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 ), and R . The maximum
ST
OVP
switching frequency relationship in Hz is:
1
-1
FMAX
<
(tON + tOFF + (tRISE,VD | tCHG) + tFALL,VD
)
•
M
Where:
• M=3 (multiplication factor for temperature and pro-
cess variations
• t and t
are CPC1580 data sheet parameters
ON
OFF
• t
is the rise time of the drain voltage and t
CHG
RISE, VD
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
or t
RISE,VD
CHG
for the calculation
• t
is the fall time across the transistor
FALL,VD
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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