FN6320 Rev 3.00

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April 18, 2007

ISL8840A, ISL8841A, ISL8842A, ISL8843A, ISL8844A, ISL8845A

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Example:

Ns/Np = 10

Lp = 8.0

H

Duty Cycle, D = 28.6%

Determine the amount of voltage, Ve, that must be added to

the current feedback signal using Equation 12.

Ve = 92.4mV

to CS.

Additional slope compensation may be considered for design

margin. The above discussion determines the minimum

external ramp that is required. The buffer transistor used to

create the external ramp from RTCT should have a sufficiently

high gain (>200) so as to minimize the required base current.

Whatever base current is required reduces the charging

current into RTCT and will reduce the oscillator frequency.

Fault Conditions

A Fault condition occurs if VREF falls below 4.65V. When a

Fault is detected OUT is disabled. When VREF exceeds 4.80V,

the Fault condition clears, and OUT is enabled.

Ground Plane Requirements

Careful layout is essential for satisfactory operation of the

device. A good ground plane must be employed. A unique

section of the ground plane must be designated for high di/dt

bypassed directly to GND with good high frequency capacitors.

References

[1] Ridley, R., “A New Continuous-Time Model for Current

Mode Control”, IEEE Transactions on Power Electronics,

Vol. 6, No. 2, April 1991.