The CS_ILIM pin is driven by a signal representative of the transformer primary-side current. The current signal has to have compatible input range of the COMP pin. As shown in Figure 8-14, the COMP pin voltage is used as the reference for the peak current. Note that the OUTB waveform is only applicable for TPS7H5005-SEP and TPS7H5008-SEP. The primary side signals, OUTA/OUTB, are turned on by the internal clock signal and turned off when sensed peak current reaches the COMP/2 pin voltage. Note that this peak sensed current signal that is compared to COMP/2 at the PWM comparator contains an offset voltage of 150 mV. The CS_ILIM pin is also used to configure the current limit for the controller.
A resistor is needed from CS_ILIM to AVSS is used to detect current for both proper PWM operation and overcurrent protection. The current limit threshold VCS_ILIM, is specified as 1.05 V (nominal) in the electrical specifications. This indicates that when the voltage on this pin reaches this threshold, the device will go into hiccup mode. Equation 10 shows the calculation for determining the value of the sense resistor for a selected current limit.
Note that the value of ILIM has to account for where and how the current is being sensed. For a forward converter with sense resistor between source of primary FET to AVSS, ILIM will be referred to the primary side of the converter.
Equation 11 shows the calculation for determining ILIM in the design of a forward converter, where:
In the design of a buck converter which senses the high side current via a current sense transformer, Equation 12 can be used for determining ILIM for this instance.
In this equation:
Regardless of the topology, the user should ensure that there is sufficient margin between the peak current during normal operation and the overcurrent trip point when determining the value of RCS.