Analog Integrated Circuit Device Data



39

Freescale Semiconductor

BC3770

6.1.2.2

Output Capacitors Characteristics

The ceramic capacitors with X5R or X7R rated, low ESR are recommended to absorb all reflected switching ripple current generated

during charging or system powering. The output capacitors need to ensure stability of the buck converter. Special attention should be

capacitance value that falls below the minimum valve given in the recommended capacitance.

The ceramic capacitor’s capacitance can vary with temperature and thickness. The capacitor type X7R, that operates over a temperature

capacitors are not recommended due to a higher ESR values.

Figure 23. Capacitor DC Bias

6.1.2.3

Output Capacitor Selection

The output capacitor of a switching converter absorbs the AC ripple current from the inductor and provides the initial response to a load

transient. The ripple voltage at the output of the converter is the product of the ripple current flowing through the output capacitor and the

impedance of the capacitor. The impedance of the capacitor can be dominated by capacitive, resistive, or inductive elements within the

capacitor, depending on the frequency of the ripple current. Ceramic capacitors have a very low ESR and remain capacitive up to high

frequencies. Their inductive component can usually be neglected at the frequency ranges where the switcher operates. For a desired

voltage ripple at the output, the output capacitance should be decided by the following formula:

6.1.2.4

Input and Output Capacitors

The input capacitors with a 2.2

F, X5R or X7R rated, low ESR are recommended to absorb all reflected switching ripple current generated

during charging or system powering. The output capacitors need to ensure stability of the charger and buck converter. A 10

F on VSYS

and a 4.7

F on CHGOUT with X5R or X7R rated with an appropriate thickness, low ESR is able to make stable operations.

6.1.2.5

PMID Output Capacitor

The capacitor for PMID is used to accurately sense input current levels for VBUS input. It is recommended to use a 2.2

F ceramic

capacitor, X5R or X7R rated, low ESR. The voltage rating for a capacitor depends on the maximum input voltage protection rating.

10

F/10 V, X5R with 0.95 mm Thickness

10

F/10 V, X5R with 1.00 mm Thickness

–pp

–

I



ripple

8f





--------------------------------

=

where I



ripple

D

VIN VOUT

–





2L



f



--------------------------------------------------

=

DDuty

VOUT

VIN

-----------------

==

and