CPC1580

R00G

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7

3.2 Transistor Selection

The CPC1580 charges and discharges an external

MOSFET transistor. The selection of the MOSFET is

determined by the user to meet the specific power

requirements for the load. The CPC1580 output

voltage is listed in the specifications, but as mentioned

earlier, there must be little or no gate leakage.

Another parameter that plays a significant role in the

selection of the transistor is the gate drive voltage

available from the part. The CPC1580 uses

photovoltaic cells to collect the optical energy

generated by the LED; to generate more voltage, the

photovoltaic diodes are stacked. The voltage change

of the photovoltaic stack reduces with increased

temperature. The user must select a transistor that will

maintain the load current at the maximum

CPC1580 Table of Electrical Specifications.

The example circuits shown in Figure 1 and Figure 2

use “logic level” MOSFETs for each design to maintain

the load described.

3.2.1 Transistor Switching Characteristics

The primary characteristics of the application

are dependent on the MOSFET selection and need to

be reviewed in light of the application requirements.

The CPC1580 turns on the MOSFET transistor to the

delay is the amount of time until the LED is turned off

and the capacitive load discharges to the level in the

CPC1580 specification. For MOSFETs with larger or

proportionately faster and slower, but it is not a linear

relationship.

To calculate the nominal rise time of the transistor's

To calculate the nominal fall time of the transistor's

(averaged over the switching voltage range) found in

by the CPC1580 unloaded discharge characteristic

and should be reviewed in light of the final application

component selections if critical.

component selection. The storage capacitor charge

recovery time (seconds) is computed as:

Which reduces to:

elements that are present in the application circuit

diagram (see Figure 2).

The term inside the logarithm reflects the discharge

component selection, this can be simplified as

described above.

Use this information to calculate the maximum

switching frequency in Section 6 below.

remote power is otherwise unavailable. If the

LED is also powered remotely, care must be

taken to ensure that parasitic transient signals

are reliably filtered from the input control signal.

Large transient currents will mutually couple

energy between cables and a simple R-C

filtering of the CPC1580 input may be sufficient

to suppress false turn-on.

~

(SECONDS)

~

(SECONDS)

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