SG684965DZ Fairchild Semiconductor, SG684965DZ Datasheet - Page 10

• Manufacturer Part Number: SG684965DZ
• Description: IC CTRLR PWM OTP UVLO CV/CC 8DIP
• Manufacturer: Fairchild Semiconductor

Specifications of SG684965DZ

• Output Isolation: Isolated
• Frequency Range: 60 ~ 70kHz
• Voltage - Input: 8 ~ 25 V
• Voltage - Output: 16.7V
• Operating Temperature: -40°C ~ 125°C
• Package / Case: 8-DIP (0.300", 7.62mm)
• Number Of Outputs: 1
• Duty Cycle (max): 80 %
• Mounting Style: Through Hole
• Switching Frequency: 70 KHz
• Maximum Operating Temperature: + 105 C
• Minimum Operating Temperature: - 40 C
• Synchronous Pin: No
• Topology: Flyback
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

© 2008 Fairchild Semiconductor Corporation
SG6849 Rev. 1.6.1
Operation Description
SG6849 devices integrate many useful functions for
low-power switch-mode power supplies. The following
descriptions highlight the key features of the SG6849.
Startup Current
The required startup current is only 5μA. This allows a
high-resistance, low-wattage startup resistor to be used
to supply the controller’s startup power. A 1.5MΩ/0.25W
startup resistor can be used over a wide input range
(90V
Operating Current
The operating current is normally 3.6mA. The low
operating current results in higher efficiency and
reduces the required V
10μF/25V V
wide input range (90V
Green-Mode Operation
The proprietary green-mode function provides off-time
modulation to linearly decrease the switching frequency
under light-load and zero-load conditions. The on-time
is limited to provide better protection against brownouts
and other abnormal conditions.
This green-mode function dramatically reduces power
consumption under light-load and zero-load conditions.
Power supplies using SG6849 can meet international
restrictions regarding standby power-consumption.
Constant Voltage (CV) and Constant
Current (CC) without Feedback
The SG6849 tightly regulates the output voltage and
provide
secondary-side feedback signals. For improved CV and
CC accuracy, the transformer leakage inductance
should be reduced as much as possible.
Over-Temperature Protection (OTP)
The SG6849 has a built-in temperature sensing circuit
to
temperature exceeds 150°C. While PWM output is shut
down, the V
voltage. Some of the SG6849’s internal circuits are shut
down and V
V
temperature sensing circuit, starts operating normally. If
the junction temperature is still higher than 150°C, the
PWM controller is shut down immediately. This situation
continues until the temperature drops below 130°C. The
PWM output then turns back on. The temperature
hysteresis window for the OTP circuit is 20°C.
DD
shut
reaches 17V, all the internal circuits, including the
AC
-264V
over-current
down
DD
DD
AC
DD
) with very little power loss.
gradually starts increasing again. When
hold-up capacitor can be used over a
voltage gradually drops to the UVLO
PWM
AC
-264V
protection
DD
output
hold-up capacitance. A
AC
) with little power loss.
once
without
the
requiring
junction
10
V
V
from over-voltage conditions. When V
PWM output is shut down. Over-voltage conditions may
be caused by an open photo-coupler loop or a short
circuit in the output.
Oscillator Operation
The oscillation frequency is fixed at 65kHz.
Leading-Edge Blanking (LEB)
Each time the power MOSFET is switched on, a turn-on
spike occurs at the sense resistor. To avoid premature
termination of the switching pulse, a 310ns leading-
edge blanking time is built in. Conventional RC filtering
is not necessary. During this blanking period, the
current-limit comparator is disabled and it cannot switch
off the gate drive.
Constant Output Power Limit
When the SENSE voltage across the sense resistor R
reaches the threshold voltage (around 1.0V), the output
GATE drive is turned off following a small propagation
delay t
additional
propagation delay is nearly constant regardless of the
input line voltage V
larger
voltages, the output power limit is higher than under low
input-line voltages.
Over a wide range of AC input voltages, the variation
can be significant. To compensate for this, the threshold
voltage is adjusted by adding a positive ramp (V
ramp
SG6849 and it flattens out at 1.05V. A smaller threshold
voltage forces the output GATE drive to terminate
earlier, reducing total PWM turn-on time and making the
output power equal to that of the low line input. This
proprietary internal compensation feature ensures a
constant output power limit over a wide range of AC
input voltages (90V
Under-Voltage Lockout (UVLO)
The turn-on / turn-off thresholds of the SG6849 are
fixed internally at 17V/8V. To enable the SG6849 during
startup, the hold-up capacitor must first be charged to
17V through the startup resistor.
The hold-up capacitor continues to supply V
energy can be delivered from the auxiliary winding of
the main transformer. V
during this startup process. This UVLO hysteresis
window ensures that the hold-up capacitor can
adequately supply V
DD
DD
). This ramp signal varies from 0.77V to 1.05V for
over-voltage clamping is built in to prevent damage
Over-Voltage Clamping
PD
additional
. This propagation delay introduces an
current
IN
AC
currents.
DD
. Higher input line voltages result in
-264V
proportional to t
during startup.
DD
AC
must not drop below 8V
).
Under
DD
high
exceeds 22.7V,
PD
•V
www.fairchildsemi.com
IN
/L
DD
input-line
P
. The
before
LIMIT
S
-