ISL8845A Intersil Corporation, ISL8845A Datasheet - Page 10

• Manufacturer Part Number: ISL8845A
• Description: (ISL8840A - ISL8845A) High Performance 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:
D
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 0V to 1.0V and has
an internal offset of 100mV.
GND - GND is the power and small signal reference ground
for all functions.
t
t
f
C
D
=
=
≈
≈
1
0.533 RT CT
–
t
RT
C
⁄
(
1.001
1.000
0.998
0.997
0.996
f ⋅
t C
FIGURE 3. EA REFERENCE vs TEMPERATURE
⋅
CT In
⋅
+
-60 -40 -20
t D
⋅
)
⋅
⎛
⎝
D
0.008 RT 3.83
-------------------------------------------- -
0.008 RT 1.71
, the switching frequency, f, and the
ISL8840A, ISL8841A, ISL8842A, ISL8843A, ISL8844A, ISL8845A
⋅
⋅
0
TEMPERATURE (°C)
–
–
20
10
40
⎞
⎠
60
80 100 120 140
(Continued)
C
, the
(EQ. 1)
(EQ. 2)
(EQ. 3)
(EQ. 4)
OUT - This is the drive output to the power switching device.
It is a high current output capable of driving the gate of a
power MOSFET with peak currents of 1.0A. This GATE
output is actively held low when V
threshold.
V
supply current will depend on the load applied to OUT. Total
I
average output current. Knowing the operating frequency, f,
and the MOSFET gate charge, Qg, the average output
current can be calculated from:
To optimize noise immunity, bypass V
ceramic capacitor as close to the V
possible.
VREF - The 5.00V reference voltage output. +1.0/-1.5%
tolerance over line, load and operating temperature. Bypass
to GND with a 0.1μF to 3.3μF capacitor to filter this output as
needed.
Functional Description
Features
The ISL884xA current mode PWM makes an ideal choice for
low-cost flyback and forward topology applications. With its
greatly improved performance over industry standard parts,
it is the obvious choice for new designs or existing designs
which require updating.
Oscillator
The ISL884xA has a sawtooth oscillator with a
programmable frequency range to 2MHz, which can be
programmed with a resistor from VREF and a capacitor to
GND on the RTCT pin. (Please refer to Figure 4 for the
resistor and capacitance required for a given frequency.)
I
FIGURE 4. RESISTANCE FOR CT CAPACITOR VALUES GIVEN
DD
OUT
DD
100
current is the sum of the operating current and the
10
- V
10
=
1
3
1
DD
Qg
×
is the power connection for the device. The total
f
RT (kΩ)
10
DD
DD
is below the UVLO
DD
and GND pins as
to GND with a
April 18, 2007
100
FN6320.3
100pF
220pF
330pF
470pF
1.0nF
2.2nF
3.3nF
4.7nF
6.8nF
(EQ. 5)