SNVSAZ0C March 2018 – October 2021 LM51501-Q1
PRODUCTION DATA
Refer to the PDF data sheet for device specific package drawings
The LM51501-Q1 features low-side current sense amplifier with a gain of 10, and provides an internal slope compensation ramp to prevent subharmonic oscillation at high duty cycle. The device generates the slope compensation ramp using a sawtooth current source with a slope of 30 µA × FSW (typical). This current flows through an internal 2-kΩ resistor and out of the CS pin. The slope compensation ramp is determined by the RT resistor and is 60 mV × FSW (typical) at the input of the current sense amplifier and 600 mV × FSW (typical) at the output of the current sense amplifier. The slope compensation ramp can be increased by adding an external slope resistor (RSL) between the sense resistor (RS) and the CS pin, but take extra care when using the RSL, because the peak current limit is affected by adding RSL. See tSection 8.3.7 for more detailed information.
According to peak current mode control theory, the slope of the compensation ramp must be greater than half of the sensed inductor current falling slope to prevent subharmonic oscillation at high duty cycle. Therefore, the minimum amount of slope compensation should satisfy the inequality in Equation 4.
VF is a forward voltage drop of D1, the external diode. 1.2 is recommended as a margin to cover non-ideal factors.
If required, RSL can be added to increase the slope of the compensation ramp from half to 82% of the slope of the sensed inductor current during the falling slope. The typical RSL value is calculated using Equation 5. The maximum RSL value is 1 kΩ.
The PWM comparator in Figure 8-2 compares the sum of the sensed inductor current, the slope compensation ramp, and a 0.3-V (typical) internal COMP-to-PWM offset with the COMP pin voltage (VCOMP), and will terminate the present cycle if the sum is greater than VCOMP.