L6732 datasheet(23/37 Pages) STMICROELECTRONICS

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L6732

Application details

23/37

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 in 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):

Equation 7

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

is given by the following formulas:

Equation 8

Equation 9

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 L6732 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:

Equation 10

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:

Equation 11

ESR

Iout

Vout

ESR

⋅

∆

)

max

min

(

Vout

Vin

Cout

Iout

Vout

COUT

−

⋅

∆

Vout

Cout

Iout

Vout

COUT

⋅

∆

)

(

Fsw

Cout

ESR

Vout

⋅

∆

)

(

Iout

Irms

−

⋅

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