SLVSHP9 Data sheet

SLVSHP9 July 2026 TPS544A28

PRODUCTION DATA

9.2.2.4 Choose the Output Capacitor

There are three considerations for selecting the value of the output capacitor:

Stability
Steady state output voltage ripple
Regulator transient response to a change load current

First, calculate the minimum output capacitance based on these three requirements. Equation 14 calculates the minimum capacitance to keep the LC double pole below the f_P(MAX) in Table 7-2 to meet stability requirements. To calculate the f_P(MAX), locate the correct K_{f_LC} in Table 7-2. By default, this part comes pre-programmed with RAMP3 and SEL_SUMCOMP = 0 as it's compensation settings. In this example, K_{f_LC} = 9.6 and V_{REF_DAC} = VOUT_COMMAND × VOSL = 3.3V × 0.125V/V = 0.4125V. After you determine the correct K_{f_LC} and V_{REF_DAC}, plug both values into Equation 2. For this example application, f_P(MAX) = 14.947kHz. Plug the calculated f_P(MAX) into Equation 14 to determine your minimum required C_out for stability. Equation 15 calculates the minimum capacitance to meet the steady state output voltage ripple requirement of 16mV. These calculations are for CCM operation and does not include the portion of the output voltage ripple caused by the ESR or ESL of the output capacitors.

Equation 14.

C_{O U T_S T A B L I T Y} > {(\frac{1}{2 π \times f_{P (M A X)}})}^{2} \times \frac{1}{L_{O U T}} = {(\frac{1}{2 π \times 14.947 k H z})}^{2} \times \frac{1}{0.55 μ H} = 207 μ F

Equation 15.

C_{O U T_R I P P L E} > \frac{I_{R I P P L E}}{8 \times V_{R I P P L E} \times f_{S W}} = \frac{5.95 A}{8 \times 16 m V \times 800 k H z} = 58.1 μ F

Equation 17and Equation 18 calculate the minimum capacitance to meet the transient response requirement of 99mV with a 10A step. These equations calculate the necessary output capacitance to hold the output voltage steady while the inductor current ramps up or ramps down after a load step.

Equation 16.

C_{O U T_U N D E R S H O O T} > \frac{L \times {I_{S T E P}}^{2} \times (\frac{V_{O U T}}{V_{I N} (m i n) \times f_{S W}} + t_{O F F_M I N} (m a x))}{2 \times V_{T R A N S} \times V_{O U T} \times (\frac{V_{I N} (m i n) - V_{O U T}}{V_{I N} (m i n) \times f_{S W}} - t_{O F F_M I N} (m a x))}

Equation 17.

C_{O U T_U N D E R S H O O T} > \frac{0.55 μ H \times {10 A}^{2} \times (\frac{3.3 V}{8 V \times 800 k H z} + 150 n s)}{2 \times 99 m V \times 3.3 V \times (\frac{8 V - 3.3 V}{8 V \times 800 k H z} - 150 n s)} = 95.9 μ F

Equation 18.

C_{O U T_O V E R S H O O T} > \frac{L \times {I_{S T E P}}^{2}}{2 \times V_{T R A N S} \times V_{O U T}} = \frac{0.55 μ H \times {10 A}^{2}}{2 \times 99 m V \times 3.3 V} = 84.2 μ F

The output capacitance needed to meet the overshoot requirement is the highest value, so this sets the required minimum output capacitance for this example. Stability requirements can also limit the maximum output capacitance. Equation 19 calculates the recommended maximum output capacitance. This calculation keeps the LC double pole above 1/100th the f_SW.

Equation 19.

C_{O U T_S T A B I L I T Y} < {(\frac{50}{π \times f_{S W}})}^{2} \times \frac{1}{L} = {(\frac{50}{π \times 800 k H z})}^{2} \times \frac{1}{0.55 μ H} = 720 μ F

Using more output capacitance is possible, but the stability must be checked through a bode plot or transient response measurement. The selected output capacitance is 6 × 47μF, 10V ceramic capacitors. When using ceramic capacitors, the capacitance must be derated due to DC and AC bias effects. The selected capacitors derate to 75% the nominal value giving an effective total capacitance of 211μF. This effective capacitance meets the minimum and maximum requirements.

This application uses all ceramic capacitors so the effects of ESR on the ripple and transient are ignored. If using non ceramic capacitors, as a starting point, the ESR must be below the values calculated in Equation 20 to meet the ripple requirement and Equation 21 to meet the transient requirement. For more accurate calculations or if using mixed output capacitors, the impedance of the output capacitors must be used to determine if the ripple and transient requirements can be met.

Equation 20.

R_{E S R_R I P P L E} < \frac{V_{R I P P L E}}{I_{R I P P L E}} = \frac{26 m V}{5.95 A} = 4.4 m Ω

Equation 21.

R_{E S R_T R A N S} < \frac{V_{T R A N S}}{I_{S T E P}} = \frac{99 m V}{10 A} = 9.9 m Ω