RHFL4913_07 Datasheet, PDF (8/16 Pages) STMicroelectronics – Rad-Hard Adjustable positive voltage regulator

Application information
5 Application information
RHFL4913 ADJUSTABLE VERSION
Adjusting Output Voltage: R2 resistor must be connected between V_O and ADJ pins. R1 resistor must be
connected between ADJ and Ground. Resistor values can be derived from the following formula:
V_O = V_ADJ (R1+ R2) / R1
V_ADJ is 1.23V, controlled by the internal temperature compensated Band Gap block.
The minimum Output voltage is therefore 1.22V and minimum Input voltage is 3V.
The RHFL4913 Adjustable is functional as soon as V_I - V_O voltage difference is slightly above the power
element saturation voltage. The Adjust pin to Ground resistor must not be bigger than 10kΩ to keep the
output feed-back error below 0.2%. A minimum 0.5mA I_O must be set to ensure perfect “no-load”
regulation. It is advisable to dissipate this current into the divider bridge resistor. All available V_I pins shall
always be externally interconnected, same thing for all available V_O pins, otherwise Device stability and
reliability cannot be granted. The INHIBIT function switches off the output current in an electronic way,
that is very quickly. According to Lenz’s Law, external circuitry reacts with LdI/dt terms which can be of
high amplitude in case somewhere an inductance exists. Large transient voltage would develop on both
Device terminals. It is advisable to protect the Device with Schottky diodes preventing negative voltage
excursions. In the worst case, a 14V Zener diode shall protect the Device Input. The Device has been
designed for high stability and low drop out operation: Minimum 1µF input and output tantalum capacitors
are therefore mandatory. Capacitor ESR range is from 0.5 Ω to over 20 Ω. Such range turns out to be
useful when ESR increases at low temperature. When large transient currents are expected, larger value
capacitors are necessary.
In case of high current operation with expected short-circuit events, caution must be considered relatively
to capacitors. They must be connected as close as possible to device terminals. As some tantalum
capacitors may permanently fail when submitted to high charge-up surge currents, it is recommended to
decouple them with 470nF polyester capacitors.
Being RHFL4913 Adjust manufactured with very high speed bipolar technology (6GHz f_T transistors), the
PCB layout must be performed with exceptional care, very low inductance, low mutually coupling lines,
otherwise high frequency parasitic signals may be picked-up by the Device resulting into system self-
oscillation. The benefit is an SVR performance extended to far higher frequencies.
5.1 Notes about 16pin hermetic package
The bottom section of 16pin packages is metallized in order to allow User to directly solder the
RHFL4913A on the equipment heat sink for enhanced heat removal.
5.2 Remote sensing operation
See Figure 2. A separate Kelvin Voltage sensing line provides ADJ pin with exact Load "high potential"
information. But variable remote Load current consumption induces variable Iq current (Iq is roughly the
I_O current divided by the h_FE of the internal PNP series power element) routed through parasitic series
line resistor symbolized as RW2. To compensate this parasitic voltage, the Resistor RW1 introduced on
purpose provides the necessary compensating voltage signal to ADJust pin.
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