RHFL4913XX25_10 Datasheet, PDF (10/19 Pages) STMicroelectronics – Rad-hard positive fixed voltage regulator

Application information
RHFL4913XX25, RHFL4913XX33, RHFL4913XX50
6
Application information
The RHFL4913 fixed voltage is functional as soon as V_I-V_O voltage difference is slightly
above the power element saturation voltage. A minimum 0.5 mA I_O ensures perfect “no-
load” regulation.
All available V_I pins must always be externally interconnected, same thing for all available
V_O pins, otherwise device stability and reliability cannot be granted. All NC pins can be
connected to ground. 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 some series-inductance exists. The effect would be
a large transient voltage developed on both device terminals. It is necessary to protect the
device with Schottky diodes preventing negative voltage excursions. In the worst case, a 14
V Zener diode shall protect the device input.
The device has been designed for high stability and low drop out operation: minimum 1 µF
input and output tantalum capacitors are therefore mandatory. Capacitor ESR range is from
0.01 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 470 nF polyester
capacitors.
Being RHFL4913 fixed voltage manufactured with very high speed bipolar technology 6 GHz
f_T transistors), the PCB lay-out must be performed with extreme care, very low inductance,
low mutually coupling lines, otherwise high frequency parasitic signals may be picked-up by
the device resulting into self-oscillation. User’s benefit is a SVR performance extended to far
higher frequencies.
6.1
Remote sensing operation
In case the load is located far from the regulator, it is recommended to comply with the
scheme below. To obtain the best regulation, it is in addition essential to care about:
– The wire connecting R2 to the load end must not be crossed by the load current
(Kelvin sense).
The noise captured by the wires between the load and the chip could bring a noisy output
voltage. In case this happens, it is recommended that shielded cables are used for these
connections. The external wrap must be used for connecting the ground of the chip with the
load ground. It is also recommended to place 1 µF tantalum capacitors between output and
ground close to the device and another next to the load.
10/19
Doc ID 10004 Rev 11