ISL8845A Intersil Corporation, ISL8845A Datasheet - Page 13

• Manufacturer Part Number: ISL8845A
• Description: (ISL8840A - ISL8845A) High Performance Industry Standard Single-Ended Current Mode PWM Controller
• Manufacturer: Intersil Corporation

Example:
V
V
L
Ns/Np = 10
Lp = 8.0μH
I
Switching Frequency, f
Duty Cycle, D = 28.6%
R
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, R
CT to CS.
R
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.
O
s
IN
O
6
CS
9
CS
= 800μH
= 200mA
= 499Ω
= 2.67kΩ
= 48V
= 12V
= 295mΩ
= 350mΩ
ISL8840A, ISL8841A, ISL8842A, ISL8843A, ISL8844A, ISL8845A
sw
= 200kHz
13
CS
(R’
CS
CS
) using Equation 18.
, using Equation 15.
9
, from
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
[1] Ridley, R., “A New Continuous-Time Model for Current
Mode Control”, IEEE Transactions on Power
Electronics, Vol. 6, No. 2, April 1991.
DD
should be
April 18, 2007
FN6320.3