ISL8843_06 INTERSIL [Intersil Corporation], ISL8843_06 Datasheet - Page 10

• Manufacturer Part Number: ISL8843_06
• Description: Industry Standard Single-Ended Current Mode PWM Controller
• Manufacturer: INTERSIL [Intersil Corporation]

Example:
V
V
L
Ns/Np = 10
Lp = 8.0µH
I
Switching Frequency, Fsw = 200kHz
Duty Cycle, D = 28.6%
R6 = 499Ω
Solve for the current sense resistor, R
R
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 = 2.67kΩ
Determine the new value of R
R’
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
bypassed directly to GND with good high frequency
capacitors.
References
O
[1] Ridley, R., “A New Continuous-Time Model for Current
s
IN
O
CS
CS
= 800µH
= 200mA
= 48V
= 12V
Mode Control”, IEEE Transactions on Power
Electronics, Vol. 6, No. 2, April 1991.
= 295mΩ
= 350mΩ
10
CS
, R’
CS
CS
, using Equation 18.
, using Equation 15.
DD
should be
ISL8843
January 3, 2006
FN9238.1