CPC1580 Datasheet, PDF (10/11 Pages) Clare, Inc. – Optically Isolated Gate Drive Circuit

CPC1580
6. Design Switching Frequency
The maximum switching frequency is the last design
value to be calculated, because the over-voltage
protection and the storage capacitor play a significant
role in determining the result. Inasmuch as those
factors are already determined, the following gives a
good approximation for the maximum switching
frequency. The maximum switching frequency is a
function of the gate charge of the MOSFET, the
storage capacitor (C_ST), and R_OVP. The maximum
switching frequency relationship in Hz is:
F_MAX <
1
M
• ^(tON ^{+ t}OFF ^{+ (t}RISE,VD ^{| t}CHG) ^{+ t}FALL,VD^)-1
Where:
• M=3 (multiplication factor for temperature and pro-
cess variations
• t_ON and t_OFF are CPC1580 data sheet parameters
• t_{RISE, VD} is the rise time of the drain voltage and t_CHG
is the charge time of the storage capacitor and the
over-voltage protection circuitry as derived in
Section 3.2: choose the greater of t_RISE,VD or t_CHG
for the calculation
• t_FALL,VD is the fall time across the transistor
There is no minimum switching frequency since the
CPC1580 uses photovoltaic diodes to keep the output
charged while LED current flows.
10
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