8.2.2.2 Selection Of External Components

R_FB1, R_FB2: V_OUT = V_FB × (R_FB1 + R_FB2) / R_FB1, and because V_FB = 2.5 V, the ratio of R_FB2 to R_FB1 calculates as 3:1. Standard values of 3.01 kΩ and 1 kΩ are chosen. Other values could be used as long as the 3:1 ratio is maintained.

F_s and R_T: The recommended operating frequency range for the LM5008A is 50 kHz to 1.1 MHz. Unless the application requires a specific frequency, the choice of frequency is generally a compromise because it affects the size of L1 and C2 and the switching losses. The maximum allowed frequency, based on a minimum on-time of 400 ns, is calculated with Equation 7.

For this exercise, F_MAX = 263 kHz. From Equation 2, R_T calculates to 274 kΩ. A standard value 324-kΩ resistor is used to allow for tolerances in Equation 2, resulting in a frequency of 223 kHz.

L1: The main parameter affected by the inductor is the output current ripple amplitude. The choice of inductor value therefore depends on both the minimum and maximum load currents, keeping in mind that the maximum ripple current occurs at maximum V_IN.

1. Minimum load current: To maintain continuous conduction at minimum Io (100 mA), the ripple amplitude (I_OR) must be less than 200 mAp-p so the lower peak of the waveform does not reach zero. L1 is calculated using Equation 8.
Equation 8.

At V_IN = 95 V, L1 (minimum) calculates to 200 µH. The next larger standard value (220 µH) is chosen and with this value I_OR calculates to 182 mAp-p at V_IN = 95 V, and 34 mAp-p at V_IN = 12 V.

2. Maximum load current: At a load current of 300 mA, the peak of the ripple waveform must not reach the minimum value of the LM5008A’s current limit threshold (410 mA). Therefore the ripple amplitude must be less than 220 mAp-p, which is already satisfied in Equation 8. With L1 = 220 µH, at maximum V_IN and I_O, the peak of the ripple is 391 mA. While L1 must carry this peak current without saturating or exceeding its temperature rating, it also must be capable of carrying the maximum value of the LM5008A’s current limit threshold (610 mA) without saturating because the current limit is reached during start-up.

The DC resistance of the inductor must be as low as possible. For example, if the inductor’s DCR is 1 Ω, the power dissipated at maximum load current is 0.09 W. While small, it is not insignificant compared to the load power of 3 W.

C3: The capacitor on the V_CC output provides not only noise filtering and stability, but its primary purpose is to prevent false triggering of the V_CC UVLO at the buck switch on and off transitions. C3 must be no smaller than 0.47 µF.

C2, and R3: When selecting the output filter capacitor C2, the items to consider are ripple voltage due to its ESR, ripple voltage due to its capacitance, and the nature of the load.

ESR and R3: A low ESR for C2 is generally desirable to minimize power losses and heating within the capacitor. However, the regulator requires a minimum amount of ripple voltage at the feedback input for proper loop operation. For the LM5008A the minimum ripple required at pin 5 is 25 mVp-p, requiring a minimum ripple at V_OUT of 100 mV. Because the minimum ripple current (at minimum V_IN) is 34 mA p-p, the minimum ESR required at V_OUT is 100 mV / 34 mA = 2.94 Ω. Because quality capacitors for SMPS applications have an ESR considerably less than this, R3 is inserted as shown in the Functional Block Diagram. R3’s value, along with C2’s ESR, must result in at least 25 mVp-p ripple at pin 5. Generally, R3 is 0.5 to 3 Ω.

R_CL: When current limit is detected, the minimum off-time set by this resistor must be greater than the maximum normal off-time, which occurs at maximum input voltage. Using Equation 4, the minimum on-time is 472 ns, yielding an off-time of 4 µs (at 223 kHz). Due to the 25% tolerance on the on-time, the off-time tolerance is also 25%, yielding a maximum off-time of 5 µs. Allowing for the response time of the current limit detection circuit (350 ns) increases the maximum off-time to 5.35 µs. This is increased an additional 25% to 6.7 µs to allow for the tolerances of Equation 5. Using Equation 5, R_CL calculates to 325 kΩ at V_FB = 2.5 V. A standard value 332-kΩ resistor is used.

D1: The important parameters are reverse recovery time and forward voltage. The reverse recovery time determines how long the reverse current surge lasts each time the buck switch is turned on. The forward voltage drop is significant in the event the output is short-circuited as it is only this diode’s voltage which forces the inductor current to reduce during the forced off-time. For this reason, a higher voltage is better, although that affects efficiency. A good choice is a Schottky power diode, such as the DFLS1100. D1’s reverse voltage rating must be at least as great as the maximum V_IN, and its current rating be greater than the maximum current limit threshold (610 mA).

C1: This capacitor’s purpose is to supply most of the switch current during the on-time, and limit the voltage ripple at V_IN, on the assumption that the voltage source feeding V_IN has an output impedance greater than zero. At maximum load current, when the buck switch turns on, the current into pin 8 suddenly increases to the lower peak of the output current waveform, ramp up to the peak value, then drop to zero at turnoff. The average input current during this on-time is the load current (300 mA). For a worst-case calculation, C1 must supply this average load current during the maximum on-time. To keep the input voltage ripple to less than 2 V (for this exercise), C1 is calculated with Equation 9.

Equation 9.

Quality ceramic capacitors in this value have a low ESR which adds only a few millivolts to the ripple. It is the capacitance which is dominant in this case. To allow for the capacitor’s tolerance, temperature effects, and voltage effects, a 1-µF, 100-V, X7R capacitor is used.

C4: The recommended value is 0.01 µF for C4, as this is appropriate in the majority of applications. A high-quality ceramic capacitor with low ESR is recommended as C4 supplies the surge current to charge the buck switch gate at turnon. A low ESR also ensures a quick recharge during each off-time. At minimum V_IN, when the on-time is at maximum, it is possible during start-up that C4 does not fully recharge during each 300-ns off-time. The circuit is not able to complete the start-up, and achieve output regulation. This can occur when the frequency is intended to be low (for example, R_T = 500 K). In this case C4 must be increased so it can maintain sufficient voltage across the buck switch driver during each on-time.

C5: This capacitor helps avoid supply voltage transients and ringing due to long lead inductance at V_IN. A low-ESR, 0.1-µF ceramic chip capacitor is recommended placed close to the LM5008A.