SRK2000DTR(2012) データシートの表示（PDF） - STMicroelectronics

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SRK2000DTR
(Rev.:2012)

Synchronous rectifier smart driver for LLC resonant converters

STMicroelectronics 

Application information

SRK2000

Note:

Equation 2

⎧

⎪

⎪⎪R1 =

VCCOn

− VEN

VCC _ G

VEN _ On

⎨

IEN

⎪

⎪R2 = R1

VEN _ On

⎪⎩

VCC _ 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 3

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 kΩ; 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 kΩ. Choose R1 = 442 kΩ (E48 standard value) and, from the second of (2), R2 =

442/4.555 = 97 kΩ; use 97.6 kΩ (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 kΩ; with 4% additional margin R1 < 156/1.04 = 150 kΩ.

Choose R1 = 147 kΩ (E48 standard value) and, from the second of (2), R2 = 147/4.555 =

32.3 kΩ; use 32.4 kΩ (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.

10/17

Doc ID 17811 Rev 2

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