8.1.1.2.1 Sizing RPULLUP

The transistor output of ISOS510-SP operates in active, saturation, reverse, and cut-off regions, just like a

regular transistor. To verify that the output does not get damaged when the output is saturated, the minimum

value of RPULLUP can be calculated for a given pull-up voltage, VPULLUP, in Equation 1:

RPULLUP> VPULLUP− VCESAT

ICMAX

(1)

For the example of a feedback loop application, we can calculate the minimum required value for RPULLUP for a

given VPULLUP of 10V, the maximum output voltage of the error amplifier (VCOMP(MAX) of 2.5V, and the maximum

output current of the error amplifier is internally clamped at 1.6mA. The equation to calculate RPULLUP is shown

in Equation 2:

RPULLUP> VPULLUP− VCOMPMAX

ICOMPCLAMP = 10V − 2.5V

1.6mA = 4.66kΩ

(2)

8.1.1.2.2 Sizing RIN

The input side of ISOS510-SP is current-driven. To limit the amount of current flowing into the AN pin, placing a

series resistor, RIN, in series with the input as shown in Figure 8-1 is recommended.

Depending on how the ISOS510-SP device is being used, the value of RIN can vary quite a bit. However, at a

high level, to make sure the input does not get damaged, the minimum value of RIN can be calculated for a given

input voltage, VIN, in Equation 3:

RIN> VIN− VF

ICMAX

(3)

However, in the use case of a feedback loop, RIN directly affects the mid-band gain of the loop. Assuming that

the TL1431SP has been configured to give a reference voltage, VREF, of 2.5V and RPULLUP is 5kΩ, Equation 4 is

used to calculate the maximum value of RIN verifying that the VCOMP voltage on the primary side can be pulled to

the saturation voltage of the ISOS510-SP, VCE(SAT).

RIN< VOUT− VREF−VF × RPULLUP ×CTRMIN

VPULLUP − VCESAT  = 5V− 2.5V−1.2V × 5kΩ ×100%

10V − 0.3V  = 670Ω

(4)

8.1.1.3 Application Curves

The following curves shows ISOS510-SP bandwidth performance over different loading conditions where VIN =

5VDC + 2VPK. See Figure 6-4 for setup details.

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ISOS510-SP

SLVSJM3 – MAY 2026

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