ISL78840ASEH Datasheet, PDF (8/15 Pages) Intersil Corporation – Radiation Hardened, High Performance Industry Standard Single-Ended Current Mode PWM Controller

ISL78840ASEH, ISL78841ASEH, ISL78843ASEH, ISL78845ASEH
Typical Performance Curves
1.01
1.00
0.99
0.98
-60 -40 -20
0 20 40 60 80 100 120 140
TEMPERATURE (°C)
FIGURE 1. FREQUENCY vs TEMPERATURE
1.001
1.000
0.999
0.998
0.997
0.996
0.995
-60 -40 -20
0 20 40 60 80 100 120 140
TEMPERATURE (°C)
FIGURE 2. REFERENCE VOLTAGE vs TEMPERATURE
1.001
10³
1.000
0.998
0.997
0.996
-60 -40 -20
0 20 40 60 80 100 120 140
TEMPERATURE (°C)
FIGURE 3. EA REFERENCE vs TEMPERATURE
100
10
11
10
RT (kΩ)
100pF
220pF
330pF
470pF
1.0nF
2.2nF
3.3nF
4.7nF
6.8nF
100
FIGURE 4. RESISTANCE FOR CT CAPACITOR VALUES GIVEN
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 V_REF 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_C, the discharge time, t_D, the switching
frequency, f, and the maximum duty cycle, D_MAX, can be
approximated from Equations 1 through 4:
t_C ≈ 0.533 ⋅ RT ⋅ CT
(EQ. 1)
t_D
≈
–RT
⋅
C
T
⋅
In
⎛
⎝
0^-0---..^-00^----00^---88^{-----⋅⋅---}RR^-----TT^{-----––-----}13^---..^--78^---13^{--⎠⎞}
(EQ. 2)
f = 1 ⁄ (tC + tD)
(EQ. 3)
D = t_C ⋅ f
(EQ. 4)
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
switching frequency for the ISL78841ASEH, ISL78845ASEH. The
value for the ISL78840ASEH, ISL78843ASEH will be twice that
shown in Figure 4.
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.
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_DD is below the UVLO threshold.
V_DD - V_DD is the power connection for the device. The total supply
current will depend on the load applied to OUT. Total I_DD current
is the sum of the operating current and the average output
current. Knowing the operating frequency, f, and the MOSFET
gate charge, Qg, the average output current can be calculated
from Equation 5:
I_OUT = Qg × f
(EQ. 5)
8
FN7952.0
May 29, 2012