NCP4308
www.onsemi.com
14
APPLICATION INFORMATION
General description
The NCP4308 is designed to operate either as a standalone
IC or as a companion IC to a primary side controller to help
achieve efficient synchronous rectification in switch mode
power supplies. This controller features a high current gate
driver along with high−speed logic circuitry to provide
appropriately timed drive signals to a synchronous
rectification MOSFET. With its novel architecture, the
NCP4308 has enough versatility to keep the synchronous
rectification system efficient under any operating mode.
The NCP4308 works from an available voltage with range
from 4 V (A, D & Q options) or 8 V (B & C options) to 35 V
(typical). The wide V
CC
range allows direct connection to
the SMPS output voltage of most adapters such as
notebooks, cell phone chargers and LCD TV adapters.
Precise turn-off threshold of the current sense comparator
together with an accurate offset current source allows the
user to adjust for any required turn-off current threshold of
the SR MOSFET switch using a single resistor. Compared
to other SR controllers that provide turn-off thresholds in the
range of −10 mV to −5 mV, the NCP4308 offers a turn-off
threshold of 0 mV. When using a low R
DS(on)
SR (1 mW)
MOSFET our competition, with a −10 mV turn off, will turn
off with 10 A still flowing through the SR FET, while our
0 mV turn off turns off the FET at 0 A; significantly
reducing the turn-off current threshold and improving
efficiency. Many of the competitor parts maintain a drain
source voltage across the MOSFET causing the SR
MOSFET to operate in the linear region to reduce turn−off
time. Thanks to the 8 A sink current of the NCP4308
significantly reduces turn off time allowing for a minimal
drain source voltage to be utilized and efficiency
maximized.
To overcome false triggering issues after turn-on and
turn−off events, the NCP4308 provides adjustable minimum
on-time and off-time blanking periods. Blanking times can
be adjusted independently of IC VCC using external
resistors connected to GND. If needed, blanking periods can
be modulated using additional components.
An extremely fast turn−off comparator, implemented on
the current sense pin, allows for NCP4308 implementation
in CCM applications without any additional components or
external triggering.
An output driver features capability to keep SR transistor
closed even when there is no supply voltage for NCP4308.
SR transistor drain voltage goes up and down during SMPS
operation and this is transferred through drain gate
capacitance to gate and may turn on transistor. NCP4308
uses this pulsing voltage at SR transistor gate (DRV pin) and
uses it internally to provide enough supply to activate
internal driver sink transistor. DRV voltage is pulled low
(not to zero) thanks to this feature and eliminate the risk of
turned on SR transistor before enough V
CC
is applied to
NCP4308.
Some IC versions include a MAX_TON circuit that helps
a quasi resonant (QR) controller to work in CCM mode
when a heavy load is present like in the example of a
printer’s motor starting up.
Current Sense Input
Figure 36 shows the internal connection of the CS
circuitry on the current sense input. When the voltage on the
secondary winding of the SMPS reverses, the body diode of
M1 starts to conduct current and the voltage of M1’s drain
drops approximately to −1 V. The CS pin sources current of
100 mA that creates a voltage drop on the R
SHIFT_CS
resistor
(resistor is optional, we recommend shorting this resistor).
Once the voltage on the CS pin is lower than V
TH_CS_ON
threshold, M1 is turned−on. Because of parasitic
impedances, significant ringing can occur in the application.
To overcome false sudden turn−off due to mentioned
ringing, the minimum conduction time of the SR MOSFET
is activated. Minimum conduction time can be adjusted
using the R
MIN_TON
resistor.
