Datasheet SRK1001 (STMicroelectronics) - 10

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SRK1001 Drain voltage sensing Figure 5. Comparison between adaptive turn-off and comparator based turn-off turn-off by comparator adaptive turn-off and fixed VTH threshold (VZCD_OFF threshold) ISR ISR TX = LS / RDS_ON IOFF anticipation on DVS sensing signal due to I stray inductance L OFF_AD S DVS TX DVS V T ZCD_OFF X VTH target TD_On T T D_On diode_off (300ns) GD GD 5.1 Drain voltage sensing
The drain voltage of the SR MOSFET is sensed through the DVS pin: this is a high voltage pin and needs to be properly routed to the MOSFET drain, through a resistor of at least 300 Ω (in order to limit dynamic current injection in any condition). The DVS signal is used to detect when current flows through the MOSFET body diode and for the internal timings.
5.2 Turn-on
After the flyback converter primary switch has been turned off, the voltage across the transformer reverses and the SR MOSFET drain voltage quickly decreases and goes negative (-VF), allowing the rectifier current to flow. Consequently, triggered on the falling edge of the DVS signal (when it decreases below the arming voltage VTH_A), the controller turns on the SR MOSFET, with a very short delay TD_On. After turn-on, the sensed DVS signal passes from the (negative) body diode forward voltage to the drop across the MOSFET channel resistance (RDS_ON). This drop is generally affected by some amount of noise, associated with the flyback transformer leakage inductance, and this could trigger a premature turn-off of the SR MOSFET.
5.3 Minimum TON programming
In order to avoid premature turn-off of the SR MOSFET due to ringing and oscillations, the IC allows user to program a blanking time after turn-on. The circuit bases on an internal timing capacitance and an external resistor RTON connected from TON pin to ground. The blanking time settlement is done according to the following expression: TON_MIN = 12*10−12*RTON (with RTON expressed in Ohms and TON_MIN in seconds, starting from when the DVS signal falls below VTH_A). This blanking time sets a minimum turn-on time of the SR MOSFET as well: hence, when load reduction causes the SR MOSFET conduction time to become shorter than the programmed blanking time, the IC must stop driving the SR MOSFET to avoid current inversion (see Section 5.7 Low consumption mode operation: sleep-mode, burst mode, disable state).
5.4 Adaptive turn-off and TIMER
The SR MOSFET can be turned off through two coexisting mechanisms (whichever triggers first), the first based on an adaptive algorithm, the second on the internal timer.
DS13155
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Rev 1 page 10/27
Document Outline 1 Block diagram and pin connection 2 Maximum ratings 3 Typical application schematic 4 Electrical characteristics 5 Operation description 5.1 Drain voltage sensing 5.2 Turn-on 5.3 Minimum TON programming 5.4 Adaptive turn-off and TIMER 5.4.1 Fixed frequency mixed DCM/CCM operation 5.4.2 QR operation (with valley skipping) 5.4.3 Fixed frequency DCM operation 5.4.4 Variable frequency CCM operation 5.5 Minimum TOFF programming 5.6 Start-up phase 5.7 Low consumption mode operation: sleep-mode, burst mode, disable state 5.8 DIS/SYNC pin functions 5.9 VAUX pin operation in CC regulation 5.9.1 Example for parameter calculations 5.10 Operation in CC regulation and short circuit 5.11 Adaptive gate drive 6 Layout guidelines 7 Package mechanical data Revision history