I
NTEGRATED
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IRCUITS
D
IVISION
CPC1580
R01 www.ixysic.com 7
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
slowly.
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
= 1F, 20% tolerance; V
CAP
= 15V
• 1F x 20% = 0.2F
• 1F + 0.2F = 1.2
F
Capacitance drift can also be due to temperature and
the dielectrics used. Therefore, the required capacitor
value is 12F 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 (>4F)
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
over the temperature
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
temperature, given the V
GS
in Section 1.7, the
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
switching are t
on
, t
off
, t
RISE
, t
FALL
, and the recovery
time of the storage capacitor, t
CHG
. These parameters
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
specified V
GS
after the t
on
delay. Similarly the t
off
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
smaller required gate charge the t
on
and t
off
will be
proportionately faster and slower, but it is not a linear
relationship.
To calculate the nominal rise time of the transistor's
drain voltage, V
D
:
To calculate the nominal fall time of the transistor's
drain voltage, V
D
:
t
RISE,VD
~
~
V
LOAD
• C
RSS
I
G_SINK
(SECONDS)
t
FALL,VD
~
~
V
LOAD
• C
RSS
I
G_SOURCE
(SECONDS)
