ADPL12002/ADPL12003

20V, 2.5A/3.5A, Fully Integrated Synchronous

Buck Converter

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Analog Devices | 13

Equation 2:

IRMS = ILOAD(MAX) × (

√VOUT × (VSUP - VOUT)

VSUP

)

IRMS has a maximum value when the input voltage equals twice the output voltage:

VSUP = 2 × VOUT

Therefore:

IRMS =

ILOAD(MAX)

2

Choose an input capacitor that exhibits less than +10°C self-heating temperature rise at the RMS input current for optimal

long-term reliability. The input-voltage ripple comprises ΔVQ (caused by the capacitor discharge) and ΔVESR (caused by

the ESR of the capacitor). Use low-ESR ceramic capacitors with high ripple-current capability at the input. Assume the

contribution from the ESR and capacitor discharge is equal to 50%. Calculate the input capacitance and ESR required

for a specified input voltage ripple using the following equations:

Equation 3:

ESRIN =

∆VESR

IOUT + ΔIL 2

⁄

Where:

ΔIL =

(

VSUP - VOUT) × VOUT

VSUP × fSW × L

and:

CIN =

IOUT × D(1 - D)

ΔVQ × fSW

D =

VOUT

VSUP

Where:

IOUT = maximum output current

D = duty cycle

Output Capacitor

The output capacitor is selected to meet output voltage ripple, load-transient response, and loop stability requirements.

During a load step, the output current changes almost instantaneously, whereas the inductor is slow to react. During this

transition time, the load-change requirements are supplied by the output capacitor, which causes an

undershoot/overshoot in the output voltage. Output capacitance also affects the control-loop stability. For recommended

output capacitor values, see Table 1 for more details.

The output ripple comprises ΔVQ (caused by the capacitor discharge) and ΔVESR (caused by the ESR of the output

capacitor). Use low-ESR ceramic or aluminum electrolytic capacitors at the output. For aluminum electrolytic capacitors,

the entire output ripple is contributed by ΔVESR. Use Equation 4 to calculate the ESR requirement and choose the

capacitor accordingly. If using ceramic capacitors, assume the contribution to the output-ripple voltage from the ESR and

the capacitor discharge to be equal. The following equations show the output capacitance and ESR requirement for a

specified output-voltage ripple.

Equation 4: