each output "ON" Zero duty cycle Maximum duty cycle 0.5 0 1 3.5 1 Pin 9=2V with error amplifier set for maximum out, TA=25C 180 200 0.2 -1 +1 220 1.8 70 3 3.8 45 3.5 0.5 2 2 80 3.4 10 10 Typ 5.0 10 20 66 100 0.3 20 300 5 1 2 1 Max 5.4 30 50 UNIT Reference section VOUT Output voltage Line regulation Load regulation Ripple rejection ISC Short circuit current limit Temperature stability Long-term stability V mV mV dB mA % mV/kHz kHz % % % VP s mV A dB V dB MHz V % V V A mV mV/C V Oscillator section fMAX Maximum frequency Initial accuracy Voltage stability Temperature stability Output amplitude Output pulse width Error amplifier section VOS IBIAS VCM CMRR BW VOUT Input offset voltage Input bias current Open-loop voltage gain Common-mode voltage Common-mode rejection ratio Small-signal bandwidth Output voltage Duty cycle Input threshold Input threshold IBIAS Input bias current Sense voltage Sense voltage T.C. VCM Common-mode voltage Comparator section Current limiting section 1994 Aug 31 2 Philips Semiconductors Product specification SMPS control circuit SG3524 DC ELECTRICAL CHARACTERISTICS (Continued) TA = 0C to +70C, VIN = 20V, and f = 20kHz, unless otherwise specified. SYMBOL PARAMETER TEST CONDITIONS LIMITS Min 40 VCE=40V IC=50mA VIN=20V RC=2k, TA=25C RC=2k, TA=25C 17 0.1 1 18 0.2 0.1 50 2 Typ Max UNIT Output section (each output) Collector-emitter voltage (breakdown) Collector-leakage current Saturation voltage Emitter output voltage tR tF Rise time Fall time (excluding oscillator charging current, error and current limit dividers, and with outputs open) V A V V s s Total standby current VIN=40V 8 10 mA THEORY OF OPERATION Voltage Reference An internal series regulator provides a nominal 5V output which is used both to generate a reference voltage and is the regulated source for all the internal timing and controlling circuitry. This regulator may be bypassed for operation from a fixed 5V supply by connecting Pins 15 and 16 together to the input voltage. In this configuration, the maximum input voltage is 6.0V. This reference regulator may be used as a 5V source for other circuitry. It will provide up to 50mA of current itself and can easily be expanded to higher currents with an external PNP as shown in Figure 3. Q1 100 +VIN 15 SG3524 REFERENCE SECTION 8 16 + 10F VREF IL to 1.0A DEPENDING ON CHOICE FOR Q1 GND SL00176 Figure 3. Expanded Reference Current Capability TEST CIRCUIT IS OSC OUT VREF 2k 1W 15 3 16 8 6 7 RAMP VIN 8-40V 2k 2 N.I. INPUT 1 INV. INPUT 9 COMP SHUT DOWN 10k 10 4 SG3524 12 13 11 5 14 CURRENT LIMIT 2k 1W OUTPUTS VIN 0.1 RT CT 10k 2k 1k SL00177 Figure 4. Test Circuit 1994 Aug 31 3 Philips Semiconductors Product specification SMPS control circuit SG3524 3.6 V / RT and should be kept within the approximate range of 30A to 2mA; i.e., 1.8k SL00178 Figure 5. Output Stage Dead Time as a Function of the Timing Capacitor Value The range of values for CT also has limits as the discharge time of CT determines the pulse-width of the oscillator output pulse. This pulse is used (among other things) as a blanking pulse to both outputs to insure that there is no possibility of having both outputs on simultaneously during transitions. This output dead time relationship is shown in Figure 5. A pulse width below approximately 0.5s may allow false triggering of one output by removing the blanking pulse prior to the flip-flop's reaching a stable state. If small values of CT must be used, the pulse-width may still be expanded by adding a shunt capacitance (100pF) to ground at the oscillator output. [(Note: Although the oscillator output is a convenient oscilloscope sync input, the cable and input capacitance may increase the blanking pulse-width slightly.)] Obviously, the upper limit to the pulse width is determined by the maximum duty cycle acceptable. Practical values of CT fall between 0.001 and 0.1 F. The oscillator period is approximately t=RTCT where t is in microseconds when RT= and CT=F. The use of Figure 6 will allow selection of RT and CT for a wide range of operating frequencies. Note that for series regulator applications, the two outputs can be connected in parallel for an effective 0-90% duty cycle and the frequency of the oscillator is the frequency of the output. For push-pull applications, the outputs are separated and the flip-flop divides the frequency such that each output's duty cycle is 0-45% and the overall frequency is one-half that of the oscillator. TIMING RESISTOR (R T ) kohms 100 50 20 10 5 2 1 5 10 20 50 100 200 5001ms2ms OSCILLATOR PERIOD (s) External Synchronization If it is desired to synchronize the SG3524 to an external clock, a pulse of +3V may be applied to the oscillator output terminal with RTCT set slightly greater than the clock period. The same considerations of pulse-width apply. The impedance to ground at this point is approximately 2k. If two or more SG3524s must be synchronized together, one must be designated as master with its RTCT set for the correct period. The slaves should each have an RTCT set for approximately 10% longer period than the master with the added requirement that CT(slave)=one-half CT (master). Then connecting Pin 3 on all units together will insure that the master output pulse--which occurs first and has a wider pulse width--will reset the slave units. SL00179 Figure 6. Oscillator Period as a Function of RT and CT RL = 30M VOLTAGE GAIN - dB 80 60 40 20 0 10 RL = RESISTANCE FROM PIN 9 TO GND 100 1k 10k 100k 1M FREQUENCY - (Hz) 10M RL = 1M RL = 300k RL = 100k RL = 30k Error Amplifier This circuit is a simple differential input transconductance amplifier. The output is the compensation terminal, Pin 9, which is a high-impedance node (RL 5M). The gain is A V + g MR L + 8 IC RL 2kT [ 0.002R L and can easily be reduced from a nominal of 10,000 by an external shunt resistance from Pin 9 to ground, as shown in Figure 7. SL00180 Figure 7. Amplifiers Open-Loop Gain as a Function of Frequency and Loading on Pin 9 In addition to DC gain control, the compensation terminal is also the place for AC phase compensation. The frequency response curves of Figure 7 show the uncompensated amplifier with a single pole at approximately 200Hz and a unity gain crossover at 5MHz. Typically, most output filter designs will introduce one or more additional poles at a significantly lower frequency. Therefore, the best stabilizing network is a series RC combination between Pin 9 and ground which introduces a zero to cancel one of the output filter poles. A good starting point is 50k plus 0.001F. Oscillator The oscillator in the SG3524 uses an external resistor (RT) to establish a constant charging current into an external capacitor (CT). While this uses more current than a series-connected RC, it provides a linear ramp voltage on the capacitor which is also used as a reference for the comparator. The charging current is equal to 1994 Aug 31 4 Philips Semiconductors Product specification SMPS control circuit SG3524 One final point on the compensation terminal is that this is also a convenient place to insert any programming signal which is to override the error amplifier. Internal shutdown and current limit circuits are connected here, but any other circuit which can sink 200A can pull this point to ground, thus shutting off both outputs. While feedback is normally applied around the entire regulator, the error amplifier can be used with conventional operational amplifier feedback and is stable in either the inverting or non-inverting mode. Regardless of the connections, however, input common-mode limits must be observed or output signal inversions may result. For conventional regulator applications, the 5V reference voltage must be divided down as shown in Figure 8. The error amplifier may also be used in fixed duty cycle applications by using the unity gain configuration shown in the open-loop test circuit. VREF 5k R2 POSITIVE OUTPUT VOLTAGES 2 1 + - 5k GND R1 VREF R1 5k 2 1 5k GND R2 + - NEGATIVE OUTPUT VOLTAGES Current Limiting The current limiting circuitry of the SG3524 is shown in Figure 9. By matching the base-emitter voltages of Q1 and Q2, and assuming a negligible voltage drop across R1: Threshold=VBE(Q1)+I1R2-VBE(Q2) =I1R2 200mV Although this circuit provides a relatively small threshold with a negligible temperature coefficient, there are some limitations to its use, the most important of which is the 1V common-mode range which requires sensing in the ground line. Another factor to consider is that the frequency compensation provided by R1C1 and Q1 provides a roll-off pole at approximately 300Hz. Since the gain of this circuit is relatively low, there is a transition region as the current limit amplifier takes over pulse width control from the error amplifier. For testing purposes, threshold is defined as the input voltage required to get 25% duty cycle with the error amplifier signaling maximum duty cycle. In addition to constant current limiting, Pins 4 and 5 may also be used in transformer-coupled circuits to sense primary current and to shorten an output pulse, should transformer saturation occur. Another application is to ground Pin 5 and use Pin 4 as an additional shutdown terminal: i.e., the output will be off with Pin 4 open and on when it is grounded. Finally, foldback current limiting can be provided with the network of Figure 10. This circuit can reduce the short-circuit current (ISC) to approximately one-third the maximum available output current (IMAX). 9 SL00181 Figure 8. Error Amplifier Biasing Circuits RAMP t1 R1 ERROR AMPLIFIER C1 R1 Q1 Q2 COMPARATOR 5 - SENSE + 4 SL00182 Figure 9. Current Limiting Circuitry of the SG3524 VO = 5V SA/SB R1 R2 RS - SENSE + 5 4 I MAX + + 1 R S V ) V 0R 2 R1 ) R2 TH I V TH R S where SC NOTE: VTH = 200mV Foldback current limiting can be used to reduce power dissipation under shorted output conditions. SL00183 Figure 10. Foldback Current Limiting 1994 Aug 31 5 |
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