CPC1580

6

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R00G

2. Introduction

The CPC1580 Isolated Gate Driver uses an efficient

optocoupler design to provide remote gate drive

isolation between the LED control current input and

the MOSFET gate drive output. To make the most

efficient use of the input control current to the

optocoupler’s LED, the CPC1580 has multiple

phototransistors in a stacked configuration. As a

result, the gate drive current to the attached MOSFET

can be switched on with just a small input control

current to the LED.

To minimize MOSFET turn-on time, the CPC1580

makes use of an external charge storage capacitor.

When control current is applied to the CPC1580 to

turn on the MOSFET, the charge held in this capacitor

is delivered to the MOSFET’s gate, which charges the

gate quickly and turns on the MOSFET. After the

capacitor charge has turned the MOSFET on and as

long as sufficient input control current is applied, the

continuing flow of phototransistor current will keep the

MOSFET turned on; in other words, there is no lower

operating frequency with the CPC1580. When the

MOSFET is turned off, the capacitor recharges,

through the internal bootstrap diode from the

MOSFET power supply, for the next turn-on cycle. In

addition, the bootstrap diode prevents the capacitor

from discharging through the MOSFET.

The CPC1580 is powered from the system bus supply

voltage of the MOSFET, which means that no

separate power supply is required. Because of this,

circuit design is greatly simplified, circuit complexity is

minimized, and added heat and cost are avoided.

By selecting a few external components, the charge

capacitor and two resistors, the designer has control

over the operating parameters of the CPC1580 circuit,

and can customize the circuit to accommodate the

requirements of a wide selection of MOSFETs.

Overvoltage protection for the CPC1580 can be

provided in a number of ways; one is discussed in the

remainder of this data sheet along with a description

of external component selection.

3. External Part Selection

3.1 Storage Capacitor Selection

on quickly by holding a reservoir of charge to be

transferred to the gate of the MOSFET. The turn-off

cycle does not depend on the storage capacitor.

The equation used to calculate the value of the charge

storage capacitor is:

(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 only

partially.

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:

μF x 20% = 0.2μF

μF + 0.2μF = 1.2μF

Capacitance drift can also be due to temperature and

the dielectrics used. Therefore, the required capacitor

value is 1

.2μF 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

μF)

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.

(FARADS)