Datasheet LM337 (ON Semiconductor) - 7

LM337 APPLICATIONS INFORMATION Basic Circuit Operation
The LM337 is a 3−terminal floating regulator. In
External Capacitors
operation, the LM337 develops and maintains a nominal A 1.0 mF tantalum input bypass capacitor (Cin) is −1.25 V reference (Vref) between its output and adjustment recommended to reduce the sensitivity to input line terminals. This reference voltage is converted to a impedance. programming current (IPROG) by R1 (see Figure 17), and this The adjustment terminal may be bypassed to ground to constant current flows through R2 from ground. improve ripple rejection. This capacitor (CAdj) prevents The regulated output voltage is given by: ripple from being amplified as the output voltage is ǒ increased. A 10 R Ǔ mF capacitor should improve ripple V 2 rejection about 15 dB at 120 Hz in a 10 V application. out + V 1 ) I R ref R Adj 2 1 An output capacitance (CO) in the form of a 1.0 mF Since the current into the adjustment terminal (IAdj) tantalum or 10 mF aluminum electrolytic capacitor is represents an error term in the equation, the LM337 was required for stability. Using the classical tantalum or designed to control IAdj to less than 100 mA and keep it aluminum electrolytic capacitor types with non−reduced constant. To do this, all quiescent operating current is ESR (Equivalent Series Resistance) value is necessary. returned to the output terminal. This imposes the Low−ESR or similar capacitor types with reduced ESR requirement for a minimum load current. If the load current value and ceramic capacitors can cause instability or is less than this minimum, the output voltage will rise. continuous oscillations in the application. Since the LM337 is a floating regulator, it is only the
Protection Diodes
voltage differential across the circuit which is important to When external capacitors are used with any IC regulator performance, and operation at high voltages with respect to it is sometimes necessary to add protection diodes to prevent ground is possible. the capacitors from discharging through low current points + V into the regulator. out Figure 18 shows the LM337 with the recommended R2 protection diodes for output voltages in excess of −25 V or IPROG IAdj + high capacitance values (CO > 25 mF, CAdj > 10 mF). Diode C D O 1 prevents CO from discharging thru the IC during an input short circuit. Diode D2 protects against capacitor CAdj Adjust R1 Vref discharging through the IC during an output short circuit. The combination of diodes D1 and D2 prevents CAdj from Vin
LM337
- Vout the discharging through the IC during an input short circuit. Vout + V V out ref = -1.25 V Typical +
Figure 17. Basic Circuit Configuration
R2 CAdj + +
Load Regulation
Cin CO The LM337 is capable of providing extremely good load Adjust R1 D2 regulation, but a few precautions are needed to obtain 1N4002 -V
LM337
- V maximum performance. For best performance, the in out V V in out programming resistor (R1) should be connected as close to the regulator as possible to minimize line drops which D1 effectively appear in series with the reference, thereby 1N4002 degrading regulation. The ground end of R2 can be returned
Figure 18. Voltage Regulator with Protection Diodes
near the load ground to provide remote ground sensing and improve load regulation.
www.onsemi.com 7