LM2696MXAX datasheet(10/19 Pages) NSC

10 / 19 page

Application Information (Continued)

It can be seen that the average output voltage is higher than

the gained up reference by exactly half the output voltage

ripple. The output voltage may then be appended according

to the voltage ripple. The appended V

OUT term may be

expressed using the relationship:

One should note that for high output voltages (>5V), a load

of approximately 15 mA may be required for the output

voltage to reach the desired value.

INPUT CAPACITOR

Because PV

IN is the power rail from which the output voltage

is derived, the input capacitor is typically selected according

to the load current. In general, package size and ESR de-

termine the current capacity of a capacitor. If these criteria

are met, there should be enough capacitance to prevent

impedance interactions with the source. In general, it is

recommended to use a low ESR, high capacitance electro-

lytic and ceramic capacitor in parallel. Using two capacitors

in parallel ensures adequate capacitance and low ESR over

the operating range. The Sanyo MV-WX series electrolytic

capacitors and a ceramic capacitor with X5R or X7R dielec-

tric are an excellent combination. To calculate the input

capacitor RMS, one may use the following relationship:

that can be approximated by,

Typical values are 470 µF for the electrolytic capacitor and

0.1 µF for the ceramic capacitor.

IN CAPACITOR

IN is the analog bias rail of the device. It should be

bypassed externally with a small (1 µF) ceramic capacitor to

prevent unwanted noise from entering the device. In a shut-

down state the current needed by AV

IN will drop to approxi-

mately 12 µA, providing a low power sleep state.

In most cases of operation, AV

IN is connected to PVIN;

however, it is possible to have split rail operation where AV

is at a higher voltage than PV

IN.AVIN should never be lower

than PV

IN. Splitting the rails allows the power conversion to

occur from a lower rail than the AV

IN operating range.

SOFT-START CAPACITOR

The SS capacitor is used to slowly ramp the reference from

0V to its final value of 1.25V (during shutdown this pin will be

discharged to 0V). This controlled startup ability eliminates

large in-rush currents in an attempt to charge up the output

capacitor. By changing the value of this capacitor, the dura-

tion of the startup may be changed accordingly. The startup

time may be calculated using the following relationship:

Where I

SS is the soft-start pin source current (1 µA typical)

that may be found in the electrical characteristics table.

While the C

SS capacitor can be sized to meet the startup

requirements, there are limitations to its size. If the capacitor

is too small, the soft-start will have little effect as the refer-

ence voltage is rising faster than the output capacitor can be

charged causing the part to go into current limit. Therefore a

minimum soft-start time should be taken into account. This

can be determined by:

20153431

FIGURE 2. Average and Ripple Output Voltages

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