RHFL7913SCA2 datasheet(10/19 Pages) STMICROELECTRONICS

10 / 19 page

Application information

RHFL7913A

10/19

Application information

Adjusting output voltage: R1 resistor must be connected between VO and ADJ pins. R2

resistor must be connected between ADJ and GND pins. Resistor values can be derived

from the following formula:

VO = VADJ (1 + R1/R2) with VADJ = -1.20 V

To access all RHFL7913A adjustable functionality, V (+) shall be set at 3.0 V (max 5 V). As a

consequence, VI (-) cannot be greater than –9.0 V (resp. –7 V) due to –12 V maximum

operation rating.

The device is functional as soon as VI - VO voltage difference is slightly above power

element saturation voltage. The adjustable pin to Ground resistor shall not be greater than

10 k

Ω to make output feedback error be below 0.2%. A minimum 0.5 mA IO 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 VI pins shall always be PCB interconnected, the same for all available VO 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 VI and VO 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 VO to

be greater than VI 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

fT 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

VO and ADJ pin increases stability margins. Benefit is SVR performances extended to far

higher frequencies.

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