FAN9611 Fairchild Semiconductor, FAN9611 Datasheet - Page 15

• Manufacturer Part Number: FAN9611
• Manufacturer: Fairchild Semiconductor

© 2008 Fairchild Semiconductor Corporation
FAN9611 / FAN9612 • Rev. 1.1.4
9. Input Voltage Sensing (V
The input AC voltage is sensed at the VIN pin. The input
voltage is used in two functions: input under-voltage
lockout
feedforward in the PWM control circuit. All the functions
require the RMS value of the input voltage waveform.
Since the RMS value of the AC input voltage is directly
proportional to its peak, it is sufficient to find the peak
instead of the more complicated and slower method of
integrating the input voltage over a half line cycle. The
internal circuit of the VIN pin works with peak detection
of the input AC waveforms. One of the important
benefits of this approach is that the peak indicates the
correct RMS value even at no load when the HF filter
capacitor at the input side of the boost converter is not
discharged around the zero crossing of the line
waveform. Another notable benefit is that during line
transients, when the peak exceeds the previously
measured value, the input-voltage feedforward circuit
can react immediately, without waiting for a valid
integral value at the end of the half line period.
Furthermore, lack of zero crossing detection could fool
The measured peak value is then used in the following
half-line cycle while a new measurement is executed to
be used in the next half line cycle. This operation is
synchronized to the zero crossing of the line waveform.
Since the input voltage measurement is held steady
during the line half periods, this technique does not feed
any AC ripple into the control loop. If line zero crossing
detection is missing, the FAN9611/12 measures the
input voltage in every 32ms; it can operate from a DC
(brownout
protection),
IN
)
and
Figure 23. Input Voltage Sensing Circuit
input
voltage
15
the integrator while the peak detector works properly
during light-load operation.
The valid range for the peak of the AC input is between
approximately 0.925V and 3.7V. This range is optimized
for universal input voltage range of operation. If the
peak of the sense voltage remains below the 0.925V
threshold, input under-voltage or brownout condition is
declared and the FAN9611/12 stops operating. When
the V
voltage sense circuit saturates and the feedforward
circuit is not able to follow the input any higher.
Consequently, the slope of the PWM ramp remains
constant corresponding to the V
amplitude for any V
The input voltage is measured by a tracking analog-to-
digital converter, which keeps the highest value (peak
voltage) of the input voltage waveform. Once a
measurement is taken, the converter tracks the input for
at least 12ms before a new value is taken. This delay
ensures at least one new peak value is captured before
the new value is used.
input as well. The following figures provide detail about
the input voltage sensing method of the controller.
As shown in the waveforms, input voltage feedforward is
instantaneous when the line voltage increases and has
a half line cycle delay when the input voltage decreases.
Any increase in input voltage would cause output over
voltage due to the slow nature of the voltage regulation
loop. This is successfully mitigated by the immediate
action of the input-voltage feedforward circuit.
IN
voltage exceeds 3.7V, the FAN9611/12 input
IN
voltage above 3.7V.
IN
= 3.7V level
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