ISL6742_14 Datasheet, PDF (12/18 Pages) Intersil Corporation – Advanced Double-Ended PWM Controller

ISL6742
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.
VIN
ERROR VOLTAGE
RAMP
CT
OUTA, OUTB
FIGURE 8. VOLTAGE FEED FORWARD BEHAVIOR
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 1V at minimum input voltage within
the duration of one half-cycle.
VIN
R3
C7
1
16
2
15
3
14
4
ISL6742 13
5
12
6 RAMP
11
7
10
8
GND 9
FIGURE 9. VOLTAGE FEED-FORWARD CONTROL
Referring to Figure 9, the charging time of the ramp
capacitor is expressed in Equation 8:
t
=
–R3
⋅
C7
⋅
⎛
ln ⎜1
⎝
–
-^V----^R---_V-^A----_IM-_N----^P-₍--_M-⁽--^P--_I-_N-^E---₎^A----^K----⁾_{⎠⎟⎞}
s
(EQ. 8)
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.
R3 = -----------------------------------–---^t----------------------------------- = -----------------–---²---^.--⁵-----^â‹---¹---⁰----^–--⁶------------------
C7
⋅
⎛
ln ⎜1
⎝
–
V_{-----R--V--A--I--MN----(-P-M---(--PI--N--E---)A--)--K----)⎠⎟⎞}
4.7 ⋅ 10^–9 ⋅ ln_⎝⎛1 – ₃----₀₁---₀--_⎠⎞
= 159 kΩ
(EQ. 9)
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.
1
16
2
15
3
14
4 CT
13
5
12
CT
ISL6742
6
11
7
10
8
GND 9
RS
FIGURE 10. SYNCHRONIZATION TO AN EXTERNAL CLOCK
12
FN9183.2
October 31, 2008