8.2.1.2.2 External Components

The procedure for calculating the external components is illustrated with a design example using the LM25011. Referring to the Functional Block Diagram, the circuit is to be configured for the following specifications:

• V_OUT = 5 V
• V_IN = 8 V to 36 V
• Minimum load current for continuous conduction mode I_OUT(min) = 300 mA
• Maximum load current I_OUT(max) = 1.5 A
• Switching frequency (F_SW) = 1.0 MHz
• Soft-start time = 5 ms

R_FB2 and R_FB1: These resistors set the output voltage, and their ratio is calculated from:

For this example, R_FB2/R_FB1 = 0.992. R_FB1 and R_FB2 should be chosen from standard value resistors in the range of 1.0 kΩ to 10 kΩ which satisfy the above ratio. For this example, 4.99 kΩ is chosen for both resistors, providing a 5.02-V output.

R_T: This resistor sets the on-time and (by default) the switching frequency. First check that the desired frequency does not require an on-time or off-time shorter than the minimum allowed values (90 ns and 150, respectively). The minimum on-time occurs at the maximum input voltage. For this example:

Equation 9.

The minimum off-time occurs at the minimum input voltage. For this example:

Equation 10.

Both the on-time and off-time are acceptable because they are significantly greater than the minimum value for each. The R_T resistor is calculated from Equation 6 using the minimum input voltage:

Equation 11.

A standard value 118-kΩ resistor is selected. The minimum on-time calculates to 152 ns at V_IN = 36 V, and the maximum on-time calculates to 672 ns at V_IN = 8 V.

L1: The parameters controlled by the inductor are the inductor current ripple amplitude (I_OR), and the ripple voltage amplitude across the current sense resistor R_S. The minimum load current is used to determine the maximum allowable ripple to maintain continuous conduction mode (the lower peak does not reach 0 mA). This is not a requirement of the LM25011, but serves as a guideline for selecting L1. For this example, the maximum ripple current should be less than:

For applications where the minimum load current is zero, a good starting point for allowable ripple is 20% of the maximum load current. In this case substitute 20% of I_OUT(max) for I_OUT(min) in Equation 12. The ripple amplitude calculated in Equation 12 is then used in Equation 13:

Equation 13.

A standard value 10-µH inductor is chosen. Using this inductor value, the maximum ripple current amplitude, which occurs at maximum V_IN, calculates to 472 mA_P-P, and the peak current is 1736 mA at maximum load current. Ensure the selected inductor is rated for this peak current. The minimum ripple current, which occurs at minimum V_IN, calculates to 200 mA_P-P.

R_S: The minimum current limit threshold is calculated at maximum load current using the minimum ripple current calculated above. The current limit threshold is the lower peak of the inductor current waveform when in current limit (see Figure 15).

Current limit detection occurs when the voltage across the sense resistor (R_S) reaches the current limit threshold. To allow for tolerances, the sense resistor value is calculated using the minimum threshold specification:

The next smaller standard value, 80 mΩ, is selected. The next step is to ensure that sufficient ripple voltage occurs across R_S with this value sense resistor. As mentioned in the Ripple Requirements section, a minimum of 15-mV_P-P voltage ripple is required across the R_S sense resistor during the off-time to ensure the regulation circuit operates properly. The ripple voltage is the product of the inductor ripple current amplitude and the sense resistor value. In this case, the minimum ripple voltage calculates to:

If the ripple voltage had calculated to less than 15 mV_P-P, the inductor value would have to be reduced to increase the ripple current amplitude. This would have required a recalculation of I_LIM and R_S in the above equations. Because the minimum requirement is satisfied in this case, no change is necessary.

The nominal current limit threshold calculates to 1.63 A. The minimum and maximum thresholds calculate to 1.44 A and 1.83 A, respectively, using the minimum and maximum limits for the current limit threshold specification. The load current is equal to the threshold current plus one-half of the ripple current. Under normal load conditions, the maximum power dissipation in R_S occurs at maximum load current, and at maximum input voltage where the on-time duty cycle is minimum. In this design example, the minimum on-time duty cycle is:

Equation 17.

At maximum load current, the power dissipation in R_S is equal to:

When in current limit the maximum power dissipation in R_S calculates to

Duty cycle is not included in this power calculation because the on-time duty cycle is typically <5% when in current limit.

C_OUT: The output capacitor should typically be no smaller than 3.3 µF, although that is dependent on the frequency and the desired output characteristics. C_OUT should be a low ESR good-quality ceramic capacitor. 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 value for C_OUT than a non-varying load.

C_IN and C_BYP: The purpose of C_IN is to supply most of the switch current during the on-time, and limit the voltage ripple at V_IN, because it is assumed the voltage source feeding V_IN has some amount of source impedance. When the buck switch turns on, the current into V_IN suddenly increases to the lower peak of the inductor ripple current, then ramps up to the upper peak, and finally drops to zero at turn-off. The average current during the on-time is the average load current. For a worst case calculation, C_IN must supply this average load current during the maximum on-time, without letting the voltage at the VIN pin drop below a minimum operating level of 5.5 V. For this exercise 0.5 V is chosen as the maximum allowed input ripple voltage. Using the maximum load current, the minimum value for C_IN is calculated from:

Equation 20.

where t_ON is the maximum on-time, and ΔV is the allowable ripple voltage at V_IN. The purpose of C_BYP is to minimize transients and ringing due to long lead inductance leading to the VIN pin. A low ESR 0.1-µF ceramic chip capacitor is recommended, and C_BYP must be located close to the VIN and SGND pins.

C_BST: The recommended value for C_BST is 0.1 µF. A high-quality ceramic capacitor with low ESR is recommended as C_BST supplies a surge current to charge the buck switch gate at each turn-on. A 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 output voltage to reach its final value (t1 in Figure 17). For a soft-start time of 5 ms, the capacitor value is determined from the following:

Equation 21.

D1: A Schottky diode is recommended. Ultra-fast recovery diodes are not recommended as the high-speed transitions at the SW pin may affect the regulator operation due to the reverse recovery transients of the diode. The diode must be rated for the maximum input voltage, the maximum load current, and the peak current which occurs when the current limit and maximum ripple current are reached simultaneously. The average power dissipation of the diode is calculated from:

where V_F is the forward voltage drop of the diode, and D is the on-time duty cycle.