ISL6742 Intersil Corporation, ISL6742 Datasheet - Page 12

• Manufacturer Part Number: ISL6742
• Description: Advanced Double-Ended PWM Controller
• Manufacturer: Intersil Corporation

Voltage Feed Forward Operation
Voltage feed forward is a technique used to regulate the
output voltage for changes in input voltage without the
intervention of the control loop. Voltage feed forward is often
implemented in voltage-mode control loops, but is redundant
and unnecessary in peak current-mode control loops.
Voltage feed forward operates by modulating the sawtooth
ramp in direct proportion to the input voltage. Figure 8
demonstrates the concept.
Input voltage feed forward may be implemented using the
RAMP input. An RC network connected between the input
voltage and ground, as shown in Figure 9, generates a
voltage ramp proportional to the amplitude of the source
voltage. At the termination of the active output pulse RAMP
is discharged to ground so that a repetitive sawtooth
waveform is created. The RAMP waveform is compared to
the VERR voltage to determine duty cycle. The selection of
the RC components depends upon the desired input voltage
operating range and the frequency of the oscillator. In typical
applications the RC components are selected so that the
ramp amplitude reaches 1.0V at minimum input voltage
within the duration of one half-cycle.
ERROR VOLTAGE
FIGURE 8. VOLTAGE FEED FORWARD BEHAVIOR
FIGURE 9. VOLTAGE FEED FORWARD CONTROL
R3
C7
OUTA, B
VIN
RAMP
VIN
CT
12
1
2
3
4
5
6
7
8
RAMP
ISL6742
GND
16
15
14
13
12
11
10
9
ISL6742
Referring to Figure 9, the charging time of the ramp
capacitor is:
For optimum performance, the maximum value of the
capacitor should be limited to 10nF. The DC current through
the resistor should be limited to 3mA. For example, if the
oscillator frequency is 400kHz, the minimum input voltage is
300V, and a 4.7nF ramp capacitor is selected, the value of
the resistor can be determined by rearranging Equation 8.
where t is equal to the oscillator period minus the deadtime.
If the deadtime is short relative to the oscillator period, it can
be ignored for this calculation.
When implemented, the voltage feed forward feature also
provides a volt-second clamp on the transformer. The
maximum duty cycle is determined by the lesser of the
oscillator period or the RAMP charge time. As the input
voltage increases, the RAMP charge time decreases,
limiting the duty cycle proportionately.
If feed forward operation is not desired, the RC network may be
connected to VREF or a buffered CT signal rather than the
input voltage. Regardless, a sawtooth waveform must be
generated on RAMP as it is required for proper PWM operation.
Implementing Synchronization
Synchronization to an external clock signal may be
accomplished in the same manner as many PWM controllers
that do not have a separate synchronization input. By
injecting a short pulse across a small resistor in series with
the timing capacitor, the oscillator sawtooth waveform may
be terminated prematurely.
The injected pulse width should be narrower than the
sawtooth discharge duration.
t
R3
FIGURE 10. SYNCHRONIZATION TO AN EXTERNAL CLOCK
=
–
=
=
R3 C7
-------------------------------------------------------------------------
C7
159
⋅
⋅
ln



⋅
Rs
CT
1
ln
k Ω
–



1
V
--------------------------------------- -
t –
RAMP PEAK
–
V
V
--------------------------------------- -
IN MIN )
RAMP PEAK
V
(
(
IN MIN
(
(
)
)
)



=
1
2
3
4
5
6
7
8
)



------------------------------------------------------------
4.7 10
CT
www.DataSheet4U.com
ISL6742
⋅
S
–
2.5
–
9
⋅
⋅
ln
GND
10


1
–
6
–
16
15
14
13
12
11
10
9
--------- -
300
July 25, 2005
1
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FN9183.1
(EQ. 8)
(EQ. 9)