CPC1580

R01

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7

(listed in the MOSFET data sheet).

The storage capacitor needs to deliver enough charge

to the gate without going below the 15V required for

equal, at a minimum, 15.7V. The part can operate at a

lower voltage, but the MOSFETs will be turned on

slowly.

The proper selection of a capacitor is important. The

rated voltage should be at least two to three times the

capacitors lose capacitance when operated at their full

rated voltages. When selecting a capacitor, be sure to

add in their tolerance because of capacitor drift. For

example:

F x 20% = 0.2F

F + 0.2F = 1.2F

Capacitance drift can also be due to temperature and

the dielectrics used. Therefore, the required capacitor

value is 1

2F or next higher value, and the capacitor

voltage rating must be at least 30 volts. It is

recommended to go higher in the voltage rating if

engineering restraints permit, such as 50V.

Temperature requirements for capacitors are

application-specific. The designer must know the

intended operating temperature when selecting

capacitors. The information given above should be

applied to other capacitors discussed in this data

sheet.

The CPC1580 can deliver 32nC at the rated operating

speed and will operate with much larger loads (>4

F)

with slower turn-on and turn-off times.

capacitor-to-ground leakage current path

between pins 7 and 8 (MOSFET gate current)

and between pins 5 and 6. Leakage currents will

discharge the storage capacitor and, even

though the device is already on, will become a

load to the photocurrent, which keeps the gate

voltage on. The gate voltage will be reduced if

>500nA of leakage is present, therefore the

combined impedance from pin 8 to pin 7, pin 5,

and pin 6, capacitor current, and MOSFET

current must be >20M

rating of the part.

4.3 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 3

use “logic level” MOSFETs for each design to maintain

the load described.

4.3.1 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

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

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