SRK2000D datasheet(8/17 Pages) STMICROELECTRONICS

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Application information

SRK2000

8/17

Doc ID 17811 Rev 2

Application information

5.1

EN pin: pin function and usage

This pin can perform three different functions: it sets the threshold VDVS1,2_Off for the drain-

to-source voltage of either synchronous rectifier (SR) Power MOSFET to determine their

turn-off in each conduction cycle; it allows the user to program the UVLO thresholds of the

gate drivers and can be used as Enable (remote on/off control).

5.1.1

Pull-up resistor configuration

At startup, an internal 10 µA current sink (IEN) is active as long as the device supply voltage

Vcc is below the startup threshold VCCOn. The moment Vcc equals VCCOn (4.5 V typ.), the

voltage VEN on the EN pin determines the turn-off threshold VDVS1,2_Off for the drain voltage

of both synchronous rectifiers during their cycle-by-cycle operation: if VEN < VEN_Th (= 0.36

V) the threshold is set at -25 mV, otherwise at -12 mV. Once the decision is made, the

setting is frozen as long as Vcc is greater than the turn-off level VCCOff (4.25 V typ.).

A simple pull-up resistor R1 to Vcc can be used to set VDVS1,2_Off turn-off threshold. The

voltage on the EN pin as the device turns on is given by:

Then, considering worst-case scenarios, we have:

Some additional margin (equal to the resistor's tolerance) needs to be considered;

assuming 5% tolerance, the use of the standard values R1 = 680 kΩ in the first case and R1

= 270 k

Ω in the second case, is suggested.

Figure 5.

EN pin biased with a pull-up resistor (for logic-level MOSFET driving)

As Vcc exceeds VCCOn, the internal current sink IEN is switched off and the enable function

is activated. The voltage on the pin is then compared to an internal reference VEN_On set at

CCOn

−

633

Off

DVS

−

→

Ω

296

Off

DVS

−

→

Ω

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