Datasheet

Application details L6730 - L6730B
34/52 Doc ID 11938 Rev 3
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. D
MAX
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 C
OUT
value can be set. The ESR and C
OUT
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, I
OUT
/2 with D = 0.5.
The losses in worst case are:
ESRIoutVout
ESR
⋅
Δ
=
Δ
(7)
)maxmin,(2
2
VoutDVinCout
LIout
Vout
COUT
−⋅⋅⋅
⋅Δ
=Δ
(8)
VoutCout
LIout
Vout
COUT
⋅⋅
⋅Δ
=Δ
2
2
(9)
)
8
1
(
FswCout
ESRIVout
L
⋅⋅
+⋅Δ=Δ
(10)
)1( DDIoutIrms −⋅⋅=
(11)
2
)5.0( IoutESRP ⋅⋅=
(12)