8.2.1.2 External Components

The following guidelines can be used to select external components.

R_FB1 and R_FB2: Choose these resistors from standard values in the range of 1 kΩ to 10 kΩ, satisfying the following ratio:

For V_OUT = 0.8 V, the FB pin can be connected to the output directly with a pre-load resistor drawing more than 20 µA. This is needed because the converter operation needs a minimum inductor current ripple to maintain good regulation when no load is connected.

R_ON:Equation 2 can be used to select R_ON if a desired operating frequency is selected. But the minimum value of R_ON is determined by the minimum on-time. It can be calculated as follows:

Equation 10.

If R_ON calculated from Equation 2 is smaller than the minimum value determined in Equation 10, select a lower frequency to re-calculate R_ON by Equation 2. Alternatively, V_IN(MAX) can also be limited in order to keep the frequency unchanged. The relationship of V_IN(MAX) and R_ON is shown in Figure 21.

On the other hand, the minimum off-time of 260 ns can limit the maximum duty ratio.

Figure 21. Maximum V_IN for Selected R_ON

L: The main parameter affected by the inductor is the amplitude of inductor current ripple (I_LR). Once I_LR is selected, L can be determined by:

Equation 11.

where

• V_IN is the maximum input voltage and
• f_SW is determined from Equation 2.

If the output current I_OUT is determined, by assuming that I_OUT = I_L, the higher and lower peak of I_LR can be determined. Beware that the higher peak of I_LR must not be larger than the saturation current of the inductor and current limits of the main and synchronous MOSFETs. Also, the lower peak of I_LR must be positive if CCM operation is required.

Figure 22. Inductor Selection for V_OUT = 3.3 V

Figure 23. Inductor Selection for V_OUT = 0.8 V

Figure 22 and Figure 23 show curves on inductor selection for various V_OUT and R_ON. For small R_ON, according to Equation 10, V_IN is limited. Some curves are therefore limited as shown in the figures.

C_VCC: The capacitor on the V_CC output provides not only noise filtering and stability, but also prevents false triggering of the V_CC UVLO at the main MOSFET on/off transitions. C_VCC should be no smaller than 680 nF for stability, and should be a good-quality, low ESR, ceramic capacitor.

C_OUT and C_OUT3: C_OUT must generally be no smaller than 10 µF. Experimentation is usually necessary to determine the minimum value for C_OUT, as the nature of the load may require a larger value. A load which creates significant transients requires a larger C_OUT than a fixed load.

C_OUT3 is a small value ceramic capacitor located close to the LMR24220 to further suppress high frequency noise at V_OUT. A 100-nF capacitor is recommended.

C_IN and C_IN3: The function of C_IN is to supply most of the main MOSFET current during the on-time, and limit the voltage ripple at the VIN pin, assuming that the voltage source connecting to the VIN pin has finite output impedance. If the voltage source’s dynamic impedance is high (effectively a current source), C_IN supplies the average input current, but not the ripple current.

At the maximum load current, when the main MOSFET turns on, the current to the VIN pin suddenly increases from zero to the lower peak of the inductor’s ripple current and ramps up to the higher peak value. It then drops to zero at turnoff. The average current during the on-time is the load current. For a worst case calculation, C_IN must be capable of supplying this average load current during the maximum on-time. C_IN is calculated from:

Equation 12.

where

• I_OUT is the load current
• t_on is the maximum on-time, and
• ΔV_IN is the allowable ripple voltage at V_IN.

The purpose of C_IN3 is to help avoid transients and ringing due to long lead inductance at the VIN pin. A low ESR 0.1-µF ceramic chip capacitor located close to the LMR24220 is recommended.

C_BST: TI recommends a 33-nF, high-quality ceramic capacitor with low ESR for C_BST because it supplies a surge current to charge the main MOSFET gate driver at turnon. Low ESR also helps ensure a complete recharge during each off-time.

C_SS: The capacitor at the SS pin determines the soft-start time; that is, the time for the reference voltage at the regulation comparator and the output voltage to reach their final value. The time is determined from Equation 13:

Equation 13.

C_FB: If the output voltage is higher than 1.6 V, use a C_FB in the DCM to reduce the output ripple. The recommended value for C_FB is 10 nF.