NCP4303B ON Semiconductor, NCP4303B Datasheet - Page 14

• Manufacturer Part Number: NCP4303B
• Description: Secondary Side Synchronous Rectification Driver
• Manufacturer: ON Semiconductor

thresholds are fully given by the CS input specification
(please refer to parametric table). Once non−zero R
resistor is used, both thresholds move down (i.e. higher
MOSFET turn off current) as the CS pin offset current
causes a voltage drop that is equal to:
Final turn−on and turn off thresholds can be then calculated
as:
VCS_turn_on + Vth_CS_on * (Rshift_cs * Ishift_cs)
VCS_turn_off + Vth_CS_off * (Rshift_cs * Ishift_cs)
If no R
shift_cs
V_Rshift_cs + Rshift_cs * Ishift_cs
resistor is used, the turn−on and turn−off
Figure 34. ZCD Comparators Thresholds and Blanking Periods Timing
(eq. 1)
(eq. 2)
(eq. 3)
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shift_cs
NCP4303A/B
14
Note that R
compare to turn−off threshold.
package which features leads), the parasitic inductance of
the package leads causes a turn−off current threshold
increase. This is because current that flows through the SR
MOSFET has quite high di(t)/dt that induces error voltage
on the SR MOSFET leads inductance. This error voltage,
that is proportional to the secondary current derivative,
shifts the CS input voltage to zero when significant current
still flows through the channel. Zero current threshold is thus
detected when current still flows through the SR MOSFET
channel – please refer to Figure 35 for better understanding.
As a result, the SR MOSFET is turned−off prematurely and
the efficiency of the SMPS is not optimized.
If using a SR MOSFET in TO220 package (or other
shift_cs
impact on turn−on threshold is less critical
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