SF6901
Fast Turn-off Intelligent Controller
December 2015
GENERAL DESCRIPTION
The SF6901 is a Low-Drop Diode Emulator IC that,
combined with an external switch replaces Schottky
diodes in high-efficiency, Flyback converters. The chip
regulates the forward drop of an external switch to about
70mV and switches it off as soon as the voltage becomes
negative. FS6901 is offered in a space saving SOT23-6
package.
FEATURES
Works with both Standard and Logic Level FETS
Compatible with Energy Star, 1W Standby
Requirements
V
CC
Range From 4.5V to 32V
Fast Turn-off Total Delay of 20ns
Max 400kHz Switching Frequency
< 1mA Low Quiescent Current
Supports CCM, DCM and Quasi-Resonant Topologies
Supports High-side and Low-side Rectification
Power Savings of Up to 1.5W in a Typical Notebook
Adapter
APPLICATIONS
Industrial Power Systems
Distributed Power Systems
Battery Powered Systems
Flyback Converters
TYPICAL APPLICATION CIRCUIT
Figure 1
Typical Application Circuit (Vout > 5V)
RevA, 11/12/2015
1
SF6901
OPERATION DISCRIPTION
The SF6901 supports operation in CCM, DCM and Quasi-
Resonant topologies. Operating in either a DCM or Quasi-
Resonant topology, the control circuitry controls the gate in
forward mode and will turn the gate off when the MOSFET
current is fairly low. In CCM operation, the control circuitry
turns off the gate when very fast transients occur.
Blanking
The control circuitry contains a blanking function. When it
pulls the MOSFET on/off, it makes sure that the on/off
state at least lasts for some time. The turn on blanking
time is ~1.6us, which determines the minimum on-time.
During the turn on blanking period, the turn off threshold is
not totally blanked, but changes the threshold voltage to
~+50mV (instead of -20mV). This assures that the part can
always be turned off even during the turn on blanking
period. (Albeit slower, so it is not recommended to set the
synchronous period less than 1.6us at CCM condition in
flyback converter, otherwise shoot through may occur)
Under-Voltage Lockout (UVLO)
When the V
CC
is below UVLO threshold, the part is in
sleep mode and the Vg pin is pulled low by a 10kΩ resistor.
Thermal Shutdown
If the junction temperature of the chip exceeds 150°C, the
Vg will be pulled low and the part stops switching. The part
will return to normal function after the junction temperature
has dropped to 115°C.
Turn-on Phase
When the synchronous MOSFET is conducting, current
will flow through its body diode which generates a negative
Vds across it. Because this body diode voltage drop (<-
500mV) is much smaller than the turn on threshold of the
control circuitry (-70mV), which will then pull the gate
driver voltage high to turn on the synchronous MOSFET
after about 100ns turn on delay.
As soon as the turn on threshold (-70mV) is triggered, a
blanking time (Minimum on-time: ~1.6us) will be added
during which the turn off threshold will be changed from -
15mV to +50mV. This blanking time can help to avoid
error trigger on turn off threshold caused by the turn on
ringing of the synchronous MOSFET.
Conducting Phase
When the synchronous MOSFET is turned on, Vds
becomes to rise according to its on resistance, as soon as
Vds rises above the turn on threshold (-70mV), the control
circuitry stops pulling up the gate driver which leads the
gate voltage is pulled down by the internal pull-down
resistance (10kΩ) to larger the on resistance of
synchronous MOSFET to ease the rise of Vds. By doing
that, Vds is adjusted to be around - 70mV even when the
current through the MOS is fairly small, this function can
make the driver voltage fairly low when the synchronous
MOSFET is turned off to fast the turn off speed (this
function is still active during turn on blanking time which
means the gate driver could still be turned off even with
very small duty of the synchronous MOSFET).
Turn-off Phase
When Vds rises to trigger the turn off threshold (- 15mV),
the gate voltage is pulled to low after about 20ns turn off
delay (defined in Fig.2) by the control circuitry. Similar with
turn-on phase, a 200ns blanking time is added after the
synchronous MOSFET is turned off to avoid error trigger.
Fig.3 shows synchronous rectification operation at heavy
load condition. Due to the high current, the gate driver will
be saturated at first. After Vds goes to above -70mV, gate
driver voltage decreases to adjust the Vds to typical -
70mV.
Fig.4 shows synchronous rectification operation at light
load condition. Due to the low current, the gate driver
voltage never saturates but begins to decrease as soon as
the synchronous MOSFET is turned on and adjust the Vds.
Figure 3 Synchronous Rectification
Operation at heavy load
Figure 2 Turn-on and Turn-off Delay
RevA, 11/12/2015
5
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