CPC1580 Datasheet, PDF (7/11 Pages) Clare, Inc. – Optically Isolated Gate Drive Circuit

CPC1580
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
temperature, given the V_GS in Section 1.6, the
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
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:
t_{RISE,VD ~} ^{VLOAD • CRSS}
I_{G_SINK}
(SECONDS)
by the CPC1580 unloaded discharge characteristic
and should be reviewed in light of the final application
component selections if critical.
The value for the charge time, T_CHG, is due to external
component selection. The storage capacitor charge
recovery time (seconds) is computed as:
( ) tCHG ~ - (400 + ROVP) • (CST + COVP) • ln (VLOAD - VFINAL) • CST
Q_GATE
Which reduces to:
t_CHG ~ - (400 + R_OVP) • (C_ST + C_OVP) • 3
R_OVP and C_OVP are optional over-voltage protection
elements that are present in the application circuit
diagram (see Figure 2).
The term inside the logarithm reflects the discharge
and recharge voltage on C_ST. For practical circuit
component selection, this can be simplified as
described above.
Use this information to calculate the maximum
switching frequency in Section 6 below.
Note: The CPC1580 is ideal to use where
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.
To calculate the nominal fall time of the transistor's
drain voltage, V_D:
t_FALL,VD ~ ^{VLOAD • CRSS}
I_{G_SOURCE}
(SECONDS)
Where C_RSS is the MOSFET gate-drain capacitance
(averaged over the switching voltage range) found in
the MOSFET data sheet, I_{G_SINK} is the gate sinking
current of the CPC1580, and I_{G_SOURCE} is the gate
driving ability. The maximum value of t_RISE is limited
R00G
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