10
FN9238.2
September 14, 2015
Example:
V
IN
= 12V
V
O
= 48V
L
s
= 800H
Ns/Np = 10
Lp = 8.0H
I
O
= 200mA
Switching Frequency, Fsw = 200kHz
Duty Cycle, D = 28.6%
R6 = 499
Solve for the current sense resistor, R
CS
, using Equation 15.
R
CS
= 295m
Determine the amount of voltage, Ve, that must be added to
the current feedback signal using Equation 12.
Ve = 92.4mV
Using Equation 17, solve for the summing resistor, R9, from
CT to CS.
R9 = 2.67k
Determine the new value of R
CS
, R’
CS
, using Equation 18.
R’
CS
= 350m
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
currents associated with the output stage. V
DD
should be
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.
ISL8843
