LM78S40 LM78S40 Universal Switching Regulator Subsystem Literature Number: SNVS021
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.
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.
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 Nat
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.
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.
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.
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.
Typical Applications (Continued) DS010057-10 FIGURE 4. Pulse Width Modulated Step-Down Regulator (fOSC = 20 kHz) www.national.
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.
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.
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