TL783_15 Datasheet, PDF (13/27 Pages) Texas Instruments – TL783 High-voltage Adjustable Regulator

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TL783
SLVS036N – SEPTEMBER 1981 – REVISED JANUARY 2015
Typical Application (continued)
9.2.2 Detailed Design Procedure
The internal reference (see Simplified Schematic) generates 1.25 V nominal (V_ref) between OUT and ADJ. This
voltage is developed across R1 and causes a constant current to flow through R1 and the programming resistor
R2, giving an output voltage of:
V_O = V_ref (1 + R2 / R1) + l_I(ADJ) (R2)
or
V_O ≉ V_ref (1 + R2 / R1)
The TL783 was designed to minimize the input current at ADJ and maintain consistency over line and load
variations, thereby minimizing the associated (R2) error term.
To maintain I_I(ADJ) at a low level, all quiescent operating current is returned to the output terminal. This quiescent
current must be sunk by the external load and is the minimum load current necessary to prevent the output from
rising. The recommended R1 value of 82 Ω provides a minimum load current of 15 mA. Larger values can be
used when the input-to-output differential voltage is less than 125 V (see the output-current curve in Figure 12) or
when the load sinks some portion of the minimum current.
9.2.2.1 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.
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