CPC1580
2. Introduction
The CPC1580 Isolated Gate Driver uses an efficient
optocoupler design to provide remote gate drive
current to a MOSFET, while providing 3750V
rms
of
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.
The equation used to calculate the value of the charge
storage capacitor is:
C
ST
>
Q
G
V
LOAD
- V
CAP
(FARADS)
Where V
CAP
> 15V and Q
G
is the total gate charge
(listed in the MOSFET data sheet).
The storage capacitor needs to deliver enough charge
to the gate without going below the 15V required for
switching the MOSFET. This means that V
LOAD
must
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
V
CAP
. The extra margin is important because some
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:
•
C
ST
= 1μF, 20% tolerance; V
CAP
= 15V
•
1μF x 20% = 0.2μF
•
1μF + 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 (>4μF)
with slower turn-on and turn-off times.
Note:
Care must be taken to minimize any
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
Ω
over the temperature
rating of the part.
3. External Part Selection
3.1 Storage Capacitor Selection
The storage capacitor (C
ST
) enables the part to turn
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.
6
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