The TPS543B25 uses input voltage, duty cycle, and low-side FET current information to generate an internal ramp. The ramp amplitude is determined by an internal ramp generation capacitor, C_RAMP. Three different values for C_RAMP can be selected with a resistor to AGND on the MSEL pin (see Section 6.3.9). The capacitor options are 1 pF, 2 pF, and 4 pF. A larger ramp capacitor results in a smaller ramp amplitude, which results in a higher control loop bandwidth. The following figures show how the loop changes with each ramp setting for the schematic in Figure 7-1.

Many applications perform best with a C_RAMP value of 4 pF, however, the up to the user to measure the loop gain and phase to determine the optimum C_RAMP value for the specific application.

1. First, calculate the RAMP time constant using Equation 5 and Table 6-4.
   Equation 5. ${\mathrm{\tau }}_{\mathrm{C}\mathrm{R}\mathrm{A}\mathrm{M}\mathrm{P}}=\frac{{\mathrm{C}}_{\mathrm{R}\mathrm{A}\mathrm{M}\mathrm{P}}\mathrm{ }\times \mathrm{ }{10}^6}{\mathrm{L}\mathrm{o}\mathrm{o}\mathrm{k}\mathrm{u}\mathrm{p}1\mathrm{ }-\mathrm{ }\mathrm{L}\mathrm{o}\mathrm{o}\mathrm{k}\mathrm{u}\mathrm{p}2\mathrm{ }\mathrm{ }\times \mathrm{ }\mathrm{ }\frac{{\mathrm{V}}_{\mathrm{O}\mathrm{U}\mathrm{T}}}{{\mathrm{V}}_{\mathrm{I}\mathrm{N}}}}$
   Table 6-4 RAMP Selection Lookup Values

   fSW (kHz)	Lookup1 Value	Lookup2 Value
   500	0.372	0.297
   750	0.548	0.445
   1000	0.719	0.594
   1500	1.04	0.891
   2200	1.46	1.31

2. Next, calculate the RAMP capacitor voltage to ensure the capacitor chosen for C_RAMP does not result in a ramp amplitude of greater than 1.25 V, which ensures the ramp does not saturate to ground during a load transient.
   Equation 6. ${\mathrm{V}}_{\mathrm{C}\mathrm{R}\mathrm{A}\mathrm{M}\mathrm{P}}=\frac{{\mathrm{V}}_{\mathrm{I}\mathrm{N}}\mathrm{ }\times {\mathrm{ }(\mathrm{t}}_{\mathrm{O}\mathrm{N}}\mathrm{ }+\mathrm{ }\mathrm{100\; ns})}{{\mathrm{\tau }}_{\mathrm{C}\mathrm{R}\mathrm{A}\mathrm{M}\mathrm{P}}}$

• A larger C_RAMP capacitance results in highest loop gain.
• A smaller C_RAMP capacitance requires fewer output capacitors, and results in a higher crossover frequency.

Figure 6-7 and Figure 6-8 show how the loop changes with each ramp setting for the schematic in Section 7.

Figure 6-7 Loop Gain vs Ramp Settings

Figure 6-8 Loop Phase vs Ramp Settings