SRK2000DTR データシートの表示（PDF） - STMicroelectronics

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SRK2000DTR

Synchronous rectifier smart driver for LLC resonant converters

STMicroelectronics 

SRK2000

Application information

Note:

6.1.3

Solving these equations for R1 and R2 we get:

Equation 4





R1 



VCCOn

 VEN

IEN

VCC _ G

VEN _ On



R2





R1 VCC

VEN _ On

_ G  VEN

_

On

If VCC_G is not too low (< 8 ÷ 9 V), its tolerance is not critical because it is related only to that

of VEN_On (± 5.6%) and of the external resistors R1, R2 (± 1% each is recommended).

Then, some care needs to be taken only as far as the selection of the -12/-25 mV threshold

is concerned: in fact, the large spread of IEN considerably affects the voltage on the EN pin

as the device turns on, a value that can be found by solving the first of (1) for VEN:

Equation 5

VEN



VCCOn  IEN R1

1

R1

R2

A couple of examples clarify the suggested calculation methodology.

Example 1 VCC_G = 10 V, VDVS1,2_Off = - 25 mV.

In this case, VEN must definitely be lower than the minimum value of VEN_Th (= 0.32 V).

From the second of (2), the nominal ratio of R1 to R2 is (10 – 1.8) / 1.8 = 4.555. Substituting

the appropriate extreme values in (3) it must be (4.75 - 7·10-6·R1) / (1 + 4.555) < 0.32;

solving for R1 yields R1 > 425 k; let us consider an additional 4% margin to take both the

tolerance and the granularity of the R1 and R2 values into account, so that:

R1 > 425·1.04 = 442 k. Choose R1 = 442 k (E48 standard value) and, from the second of

(2), R2 = 442/4.555 = 97 k; use 97.6 k (E48 standard value).

Example 2 VCC_G = 10 V, VDVS1,2_Off = - 12 mV.

In this case, VEN must definitely be higher than the maximum value of VEN_Th (= 0.40 V).

From the second of (2), the nominal ratio of R1 to R2 is (10 – 1.8) / 1.8 = 4.555. Substituting

the appropriate extreme values in (3) it must be (4.25 - 13·10-6·R1) / (1 + 4.555) > 0.4;

solving for R1 yields R1 < 156 k; with 4% additional margin R1 < 156/1.04 = 150 k.

Choose R1 = 147 k (E48 standard value) and, from the second of (2),

R2 = 147/4.555 = 32.3 k; use 32.4 k (E48 standard value).

In both examples the gate drivers are disabled as VCC falls below 9.75 V (nominal value), as

the voltage on the EN pin falls 45 mV below VEN_On.

Remote on/off control

Whichever configuration is used, since a voltage on the EN pin 45 mV below VEN_On

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: < VCCOff in

DocID17811 Rev 4

11/19

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