ADPL12005/ADPL12006

20V, 5A/6A Fully Integrated Synchronous Buck

Converters

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Analog Devices | 13

Input Capacitor

The input filter capacitor reduces peak currents drawn from the power source and reduces noise and voltage ripple on

the input caused by the circuit's switching. The ADPL12005/ADPL12006 incorporates a symmetrical pinout that can be

leveraged for better EMI performance. Connect two high-frequency 0603 or smaller capacitors on two SUP pins on either

side of the package for good EMI performance. Connect a high-quality, 4.7μF (or larger) low-ESR ceramic capacitor on

the SUP pin for low-input voltage ripple.

A bulk capacitor with higher Equivalent series resistance (ESR), such as an electrolytic capacitor, is normally required as

well to lower the Q of the front-end circuit and provide the remaining capacitance needed to minimize input-voltage ripple.

The input capacitor RMS current requirement (IRMS) is defined by the following equation:

Equation 6:

IRMS = ILOADMAX × (

√

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 consists of 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 7:

ESRIN =

∆VESR

IOUT + ∆IL 2

⁄

Where:

∆IL=

(

VSUP - VOUT) × VOUT

VSUP × fSW × L

and:

CIN =

IOUT × D(1 - D)

∆VQ × fSW

D =

VOUT

VSUP

where:

IOUT = maximum output current

D = duty cycle