ISL8843 Intersil Corporation, ISL8843 Datasheet - Page 7

• Manufacturer Part Number: ISL8843
• Description: Industry Standard Single-Ended Current Mode PWM Controller
• Manufacturer: Intersil Corporation

Typical Performance Curves
Pin Descriptions
RTCT - This is the oscillator timing control pin. The
operational frequency and maximum duty cycle are set by
connecting a resistor, RT, between VREF and this pin and a
timing capacitor, CT, from this pin to GND. The oscillator
produces a sawtooth waveform with a programmable
frequency range up to 2.0MHz. The charge time, T
discharge time, T
maximum duty cycle, Dmax, can be approximated from the
following equations:
T
T
f
D
C
D
=
=
≈
≈
1
T C
0.533 RT CT
–
⁄
RT
(
1.001
1.000
0.998
0.997
0.996
T
1.01
0.99
0.98
FIGURE 3. EA REFERENCE vs TEMPERATURE
•
C
FIGURE 1. FREQUENCY vs TEMPERATURE
•
f
1
•
+
-60 -40 -20
CT
-60 -40 -20
T
D
•
)
•
ln
D


0.008 RT 3.83
----------------------------------------------
0.008 RT 1.71
, the switching frequency, f, and the
•
•
0
0
TEMPERATURE (°C)
TEMPERATURE (°C)
20
20
–
–
7
40
40


60
60
80 100 120 140
80 100 120 140
C
, the
(EQ. 1)
(EQ. 2)
(EQ. 3)
(EQ. 4)
ISL8843
The formulae have increased error at higher frequencies due
to propagation delays. Figure 4 may be used as a guideline
in selecting the capacitor and resistor values required for a
given frequency.
COMP - COMP is the output of the error amplifier and the
input of the PWM comparator. The control loop frequency
compensation network is connected between the COMP and
FB pins.
FB - The output voltage feedback is connected to the
inverting input of the error amplifier through this pin. The
non-inverting input of the error amplifier is internally tied to a
reference voltage.
CS - This is the current sense input to the PWM comparator.
The range of the input signal is nominally 0 to 1.0V and has
an internal offset of 100mV.
GND - GND is the power and small signal reference ground
for all functions.
1•10
100
FIGURE 2. REFERENCE VOLTAGE vs TEMPERATURE
10
1
3
1
1.001
1.000
0.999
0.998
0.997
0.996
0.995
-60 -40 -20
FIGURE 4. RTCT vs FREQUENCY
0
TEMPERATURE (°C)
RT (kΩ)
20
10
40
60
80 100
120
January 3, 2006
100
140
FN9238.1
CT =
100pF
220pF
330pF
470pF
1.0nF
2.2nF
3.3nF
4.7nF
6.8nF