SG6961 Fairchild Semiconductor, SG6961 Datasheet - Page 8

• Manufacturer Part Number: SG6961
• Description: Power Factor Controller
• Manufacturer: Fairchild Semiconductor

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Power Factor Controller
OPERATION DESCRIPTION
Error Amplifier
The inverting input of the error amplifier is referenced to
INV. The output of the error amplifier is referenced to
COMP. The non-inverting input is internally connected to
a fixed 2.5V ± 2% voltage. The output of the error
amplifier is used to determine the on-time of the PWM
output and regulate the output voltage. To achieve a low
input current THD, the variation of the on-time within one
input AC cycle should be very small. A multi-vector error
amplifier is built in to provide fast transient response and
precise output voltage clamping.
For SG6961, connecting a capacitance, such as 1µF,
between COMP and GND is suggested. The error
amplifier is a transconductance amplifier that converts
voltage to current with a 125µmho.
Start-Up Current
Typical start-up current is less than 20µA. This ultra-low
start-up current allows the usage of a high resistance,
low-wattage start-up resistor. For example, 1MΩ /0.25W
start-up resistor and a 10µF/25V (V
are recommended for an AC-to-DC power adaptor with a
wide input range 85 to 265V
Operating Current
Operating current is typically 4.5mA. The low operating
current enables better efficiency and reduces the
requirement of V
Maximum On-Time Operation
Given a fixed inductor value and maximum output power,
the relation between on-time and line voltage is:
If the line voltage is too low or the inductor value is too
high, T
frequency and achieve brownout protection, the
maximum value of T
connected between MOT and GND. A 24kΩ resistor R
generates corresponds to 25µs maximum on-time.
The range of the maximum on-time is designed as
10 ~ 50µs.
© System General Corp.
Version 1.3.1 (IRO33.0028.B6)
t
t
on
on
(max)
=
V
2
ON
=
rms
⋅ ⋅
L P
R k
2
is too long. To avoid extra low operating
⋅
I
η
(
o
--------------------------------------
Ω ⋅
CC
)
hold-up capacitance.
ON
25
24
is programmable by a resistor, R
(µs)---------------
AC
.
CC
hold-up) capacitor
(1)
(2)
I
- 8 -
,
I
Peak Current Limiting
The switch current is sensed by one resistor. The signal is
fed into CS pin and an input terminal of a comparator. A
high voltage in the CS pin terminates a switching cycle
immediately and cycle-by-cycle current limit is achieved.
The designed threshold of the protection point is 0.82V.
Leading-Edge Blanking
A turn on spike on CS pin occurs when the power
MOSFET is switched on. At the beginning of each
switching pulse, the current-limit comparator is disabled
for ~400ns to avoid premature termination. The gate drive
output cannot be switched off during the blanking period.
Conventional RC filtering is not necessary; the
propagation delay of current limit protection can be
minimized.
Under-Voltage Lockout (UVLO)
The turn-on and turn-off threshold voltages are fixed
internally at 12V/9.5V for SG6961. This hysteresis
behavior guarantees a one-shot start-up with proper
start-up resistor and hold-up capacitor. With an ultra–low
start-up current of 20µA, one 1M Ω resistor, R
sufficient for start-up under low input line voltage,
85V
under high line (V
Output Driver
With low on resistance and high current driving capability,
the output driver can drive an external capacitive load
larger than 3000pF. Cross conduction current is avoided
to minimize heat dissipation, such that efficiency and
reliability can be improved. This output driver is
internally equipped with clamped by a 16.5V Zener diode.
RMS
. Power dissipation on R
www.sg.com.tw • www.fairchildsemi.com
AC
=265V
RMS
) conditions.
Product Specification
IN
is less than 0.1W even
September 24, 2007
SG6961
IN
, is