sg6741tp Fairchild Semiconductor, sg6741tp Datasheet - Page 10

• Manufacturer Part Number: sg6741tp
• Description: Sg6741-highly-integrated Green-mode Pwm Controller
• Manufacturer: Fairchild Semiconductor

Highly-Integrated Green-Mode PWM Controller
Built-in Slope Compensation
The sensed voltage across the current-sense resistor is
used for peak-current-mode control and pulse-by-pulse
current limiting. Built-in slope compensation improves
stability and prevents sub-harmonic oscillation. SG6741
inserts a synchronized positive-going ramp at every
switching cycle.
Constant Output Power Limit
When the SENSE voltage, across the sense resistor R
reaches the threshold voltage around 0.9V, the output
GATE drive is turned off after a small delay, t
delay introduces an additional current, proportional to t
V
input voltage V
additional current and the output power limit is also higher
than that under low input line voltage. To compensate this
variation for wide AC input range, a sawtooth
power-limiter is designed to solve the unequal
power-limit problem. The power limiter is designed as a
positive ramp signal and is fed to the inverting input of the
OCP comparator. This results in a lower current limit at
high-line inputs than at low-line inputs.
V
V
damage due to abnormal conditions. Once the V
is over the V
and lasts for t
the V
again. Over-voltage conditions are usually caused by open
feedback loops.
© System General Corp.
Version 1.3.1 (IAO33.0047.B3)
IN
DD
DD
/ L
over-voltage protection has been built in to prevent
Over-Voltage Protection
DD
P
. The delay is nearly constant, regardless of the
voltage drops below the UVLO, then starts up
DD
D-VDDOVP
over-voltage protection voltage (V
IN.
Higher input voltage results in a larger
, the PWM pulses is disabled until
DD
PD
DD-OVP
voltage
. This
PD
- 10 -
S
),
•
,
Limited Power Control
The FB voltage increases every time the output of the
power supply is shorted or overloaded. If the FB voltage
remains higher than a built-in threshold for longer than
t
turned off, the supply voltage V
When V
the controller totally shuts down. V
turn-on threshold voltage of 16V through the start-up
resistor until PWM output is restarted. This protection is
activated as long as the overloading condition persists.
This prevents the power supply from overheating.
Noise Immunity
Noise on the current sense or control signal may cause
significant pulse width jitter, particularly in the
continuous-conduction mode. Slope compensation helps
alleviate this problem. Good placement and layout
practices should be followed. Avoiding long PCB traces
and component leads, locating compensation and filter
components near to the SG6741, and increasing the power
MO gate resistance improves performance.
D-OLP
, PWM output is turned off. As PWM output is
DD
goes below the turn-off threshold (eg, 10.5V)
www.sg.com.tw • www.fairchildsemi.com
Product specification
DD
begins decreasing.
DD
is charged up to the
October 30, 2007
SG6741