RHFL7913A_11 Datasheet, PDF (10/19 Pages) STMicroelectronics – Rad-hard adjustable negative voltage regulator

Application information
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Application information
RHFL7913A
Adjusting output voltage: R1 resistor must be connected between V_O and ADJ pins. R2
resistor must be connected between ADJ and GND pins. Resistor values can be derived
from the following formula:
V_O = V_ADJ (1 + R1/R2) with V_ADJ = -1.20 V
To access all RHFL7913A adjustable functionality, V (+) shall be set at 3.0 V (max 5 V). As a
consequence, V_I (-) cannot be greater than –9.0 V (resp. –7 V) due to –12 V maximum
operation rating.
The device is functional as soon as V_I - V_O voltage difference is slightly above power
element saturation voltage. The adjustable pin to Ground resistor shall not be greater than
10 kΩ to make output feedback error be below 0.2%. A minimum 0.5 mA I_O is necessary to
set to ensure perfect “no-load” regulation. It can be wise to dissipate this current into the
divider bridge resistor.
All available V_I pins shall always be PCB interconnected, the same for all available V_O pins,
otherwise device stability and reliability cannot be granted. The INHIBIT function switches
off output current in an electronic way, is very quickly. According to Lenz’ Law, external
circuitry reacts with –LdI/dt terms which can be of high amplitude in case some series-
inductance exists. Effect would be large transient voltage developed on both device
terminals. External Schottky diodes set on V_I and V_O may prevent voltage excursions
beyond max ratings. In the worst case, a 14 V Zener diode shall protect the device input. In
case of capacitive load, a input-output protection diode can be necessary to prevent V_O to
be greater than V_I during transient operation.
The device has been designed for high stability and low drop out operation: minimum 1 µF
input and output tantalum capacitors are therefore mandatory. Typical industry PCB design
practices 10 µF capacitors values are acceptable. Capacitor ESR range is from 0.020 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 shall be
considered relatively to capacitors. They shall be connected as close as possible form
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 or
ceramic capacitors.
RHFL7913A adjustable being manufactured with very high speed bipolar technology (6 GHz
f_T transistors), the PCB layout shall be performed with unprecedented 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. In difficult high current
circumstances (coming from PCB layout) a 470 nF polyester capacitor connected between
V_O and ADJ pin increases stability margins. Benefit is SVR performances extended to far
higher frequencies.
10/19
Doc ID 12344 Rev 7