DocID029495 Rev 1

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SRK2001A

Operation description

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Operation description

The device block diagram is shown in Figure 1 on page 3. The SRK2001A can be supplied

through the VCC pin by the same converter output voltage, within a wide voltage range

(undervoltage lockout) circuit with hysteresis keeps the device switched off at supply voltage

pins DVS1,2 are switching is not allowed: this in order to avoid that a dV/dt on the DVS pin

may cause a high flowing current with possible damage of the IC.

The core of the device is the control logic block, implemented by asynchronous logic: this

digital circuit generates the logic signals to the output drivers, so that the two external power

MOSFETs are switched on and off, depending on the evolution of their drain-source

voltages, sensed on the DVS-SVS pin pairs through the comparators block.

The logic that controls the driving of the two SR MOSFETs is based on two gate-driver state

machines working in parallel in an interlocked way to avoid switching on both gate drivers at

the same time. A third state machine manages the transitions from the normal operation to

the low consumption mode and vice versa.

7.1

Drain voltage sensing

The SRK2001A basic operation is such that each synchronous rectifier MOSFET is

switched on whenever the corresponding transformer half-winding starts conducting (i.e.

when the MOSFET body diode, or an external diode in parallel, starts conducting) and it is

then switched off when the flowing current approaches zero. To understand the polarity and

the level of this current, the IC is provided with two pairs of pins (DVS1-SVS1 and DVS2-

SVS2) that sense the drain-source voltage of either MOSFET (Kelvin sensing). In order to

limit dynamic current injection in any condition, at least 100

 resistors in series to DVS1,2

pins must be used.

Referring to the typical waveforms in Figure 5, there are three significant voltage thresholds:

edges provides a pre-trigger of the gate driver and sets the internal clock; the third one

MOSFET starts conducting.