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ICORMS =∆ILO12= 312=0.86A

(39)

7.2.1.2.5 Input Capacitors

A power supply input typically has a relatively high source impedance at the switching frequency. Good-quality

input capacitors are necessary to limit the input ripple voltage. In general, the ripple current splits between the

input capacitors based on the relative impedance of the capacitors at the switching frequency.

1. Select the input capacitors with sufficient voltage and RMS ripple current ratings.

2. Use Equation 40 to calculate the input capacitor RMS ripple current assuming a worst-case duty-cycle

operating point of 50%.

ICINRMS =IOUT× D× 1−D =10A× 0.5× 1−0.5 =5A

(40)

3. Use Equation 41 to find the required input capacitance.

CIN≥ D× 1−D ×IOUT

FSW× ∆VIN−RESRIOUT = 0.5× 1−0.5 ×10A

1MHz× 240mV−2mΩ×10A =11.4μF

(41)

where

• ΔVIN is the input peak-to-peak ripple voltage specification.

• RESR is the input capacitor ESR.

4. Recognizing the voltage coefficient of ceramic capacitors, select two 10-µF, 50V, X7R, 1210 ceramic input

capacitors. Place these capacitors adjacent to the power MOSFETs. See Section 7.4.1.1 for more detail.

5. Use four 10nF, 50V, X7R, 0603 ceramic capacitors near the high-side MOSFET to supply the high di/dt

current during MOSFET switching transitions. Such capacitors offer high self-resonant frequency (SRF) and

low effective impedance above 100MHz. The result is lower power loop parasitic inductance, thus minimizing

switch-node voltage overshoot and ringing for lower conducted and radiated EMI signature. Refer to Section

7.4.1 for more detail.

7.2.1.2.6 Feedback Resistors

Determine the feedback resistor values using Equation 42.

RFB1=RFB2× VOUTVREF−1 =715kΩ× 12V0.8V−1 =100.1kΩ

(42)

7.2.1.2.7 Frequency Set Resistor

Use Equation 43 to calculate the RT resistance for a switching frequency of 1MHz. Choose a standard value of

23.2kΩ.

RT= 23759

FSWkHz −0.72= 23759

1000kHz− 0.72=23kΩ

(43)

7.2.1.2.8 Compensation Components

Choose compensation components for a stable control loop using the procedure outlined as follows:

1. Based on a specified loop gain crossover frequency, fC, of 55kHz, use Equation 44 to calculate RCOMP,

assuming an effective output capacitance of 100 µF. Choose a standard value for RCOMP of 20.5kΩ.

RCOMP=2π×fC×VOUTVREF×RS×GCSgm×COUT=2π×55kHz×12V0.8V×3mΩ×10

1100μS ×150μF=21.2kΩ

(44)

2. To provide adequate phase boost at crossover while also allowing a fast settling time during a load or line

transient, select CCOMP to place a zero at the higher of (1) one tenth of the crossover frequency, or (2)

the load pole. Calculate the necessary value using Equation 45 and choose a standard value for CCOMP of

1.5nF.

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