L6730 datasheet(34/52 Pages) STMICROELECTRONICS

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Application details

L6730 - L6730B

34/52

6.2

Output capacitors

The output capacitors are basic components 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 a load transient, the output capacitors

supply the current to the load or absorb the current stored into the inductor until the converter

reacts. In fact, even if the controller recognizes immediately 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 first drop due to the current variation inside the capacitor (neglecting the effect of the ESL):

Moreover, there is an additional drop due to the effective capacitor discharge or charge that is

given by the following formulas:

Formula (8) is valid in case of positive load transient while the formula (9) is valid in case of

negative load transient. DMAX is the maximum duty cycle value that in the L6730/B is 100%. For

a given inductor value, minimum input voltage, output voltage and maximum load transient, a

maximum ESR, and a minimum COUT value can be set. The ESR and COUT values also affect

the static output voltage ripple. In the worst case the output voltage ripple can be calculated

with the following formula:

Usually the voltage drop due to the ESR is the biggest one while the drop due to the capacitor

discharge is almost negligible.

6.3

Input capacitors

The input capacitors have to sustain the RMS current flowing through them, that is:

Where D is the duty cycle. The equation reaches its maximum value, IOUT/2 with D = 0.5. The

losses in worst case are:

ESR

Iout

Vout

ESR

⋅

∆

(7)

)

max

min

(

Vout

Vin

Cout

Iout

Vout

COUT

−

⋅

∆

(8)

Vout

Cout

Iout

Vout

COUT

⋅

∆

(9)

)

(

Fsw

Cout

ESR

Vout

⋅

∆

(10)

)

(

Iout

Irms

−

⋅

(11)

)

(

Iout

ESR

⋅

(12)

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