SG6858 Fairchild Semiconductor, SG6858 Datasheet - Page 8

• Manufacturer Part Number: SG6858
• Description: This highly integrated PWM controller provides several special enhancements designed to meet the low standby power needs of low-power SMPS
• Manufacturer: Fairchild Semiconductor

© 2008 Fairchild Semiconductor Corporation
SG6858 • Rev. 1.2.4
Operation Description
SG6858 devices integrate many useful designs into one
controller for low-power switch-mode power supplies.
The following descriptions highlight some of the
features of the SG6858 series.
Startup Operation
The startup current is only 10µA. Low startup current
allows a startup resistor with a high resistance and low
wattage to supply the startup power for the controller. A
1.5MΩ, 0.25W, startup resistor and a 10µF/25V V
hold-up capacitor would be sufficient for an AC-to-DC
power adapter with a wide input range (100V
240V
Operating Current
The operating current has been reduced to 2.5mA. The
low operating current results in higher efficiency and
reduces the V
Green-Mode Operation
The proprietary green-mode function provides off-time
modulation to linearly decrease the switching frequency
under light-load conditions. On-time is limited to provide
stronger protection against brownouts and other
abnormal conditions. The feedback current, which is
sampled from the voltage feedback loop, is taken as the
reference. Once the feedback current exceeds the
threshold current, the switching frequency starts to
decrease.
reduces power consumption under light-load and zero-
load conditions. Power supplies using the SG6858 meet
the strictest regulations regarding standby power
consumption.
Oscillator Operation
A resistor connected from the RI pin to ground
generates a constant current source for the SG6858.
This current is used to charge an internal capacitor. The
charge-time determines the internal clock speed and
the switching frequency. Increasing the resistance
reduces the amplitude of the input current and reduces
the switching frequency. A 95kΩ resistor R
50µA constant current I
frequency. The relationship between R
switching frequency is:
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 300ns leading-
edge blanking time is built in. Conventional RC filtering
can therefore be omitted. During this blanking period,
the current-limit comparator is disabled and it cannot
switch off the gate driver.
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Constant Output Power Limit
When the SENSE voltage across the sense resistor R
reaches the threshold voltage (around 0.96V), the
output GATE drive is turned off following a short
propagation delay t
This propagation delay introduces an additional current
proportional to t
nearly constant regardless of the input line voltage V
Higher input line voltages result in larger additional
currents. At high input line voltages, the output power
limit is higher than at low input line voltages.
To compensate for this output power limit variation
across a wide AC input range, the threshold voltage is
adjusted by adding a positive ramp (V
This ramp signal rises from 0.80V to 0.96V, then
flattens out at 0.96V. A smaller threshold voltage forces
the output GATE drive to terminate earlier. This reduces
the total PWM turn-on time and makes the output power
equal to that of low line input. This proprietary internal
compensation ensures a constant output power limit for
a wide AC input voltage range (90V
Under-Voltage Lockout (UVLO)
The turn-on and turn-off thresholds are fixed internally
at 16.5V/11.5V. During startup, the hold-up capacitor
must be charged to 16.5V through the startup resistor,
so that the SG6858 is enabled. The hold-up capacitor
continues to supply V
from the auxiliary winding of the main transformer. V
must not drop below 11.5V during this startup process.
This UVLO hysteresis window ensures that the hold-up
capacitor is adequate to supply V
Figure 16. Current Sense R-C Filter
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