ISL6744 Intersil Corporation, ISL6744 Datasheet - Page 7

• Manufacturer Part Number: ISL6744
• Description: Intermediate Bus PWM Controller
• Manufacturer: Intersil Corporation

Pin Descriptions
V
noise immunity, bypass V
capacitor as close to the V
The total supply current, I
applied to outputs OUTA and OUTB. Total I
sum of the quiescent current and the average output current.
Knowing the operating frequency, Fsw, and the output
loading capacitance charge, Q, per output, the average
output current can be calculated from:
R
control pin. A resistor is connected between this pin and
GND. The current flowing through the resistor determines
the magnitude of the discharge current. The discharge
current is nominally 55x this current. The PWM deadtime is
determined by the timing capacitor discharge duration.
C
this pin and GND.
CS - This is the input to the overcurrent protection comparator.
The overcurrent comparator threshold is set at 0.600V nominal.
The CS pin is shorted to GND at the end of each switching
cycle. Depending on the current sensing source impedance, a
series input resistor may be required due to the delay between
the internal clock and the external power switch.
Exceeding the overcurrent threshold will start a delayed
shutdown sequence. Once an overcurrent condition is
detected, the soft-start charge current source is disabled.
The soft-start capacitor begins discharging through a 15µA
current source, and if it discharges to less than 3.9V
(Sustained Overcurrent Threshold), a shutdown condition
occurs and the OUTA and OUTB outputs are forced low.
When the soft-start voltage reaches 0.27V (Reset
Threshold) a soft-start cycle begins.
If the overcurrent condition ceases, and then an additional
50µs period elapses before the shutdown threshold is
reached, no shutdown occurs. The SS charging current is
re-enabled and the soft-start voltage is allowed to recover.
GND - Reference and power ground for all functions on this
device. Due to high peak currents and high frequency
operation, a low impedance layout is necessary. Ground
planes and short traces are highly recommended.
OUTA and OUTB - Alternate half cycle output stages. Each
output is capable of 1A peak currents for driving power
MOSFETs or MOSFET drivers. Each output provides very
low impedance to overshoot and undershoot.
SS - Connect the soft-start timing capacitor between this pin
and GND to control the duration of soft-start. The value of the
capacitor determines the rate of increase of the duty cycle
during start-up, controls the overcurrent shutdown delay, and
the overcurrent and short circuit hiccup restart period.
I
OUT
DD
TD
T
- The oscillator timing capacitor is connected between
- This is the oscillator timing capacitor discharge current
- V
=
DD
2 Q Fsw
•
is the power connection for the IC. To optimize
•
DD
DD
DD
, will be dependent on the load
to GND with a ceramic
7
and GND pins as possible.
DD
current is the
(EQ. 1)
ISL6744
Functional Description
Features
The ISL6744 PWM is an excellent choice for low cost bridge
topologies for applications requiring accurate frequency and
deadtime control. Among its many features are 1A FET
drivers, adjustable soft-start, overcurrent protection and
internal thermal protection, allowing a highly flexible design
with minimal external components.
Oscillator
The ISL6744 has an oscillator with a frequency range to
2MHz, programmable using a resistor R
The switching period may be considered to be the sum of
the timing capacitor charge and discharge durations. The
charge duration is determined by C
source (assumed to be 160
duration is determined by R
where T
times, respectively, T
period, and F
switching cycle requires two oscillator cycles. The actual
times will be slightly longer than calculated due to internal
propagation delays of approximately 5ns/transition. This
delay adds directly to the switching duration, and also
causes overshoot of the timing capacitor peak and valley
voltage thresholds, effectively increasing the peak-to-peak
voltage on the timing capacitor. Additionally, if very low
charge and discharge currents are used, there will be an
increased error due to the input impedance at the C
The above formulae help with the estimation of the
frequency. Practically, effects like stray capacitances that
affect the overall C
and charge current over temperature, etc. exist, and are best
evaluated in-circuit. Equation 2 follows from the basic
capacitor current equation,
variation in dV with R
(Figure 4), results from Equation 2 would differ from the
calculated frequency. The typical performance curves may
be used as a tool along with the above equations as a more
accurate tool to estimate the operating frequency more
accurately.
The maximum duty cycle, D, and deadtime, DT, can be
calculated from:
T
DT
D
T
T
C
D
OSC
=
≈
≈
=
1.25
T
---------------------------------------------------------------------------- - R
CTDisch
(
C
1 D
=
C
⁄
–
×10
T
T
and T
OSC
C
) T
4
+
⋅
OSC
•
T
arg
C
D
OSC
D
T
eCurrentGain
=
1
are the approximate charge and discharge
is the oscillator frequency. One output
T
--------------- -
F
capacitance, variation in R
OSC
OSC
TD
1
s
(Figure 5), and in charge current
s
is the oscillator free running
µA
TD
i
=
in the formula). The discharge
s
and C
C
•
×
www.DataSheet4U.com
dV
t d
T
TD
and the internal current
T
. In this case, with
•
.
C
TD
T
and capacitor C
September 22, 2005
s
TD
voltage
T
(EQ. 6)
pin.
(EQ. 5)
FN9147.8
(EQ. 2)
(EQ. 3)
(EQ. 4)
T
.