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ZHCSJK0A April 2019 – September 2019 UC1843B-SP

PRODUCTION DATA.

8.2.2.7 Sense Resistor and Slope Compensation

The sense resistor is used to sense the ripple current from the transformer as well as shutdown the switching cycle if the peak current of the converter is allowed to get too high. The voltage threshold of the CS pin is around 1 V, thus the equation to find the sense resistor from the peak current is shown in Equation 44.

Equation 44.

R_{c s} = \frac{V_{C S T h r e s h o l d} - V_{S l o p e C o m p O f f s e t}}{I_{l i m i t}}

Equation 45.

R_{c s} = \frac{1 V - 0.1 V}{12 A} = 0.075 Ω

Note that I_limit should be greater than I_PriPeak, and that the voltage offset from the slope compensation will be dependant on the amount of slope compensation in the design. The value of 0.075 Ω for the sense resistance was found to be the optimum value adding some headroom for slope compensation offset of 0.1 V. Slope compensation was implemented with a BJT being turned off and on by the RC pin of the device. The BJT was placed between the REF pin and a resistor divider to the CS pin. The optimum slope compensation value can be found from the following equations after picking a value for the top of the divider:

Equation 46.

S_{c} = \frac{V_{o u t} \times R_{c s} \times G_{c s}}{L_{P R I} \times N_{p s}}

Equation 47.

S_{c} = \frac{5 V \times 0.075 Ω \times 3}{21 μ H \times 3.33} = 16088

Equation 48.

S_{o s c} = \frac{f_{o s c} \times V_{o s c p p}}{D_{M I N}}

Equation 49.

S_{o s c} = \frac{200 k H z \times 1.7 V}{0.25} = 1360000

Equation 50.

R_{c s f} = \frac{R_{t o p}}{\frac{S_{o s c}}{S_{c}} - 1}

Equation 51.

R_{c s f} = \frac{11.8 k Ω}{\frac{1360000}{16088} - 1} = 141 Ω

The UC1843B-SP design uses a much higher resistor of 1.47 kΩ, but this is an attempt to be very conservative. Note that the bottom resistor can be used as part of a filter to the CS pin as well, which is implemented in the design using a capacitor near the CS pin. Care was taken such that the RC filter would not filter the switching frequency by having the RC time constant be a decade less than the switching frequency.