TL783 Datasheet, PDF (9/13 Pages) Texas Instruments – HIGH-VOLTAGE ADJUSTABLE REGULATOR

TL783
HIGH-VOLTAGE ADJUSTABLE REGULATOR
SLVS036E – SEPTEMBER 1981 – REVISED FEBRUARY 2000
DESIGN CONSIDERATIONS
bypass capacitors
The TL783 regulator is stable without bypass capacitors; however, any regulator becomes unstable with certain
values of output capacitance if an input capacitor is not used. Therefore, the use of input bypassing is
recommended whenever the regulator is located more than four inches from the power-supply filter capacitor.
A 1-µF tantalum or aluminum electrolytic capacitor usually is sufficient.
Adjustment-terminal capacitors are not recommended for use on the TL783 because they can seriously
degrade load transient response as well as create a need for extra protection circuitry. Excellent ripple rejection
presently is achieved without this added capacitor.
Due to the relatively low gain of the MOS output stage, output voltage dropout may occur under large load
transient conditions. The addition of an output bypass capacitor greatly enhances load transient response and
prevents dropout. For most applications, it is recommended that an output bypass capacitor be used, with a
minimum value of:
C_o (µF) = 15/V_O
Larger values provide proportionally better transient-response characteristics.
protection circuitry
The TL783 regulator includes built-in protection circuits capable of guarding the device against most overload
conditions encountered in normal operation. These protective features are current limiting, safe-operating-area
protection, and thermal shutdown. These circuits protect the device under occasional fault conditions only.
Continuous operation in the current limit or thermal shutdown mode is not recommended.
The internal protection circuits of the TL783 protect the device up to maximum-rated V_I as long as certain
precautions are taken. If V_l is instantaneously switched on, transients exceeding maximum input ratings may
occur, which can destroy the regulator. These are usually caused by lead inductance and bypass capacitors
causing a ringing voltage on the input. In addition, when rise times in excess of 10 V/ns are applied to the input,
a parasitic npn transistor in parallel with the DMOS output can be turned on, causing the device to fail. If the
device is operated over 50 V and the input is switched on rather than ramped on, a low-Q capacitor, such as
tantalum or aluminum electrolytic should be used rather than ceramic, paper, or plastic bypass capacitors. A
Q factor of 0.015 or greater usually provides adequate damping to suppress ringing. Normally, no problems
occur if the input voltage is allowed to ramp upward through the action of an ac line rectifier and filter network.
Similarly, when an instantaneous short circuit is applied to the output, both ringing and excessive fall times can
result. A tantalum or aluminum electrolytic bypass capacitor is recommended to eliminate this problem.
However, if a large output capacitor is used and the input is shorted, addition of a protection diode may be
necessary to prevent capacitor discharge through the regulator. The amount of discharge current delivered is
dependent on output voltage, size of capacitor, and fall time of V_l. A protective diode (see Figure 17) is required
only for capacitance values greater than:
C_o (µF) = 3 x 10⁴/(V_O)²
Care always should be taken to prevent insertion of regulators into a socket with power on. Power should be
turned off before removing or inserting regulators.
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