SG6846A Fairchild Semiconductor, SG6846A Datasheet - Page 11

• Manufacturer Part Number: SG6846A
• Description: The highly integrated SG6846A series of PWM controllers provides features to enhance the performance of flyback converters
• Manufacturer: Fairchild Semiconductor

© 2008 Fairchild Semiconductor Corporation
SG6846A • Rev. 1.1.5
Operation Description
Startup Operation
The turn-on/turn-off thresholds are fixed internally at
16.5V and 10.5V. To enable the SG6846A during
startup, the hold-up capacitor must first be charged to
16.5V through the startup resistor.
The hold-up capacitor continues to supply V
energy can be delivered from the auxiliary winding of the
main transformer. The V
during this startup process. This UVLO hysteresis
window ensures that the hold-up capacitor can
adequately supply V
The typical startup current is only 8µA, which allows a
high-resistance, low-wattage startup resistor to be used.
For constant output power limit over a universal input-
voltage range, the peak-current threshold is adjusted by
the voltage of the VIN pin. Since the VIN pin is
connected to the rectified AC input line voltage through
the resistive divider, a higher line voltage generates a
higher V
the V
high line input voltage equal to that at low line input. The
value of R-C network should not be so large it affects
the power limit (shown in Figure 20). R and C should put
on less than 300 and 1000pF, respectively, to
minimize power loss. A 1.5M/0.25W startup resistor
and a 10µF/25V V
a universal input range.
The required operating current has been reduced to
3.7mA, which enables higher efficiency and reduces the
V
Green-Mode Operation
The proprietary green-mode function provides off-time
modulation to continuously decrease the switching
frequency under light-load conditions. Maximum on-time
is limited to provide protection against abnormal
conditions. To further reduce power consumption under
zero-load condition, the PWM oscillator is completely
turned off and the power supply enters burst-mode. This
green mode dramatically reduces power consumption
under light-load and zero-load conditions. Power
supplies using SG6846A can meet restrictive international
regulations regarding standby power consumption.
Oscillator Operation
A resistor connected from the RI pin to GND generates
a reference current source, inside the SG6846A, used to
determine
resistance decreases the amplitude of the current
source and reduces the PWM frequency. Using a 26k
resistor results in a corresponding 65kHz switching
frequency. The relationship between R
switching frequency is:
f
DD
PWM
hold-up capacitance requirement.
IN
(kHz)
increases, making the maximum output power at
IN
voltage. The threshold voltage decreases as

the
R
1690
I
(k

DD
PWM
)
DD
hold-up capacitor are sufficient for
during startup.
DD
frequency.
must not drop below 10.5V
Increasing
I
DD
and the
before
(2)
the
11
If an open-circuit or short-circuit to ground occurs at the
RI pin, the internal protection circuit immediately shuts
down the controller.
Two-level Over-current Protection (OCP)
The cycle-by-cycle current limiting shuts down the PWM
immediately when the switching current is over the
peak-current threshold. Additionally, when the switching
current is higher than the over-current threshold, the
internal
accumulated counting time is more than ~1600ms
(R
counter counts up. When the switching current is lower
than the over-current threshold, the internal counter
counts down. When the total accumulated counting time
is more than ~1600ms (R
latched off.
This two-level OCP protection and up/down counter are
especially designed for SMPS with surge current output,
such as those for printers, scanners, and motor drivers.
Constant Output Power Limit
For constant output power limit over universal input-
voltage range, the peak-current threshold is adjusted by
the voltage of the VIN pin. Since the VIN pin is
connected to the rectified AC input line voltage through
the resistive divider, a higher line voltage generates a
higher V
V
high-line input voltage equal to that at low-line input.
Brownout Protection
Since the VIN pin is connected through a resistive
divider to the rectified AC input line voltage, it can also
be used for brownout protection. If the V
than 0.7V, the PWM output is shut off. If the V
0.9V, the PWM output is turned on again. The
hysteresis window for on/off is ~0.2V.
V
V
damage. If V
Over-Temperature Protection (OTP)
An external NTC thermistor can be connected from the
RT pin to GND. The impedance of the NTC decreases
at high temperatures. When the voltage of the RT pin
drops below 1.05V, the SG6846A is turned off. For
protection-mode options, see Ordering Information.
IN
DD
DD
I
= 26k), the controller is latched off and the internal
increases, making the maximum output power at
Over-voltage Protection (OVP)
over-voltage protection is built in to prevent
IN
counter
voltage. The threshold voltage decreases as
DD
is over 23.6V, SG6846A is latched off.
counts
I
down.
= 26k), the controller is
When
IN
voltage is less
www.fairchildsemi.com
the
IN
is over
total