LM78S40 Universal Switching Regulator Subsystem General Description Features The LM78S40 is a monolithic regulator subsystem consisting of all the active building blocks necessary for switching regulator systems. The device consists of a temperature compensated voltage reference, a duty-cycle controllable oscillator with an active current limit circuit, an error amplifier, high current, high voltage output switch, a power diode and an uncommitted operational amplifier. The device can drive external NPN or PNP transistors when currents in excess of 1.5A or voltages in excess of 40V are required. The device can be used for step-down, step-up or inverting switching regulators as well as for series pass regulators. It features wide supply voltage range, low standby power dissipation, high efficiency and low drift. It is useful for any stand-alone, low part count switching system and works extremely well in battery operated systems. n n n n n n n n Step-up, step-down or inverting switching regulators Output adjustable from 1.25V to 40V Peak currents to 1.5A without external transistors Operation from 2.5V to 40V input Low standby current drain 80 dB line and load regulation High gain, high current, independent op amp Pulse width modulation with no double pulsing Block and Connection Diagrams DS010057-2 16-Lead DIP DS010057-1 Top View © 1998 National Semiconductor Corporation DS010057 www.national.com LM78S40 Universal Switching Regulator Subsystem April 1998 Ordering Information Part Number NS Package Temperature Range LM78S40J/883 J16A Ceramic DIP −55˚C to +125˚C LM78S40N N16E Molded DIP −40˚C to +125˚C LM78S40CN N16E Molded DIP 0˚C to +70˚C www.national.com 2 Absolute Maximum Ratings (Note 1) to GND Common Mode Input Range (Comparator and Op Amp) Differential Input Voltage (Note 4) Output Short Circuit Duration (Op Amp) Current from VREF Voltage from Switch Collectors to GND Voltage from Switch Emitters to GND Voltage from Switch Collectors to Emitter Voltage from Power Diode to GND Reverse Power Diode Voltage Current through Power Switch Current through Power Diode ESD Susceptibility If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Storage Temperature Range Ceramic DIP Molded DIP Operating Temperature Range Extended (LM78S40J) Industrial (LM78S40N) Commercial (LM78S40CN) Lead Temperature Ceramic DIP (Soldering, 60 sec.) Molded DIP (Soldering, 10 sec.) Internal Power Dissipation (Note 2) (Note 16L-Ceramic DIP 16L-Molded DIP Input Voltage from VIN to GND Input Voltage from V+ (Op Amp) −65˚C to +175˚C −65˚C to +150˚C −55˚C to +125˚C −40˚C to +125˚C 0˚C to +70˚C 300˚C 265˚C 3) 1.50W 1.04W 40V 40V −0.3 to V+ ± 30V Continuous 10 mA 40V 40V 40V 40V 40V 1.5A 1.5A (to be determined) LM78S40 Electrical Characteristics (Note 5) TA = Operating temperature range, VIN = 5.0V, V+(Op Amp) = 5.0V, unless otherwise specified. Symbol Parameter Conditions Min Typ Max Units 1.8 3.5 mA 2.3 5.0 mA 4.0 mA 5.5 mA 1.245 1.310 V 0.04 0.2 mV/V 0.2 0.5 mV/mA 20 50 µA 20 70 µA 150 250 µA 350 µA GENERAL CHARACTERISTICS ICC Supply Current (Op Amp Disconnected) ICC Supply Current (Op Amp Connected) VIN = 5.0V VIN = 40V VIN = 5.0V VIN = 40V REFERENCE SECTION VREF Reference Voltage IREF = 1.0 mA Extend −55˚C < TA < +125˚C, Comm 0 < TA < +70˚C, VR LINE Reference Voltage Indus −40˚C < TA < +85˚C VIN = 3.0V to VIN = 40V, IREF = 1.0 mA, TA = 25˚C IREF = 1.0 mA to IREF = 10 mA, Load Regulation TA = 25˚C Reference Voltage Line Regulation VR LOAD 1.180 OSCILLATOR SECTION ICHG Charging Current ICHG Charging Current IDISCHG Discharge Current IDISCHG Discharge Current VOSC Oscillator Voltage Swing ton/toff Ratio of Charge/ VIN = 5.0V, TA = 25˚C VIN = 40V, TA = 25˚C VIN = 5.0V, TA = 25˚C VIN = 40V, TA = 25˚C VIN = 5.0V, TA = 25˚C 150 0.5 V 6.0 µs/µs Discharge Time CURRENT LIMIT SECTION VCLS Current Limit Sense Voltage TA = 25˚C 250 350 mV 1.1 1.3 V 0.45 0.7 V OUTPUT SWITCH SECTION VSAT 1 Output Saturation Voltage 1 VSAT 2 Output Saturation Voltage 2 ISW = 1.0A (Figure 1) ISW = 1.0A (Figure 2) 3 www.national.com LM78S40 Electrical Characteristics (Note 5) (Continued) TA = Operating temperature range, VIN = 5.0V, V+(Op Amp) = 5.0V, unless otherwise specified. Symbol Parameter Conditions Min Typ Max Units OUTPUT SWITCH SECTION hFE Output Transistor Current Gain IC = 1.0A, VCE = 5.0V, TA = 25˚C 70 IL Output Leakage Current VO = 40V, TA = 25˚C 10 ID = 1.0A VD = 40V, TA = 25˚C 1.25 VCM = VREF VCM = VREF 1.5 15 35 200 nA VCM = VREF TA = 25˚C 5.0 75 nA 0 VIN–2 V VIN = 3.0V to 40V, TA = 25˚C 70 nA POWER DIODE VFD Forward Voltage Drop IDR Diode Leakage Current 1.5 10 V nA COMPARATOR VIO Input Offset Voltage IIB Input Bias Current IIO Input Offset Current VCM Common Mode Voltage Range PSRR Power Supply Rejection Ratio 96 mV dB OPERATIONAL AMPLIFIER VIO Input Offset Voltage IIB Input Bias Current IIO Input Offset Current AVS+ Voltage Gain+ AVS− Voltage Gain− VCM Common Mode Voltage Range CMR Common Mode Rejection PSRR Power Supply Rejection Ratio IO+ Output Source Current IO− Output Sink Current SR Slew Rate VOL Output Voltage LOW VOH Output Voltage High VCM = 2.5V VCM = 2.5V VCM = 2.5V 4.0 15 30 200 mV nA 5.0 75 nA RL = 2.0 kΩ to GND; VO = 1.0V to 2.5V, TA = 25˚C RL = 2.0 kΩ to V+ (Op Amp) 25 250 V/mV 25 250 V/mV VO = 1.0V to 2.5V, TA = 25˚C TA = 25˚C 0 VCM = 0V to 3.0V, TA = 25˚C V+ (Op Amp) = 3.0V to 40V, TA = 25˚C 76 100 dB 76 100 dB 75 150 mA 10 35 mA TA = 25˚C TA = 25˚C TA = 25˚C VCC − 2 0.6 IL = −5.0 mA, TA = 25˚C IL = 50 mA, TA = 25˚C V/µs 1.0 V + (Op V V V Amp) − 3V Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Electrical specifications do not apply when ordering the device beyond its rated operating conditions. Note 2: TJ Max = 150˚C for the Molded DIP, and 175˚C for the Ceramic DIP. Note 3: Ratings apply to ambient temperature at 25˚C. Above this temperature, derate the 16L-Ceramic DIP at 10 mW/˚C, and the 16L-Molded DIP at 8.3 mW/˚C. Note 4: For supply voltages less than 30V, the absolute maximum voltage is equal to the supply voltage. Note 5: A military RETS specification is available on request. At the time of printing, the LM78S40 RETS specification complied with the Min and Max limits in this table. The LM78S40J may also be procured as a Standard Military Drawing. www.national.com 4 Typical Performance Characteristics CT vs OFF Time Reference Voltage vs Junction Temperature DS010057-6 DS010057-7 Discharge Current vs Input Voltage Current Limit Sense Voltage vs Input Voltage DS010057-8 DS010057-9 Design Formulas Characteristic Step-Down Step-Up Inverting (ton + toff) Max Units µs CT 4 x 10−5 ton Ipk 2 IO Max 4 x 10−5 ton 4 x 10−5 ton µF A LMin µH RSC 0.33/Ipk 0.33/Ipk 0.33/Ipk Ω CO µF Note 6: VSAT = Saturation voltage of the switching element. VD = Forward voltage of the flyback diode. 5 www.national.com Functional Description Typical Applications SWITCHING FREQUENCY CONTROL The LM78S40 is a variable frequency, variable duty cycle device. The initial switching frequency is set by the timing capacitor. (Oscillator frequency is set by a single external capacitor and may be varied over a range of 100 Hz to 100 kHz). The initial duty cycle is 6:1. This switching frequency and duty cycle can be modified by two mechanisms — the current limit circuitry (Ipk sense) and the comparator. The comparator modifies the OFF time. When the output voltage is correct, the comparator output is in the HIGH state and has no effect on the circuit operation. If the output voltage is too high then the comparator output goes LOW. In the LOW state the comparator inhibits the turn-on of the output stage switching transistors. As long as the comparator is LOW the system is in OFF time. As the output current rises the OFF time decreases. As the output current nears its maximum the OFF time approaches its minimum value. The comparator can inhibit several ON cycles, one ON cycle or any portion of an ON cycle. Once the ON cycle has begun the comparator cannot inhibit until the beginning of the next ON cycle. The current limit modifies the ON time. The current limit is activated when a 300 mV potential appears between lead 13 (VCC) and lead 14 (Ipk). This potential is intended to result when designed for peak current flows through RSC. When the peak current is reached the current limit is turned on. The current limit circuitry provides for a quick end to ON time and the immediate start of OFF time. Generally the oscillator is free running but the current limit action tends to reset the timing cycle. Increasing load results in more current limited ON time and less OFF time. The switching frequency increases with load current. DS010057-3 FIGURE 1. Typical Step-Down Regulator and Operational Performance (TA = 25˚C) USING THE INTERNAL REFERENCE, DIODE, AND SWITCH The internal 1.245V reference (pin 8) must be bypassed, with 0.1 µF directly to the ground pin (pin 11) of the LM78S40, to assure its stability. VFD is the forward voltage drop across the internal power diode. It is listed on the data sheet as 1.25V typical, 1.5V maximum. If an external diode is used, then its own forward voltage drop must be used for VFD. VSAT is the voltage across the switch element (output transistors Q1 and Q2) when the switch is closed or ON. This is listed on the data sheet as Output Saturation Voltage. “Output saturation voltage 1” is defined as the switching element voltage for Q2 and Q1 in the Darlington configuration with collectors tied together. This applies to Figure 1, the step down mode. “Output saturation voltage 2” is the switching element voltage for Q1 only when used as a transistor switch. This applies to Figure 2, the step up mode. For the inverting mode, Figure 3, the saturation voltage of the external transistor should be used for VSAT. www.national.com Characteristic Condition Typical Value Output Voltage IO = 200 mA Line Regulation 20V ≤ VI ≤ 30V 1.5 mV Load Regulation 5.0 mA ≤ IO 3.0 mV Max Output Current IO ≤ 300 mA VO = 9.5V 500 mA Output Ripple Efficiency Standby Current IO = 200 mA IO = 200 mA IO = 200 mA 10V 50 mV 74% 2.8 mA Note 7: For IO ≥ 200 mA use external diode to limit on-chip power dissipation. 6 Typical Applications (Continued) DS010057-4 DS010057-5 FIGURE 2. Typical Step-Up Regulator and Operational Performance (TA = 25˚C) Characteristic Condition FIGURE 3. Typical Inverting Regulator and Operational Performance (TA = 25˚C) Typical Characteristic Value Output Voltage IO = 50 mA Line Regulation 5.0V ≤ VI ≤ 15V 4.0 mV Load Regulation 5.0 mA ≤ IO 2.0 mV Max Output Current IO ≤ 100 mA VO = 23.75V Output Ripple Efficiency Standby Current IO = 50 mA IO = 50 mA IO = 50 mA Condition Typical Value 25V 160 mA 30 mV Output Voltage IO = 100 mA Line Regulation 8.0V ≤ VI ≤ 18V 5.0 mV Load Regulation 5.0 mA ≤ IO 3.0 mV Max Output Current IO ≤ 150 mA VO = 14.25V 160 mA Output Ripple 79% Efficiency 2.6 mA Standby Current 7 IO = 100 mA IO = 100 mA IO = 100 mA −15V 20 mV 70% 2.3 mA www.national.com Typical Applications (Continued) DS010057-10 FIGURE 4. Pulse Width Modulated Step-Down Regulator (fOSC = 20 kHz) www.national.com 8 Physical Dimensions inches (millimeters) unless otherwise noted 16-Lead Ceramic Dual-In-Line Package (J) Order Number LM78S40J/883 NS Package Number J16A 16-Lead Molded Dual-In-Line Package (N) Order Number LM78S40N or LM78S40CN NS Package Number N16A 9 www.national.com LM78S40 Universal Switching Regulator Subsystem LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 2. A critical component in any component of a life support 1. Life support devices or systems are devices or sysdevice or system whose failure to perform can be reatems which, (a) are intended for surgical implant into sonably expected to cause the failure of the life support the body, or (b) support or sustain life, and whose faildevice or system, or to affect its safety or effectiveness. ure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. 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