8.1.1.1 R_FB1 and R_FB2

The ratio of these resistors is calculated from:

Equation 7.

R_FB1 and R_FB2 should be chosen from standard value resistors in the range of 1.0 kΩ - 10 kΩ which satisfy the above ratio.

For V_OUT = 0.8 V, the FB pin can be connected to the output directly. However, the converter operation needs a minimum inductor current ripple to maintain good regulation when no load is connected. This minimum load is about 10 µA and can be implemented by adding a pre-load resistor to the output.

8.1.1.2 R_ON

The minimum value for R_ON is calculated from:

Equation 8.

Equation 2 in Hysteretic Control Circuit Overview section can be used to select R_ON if a specific frequency is desired as long as the above limitation is met.

8.1.1.3 L

The main parameter affected by the inductor is the output current ripple amplitude (I_OR). The maximum allowable (I_OR) must be determined at both the minimum and maximum nominal load currents. At minimum load current, the lower peak must not reach 0 A. At maximum load current, the upper peak must not exceed the current limit threshold (1.9 A). The allowable ripple current is calculated from the following equations:

or

The lesser of the two ripple amplitudes calculated above is then used in the following equation:

Equation 11.

where V_IN is the maximum input voltage and Fs is determined from Equation 1. This provides a value for L. The next larger standard value should be used. L should be rated for the I_PK current level shown in Figure 20.

8.1.1.4 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 buck switch on/off transitions. For this reason, C_VCC should be no smaller than 680 nF for stability, and should be a good quality, low ESR, ceramic capacitor.

8.1.1.5 C_O and C_O3

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

C_O3 is a small value ceramic capacitor to further suppress high frequency noise at V_OUT. A 47 nF is recommended, located close to the LM3100.

8.1.1.6 C_IN and C_IN3

C_IN’s purpose is to supply most of the switch current during the on-time, and limit the voltage ripple at V_IN, assume the voltage source feeding V_IN has an output impedance greater than zero. If the source’s dynamic impedance is high (effectively a current source), C_IN supplies the average input current, but not the ripple current.

At maximum load current, when the buck switch turns on, the current into V_IN suddenly increases to the lower peak of the inductor’s ripple current, ramps up to the peak value, then drop to zero at turn-off. The average current during the on-time is the load current. For a worst case calculation, C_IN must supply 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 is the allowable ripple voltage at V_IN.

C_IN3’s purpose is to help avoid transients and ringing due to long lead inductance at V_IN. A low ESR, 0.1 µF ceramic chip capacitor is recommended, located close to the LM3100.

8.1.1.7 C_BST

The recommended value for C_BST is 33 nF. A high quality ceramic capacitor with low ESR is recommended as C_BST supplies a surge current to charge the buck switch gate at turn-on. A low ESR also helps ensure a complete recharge during each off-time.

8.1.1.8 C_SS

The capacitor at the SS pin determines the soft-start time, i.e. the time for the reference voltage at the regulation comparator, and the output voltage, to reach their final value. The time is determined from the following:

Equation 13.

8.1.1.9 C_FB

If output voltage is higher than 1.6 V, this feedback capacitor is needed for Discontinuous Conduction Mode to improve the output ripple performance, the recommended value for C_FB is 10 nF.