7.3.1.5 CS (Current Sense)

The current sense pin is connected through a series resistor (R_LC) to the current-sense resistor (R_CS). The controller varies the internal current sense threshold between 190 mV and 770 mV, setting a corresponding control range for the peak-primary winding current to a 4-to-1 range. The series resistor R_LC provides an input voltage feed-forward function. The voltage drop across this resistor reduces primary-side peak current as the line voltage increases, compensating for the increased di/dt and delays in the MOSFET turn-off. There is an internal leading-edge blanking time of 270 ns to eliminate sensitivity to the MOSFET turn-on leading edge current spike. If additional blanking time is needed, a small bypass capacitor, up to 30 pF, can be placed on between CS pin and GND pin. The value of R_CS is determined by the target output current in constant current (CC) regulation. The values of R_CS and R_LC can be determined by the equations below. The term η_XFMR is intended to account for the energy stored in the transformer but not delivered to the secondary. This includes transformer core and copper losses, bias power, and primary leakage inductance losses.

Example: With a transformer core and copper losses of 3%, leakage inductance caused power losses 2%, and bias power to output power ratio of 0.5%. The transformer power transfer efficiency is estimated as η_XFMR = 100% - 3% - 2% - 0.5% = 94.5%

Equation 3.

where

• V_CCR is a current regulation constant (see the Electrical Characteristics table),
• N_PS is the transformer primary-to-secondary turns ratio (a typical turns-ratio of 12 to 15 is recommended for 5-V output as an example),
• I_OCC is the target output current in constant-current limit (refer to Constant Current Limit and Delayed Shutdown for more detail),
• η_XFMR is the transformer efficiency.

Equation 4.

where

• R_S1 is the VS pin high-side resistor value,
• R_CS is the current-sense resistor value,
• t_D is the current-sense delay (typical 50 ns) plus MOSFET turn-off delay,
• t_{GATE_OFF} is the primary-side main MOSFET turn-off time,
• N_PA is the transformer primary-to-auxiliary turns-ratio,
• L_P is the transformer primary inductance,
• K_LC is a current-scaling constant (see the Electrical Characteristics table).