7

Application information

must be connected between ADJ and ground. Resistor values can be derived from the following formula:

regulation. It is advisable to dissipate this current into the divider bridge resistor.

The inhibit function switches off the output current very quickly. According to Lenz’s law, the external circuitry

reacts with LdI/dt terms which can be of high amplitude in case somewhere a serial coil inductance exists. Large

transient voltage would develop on both device terminals. It is advisable to protect the device with Schottky

diodes to prevent negative voltage excursions. In the worst case, a 14 V Zener diode could protect the device

input.

All available VO pins should always be externally interconnected, otherwise the stability and reliability of the

device cannot be guaranteed.

To ensure regulator stability, input and output capacitors with a minimum 10 μF are mandatory. These capacitors

must be connected as close as possible to the device terminals.

In the case of high-current operation, an important factor to look at for the reliability target of the space application

is the sustainable surge current of the capacitors used. The surge current is known to be one of the major failure

mechanisms for these parts, especially when the equipment is turned ON. Tantalum capacitors manufactured per

military specifications (MIL-PRF-55365) are established reliability parts targeted for less than 0.001% of failures

per 1000 hours (failure rate< 10 FIT).

Derating is a means for application engineers of space systems to further reduce the probability of failures by

limiting the level of stresses to capacitors during application. Typical derating requirements for solid tantalum

capacitors limit the maximum applied voltage to 50% of the rated voltage (VR) and the inrush currents are

bounded by additional resistors used in series with the capacitors.

In addition, a ceramic capacitor of at least 100nF in parallel to the input and output bulk capacitors must be used

for decoupling purposes. A 470 nF polyester capacitors, put close to the regulator between input and ground,

helps further improving the LEO3910 reliability by filtering potentially dangerous over voltages spikes coming out

during particular conditions.

A separate kelvin voltage sensing line provides the ADJ pin with exact load "high potential" information (see

Figure 4. Application diagram for remote sensing operation). But variable remote load current consumption

induces variable Iq current (Iq is roughly the IO current divided by the hFE of the internal PNP series power

element) routed through the parasitic series line resistor RW2. To compensate for this parasitic voltage, resistor

RW1 can be introduced to provide the necessary compensating voltage signal to the ADJ pin. A ceramic or

polyester 47nF CBYP capacitor between ADJ and VO pins is recommended when the remote sensing technique

is implemented.

Since the LEO3910 adjustable voltage regulator is manufactured with very high speed bipolar technology (6 GHz

fT transistors), the PCB layout must be designed with exceptional care, with very low inductance and low mutually

coupling lines. Otherwise, high frequency parasitic signals may be picked up by the device resulting in system

self-oscillation. The benefit is an SVR performance extended to far higher frequencies.

LEO3910

Application information

DS13691 - Rev 2

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