PM8903A

Application information

Doc ID 024147 Rev 1

21/33

6.3

Inductor design

The inductance value is defined by a compromise between the dynamic response time, the

efficiency, the cost, and the size. The inductor must be calculated to maintain the ripple

current (

value can be calculated with the following relationship:

voltage.

Increasing the value of the inductance reduces the current ripple but, at the same time,

increases the converter response time to a dynamic load change. The response time is the

time required by the inductor to change its current from the initial to the final value. Until the

inductor finishes its charging time, the output current is supplied by the output capacitors.

Minimizing the response time can minimize the output capacitance required. If the

compensation network is well designed, during a load variation the device is able to set a

duty cycle value very different (0% or 100%) from the steady-state one. When this condition

is reached, the response time is limited by the time required to change the inductor current.

6.4

Output capacitors

The output capacitors are basic components to define the ripple voltage across the output

and for the fast transient response of the power supply. They depend on the output voltage

ripple requirements, as well as any output voltage deviation requirement during a load

transient.

During steady-state conditions, the output voltage ripple is influenced by both the ESR and

the capacitive value of the output capacitors as follows:

where

takes into consideration the output capacitor charge and discharge as a consequence of the

inductor current ripple.

During a load variation, the output capacitor supplies the current to the load or absorbs the

current stored in the inductor until the converter reacts. In fact, even if the controller

immediately recognizes the load transient and sets the duty cycle at 100% or 0%, the

current slope is limited by the inductor value. The output voltage has a drop that, also in this

case, depends on the ESR and capacitive charge/discharge as follows:

where

(

MLCC capacitors have typically low ESR to minimize the ripple but also have low

capacitance that does not minimize the voltage deviation during dynamic load variations.

L

V

IN

V

OUT

–

F

SW

ΔI

L

⋅

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

V

OUT

V

IN

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

⋅

=

ΔV

OUT_ESR

ΔI

L

ESR

⋅

=

ΔV

OUT_C

ΔI

L

1

8C

OUT

F

SW

⋅⋅

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

⋅

=

ΔV

OUT_ESR

ΔI

OUT

ESR

⋅

=

ΔV

OUT_C

ΔI

OUT

L

ΔI

OUT

⋅

2C

OUT

ΔV

L

⋅⋅

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

⋅

=

D

MAX

V

IN

V

OUT

–

⋅