L6738ATR datasheet(27/32 Pages) STMICROELECTRONICS

27 / 32 page

L6738A

Application information

Doc ID 18134 Rev 2

27/32

10.2

Output capacitor(s)

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

capacitive value of the output capacitors as follows:

where

∆IL is the inductor current ripple. In particular, the expression that defines ∆VOUT_C

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

inductor current ripple.

During a load variation, the output capacitors supply the current to the load or absorb 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 80% 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

∆VL is the voltage applied to the inductor during the transient response

(

for the load appliance or VOUT for the load removal).

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

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

On the contrary, electrolytic capacitors have large capacitance to minimize voltage deviation

during load transients while they do not show the same ESR values as the MLCC resulting

∆V

OUT_ESR

∆I

ESR

⋅

∆V

OUT_C

∆I

OUT

⋅⋅

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

⋅

∆V

OUT_ESR

∆I

OUT

ESR

⋅

∆V

OUT_C

∆I

OUT

∆I

OUT

⋅

OUT

∆V

⋅⋅

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

⋅

MAX

OUT

–

⋅

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