SRK2000 STMicroelectronics, SRK2000 Datasheet - Page 11

• Manufacturer Part Number: SRK2000
• Description: SYNCHRONOUS RECTIFIER SMART DRIVER FOR LLC RESONANT CONVERTER
• Manufacturer: STMicroelectronics

SRK2000
5.1.3
5.2
Remote on/off control
Whichever configuration is used, since a voltage on the EN pin 45 mV below V
disables the gate drivers, any small-signal transistor can be used to pull down the EN pin
and force the gate drivers into an off state.
Finally, it should be noted that during power-up, power-down, and under overload or short-
circuit conditions, the gate drivers are shut down if the Vcc voltage is insufficient: < V
case of pull-up resistor configuration, < 0.975 ÞV
configuration (the coefficient 0.975 depends on the hysteresis on the Enable pin threshold).
Drain voltage sensing
In the following explanations it is assumed that the reader is familiar with the LLC resonant
half bridge topology and its waveforms, especially those on the secondary side with a
center-tap transformer winding for full-wave rectification.
To understand the polarity and the level of the current flowing in the SR MOSFETs (or their
body diodes, or diodes in parallel to the MOSFETs) the IC is provided with two pins, DVS1-
2, able to sense the voltage level of the MOSFET drains.
Figure 7.
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 both gate drivers being switched
on at the same time.
There are four significant drain voltage thresholds: the first one, V
sensitive to positive-going edges, arms the opposite gate driver (interlock function); the
second, V
gate driver; the third is the (negative) threshold V
body diode of the SR MOSFET starts conducting; the fourth is the internal (negative)
threshold V
-25 mV by properly biasing the EN pin).
The value of the ON threshold V
DVS1-2 pin needed essentially to limit the current that might be injected into the pins when
one SR MOSFET is off and the other SR MOSFET is conducting. In fact, on the one hand,
when one MOSFET is off (and the other one is conducting), its drain-to-source voltage is
DVS1,2_PT
DVS1,2_Off
Typical waveform seen on the drain voltage sensing pins
(=0.7 V), sensitive to negative-going edges, provides a pre-trigger of the
where the SR MOSFET is switched off (selectable between -12 mV or
Doc ID 17811 Rev 2
TH-ON
is affected by the external resistor in series to each
CC_G
TH-ON
in case of resistor divider
that triggers the gate driver as the
DVS1,2_A
Application information
(= 1.4 V),
EN_On
CCOff
11/17
in