l6712a STMicroelectronics, l6712a Datasheet - Page 18

• Manufacturer Part Number: l6712a
• Description: Two-phase Interleaved Dc/dc Controller
• Manufacturer: STMicroelectronics

L6712A L6712
OUTPUT CAPACITOR
The output capacitor is a basic component for the fast response of the power supply.
Two-phase topology reduces the amount of output capacitance needed because of faster load transient
response (switching frequency is doubled at the load connections). Current ripple cancellation due to the
180° phase shift between the two phases also reduces requirements on the output ESR to sustain a spec-
ified voltage ripple.
Moreover, if DROOP function is enabled, bigger ESR can be used still keeping the same transient toler-
ances. In fact, when a load transient is applied to the converter's output, for first few microseconds the
current to the load is supplied by the output capacitors. The controller recognizes immediately the load
transient and increases the duty cycle, but 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):
A minimum capacitor value is required to sustain the current during the load transient without discharge
it. The voltage drop due to the output capacitor discharge is given by the following equation:
Where D
load transient and the lower is the output voltage static ripple.
INDUCTOR DESIGN
The inductance value is defined by a compromise between the transient response time, the efficiency, the
cost and the size. The inductor has to be calculated to sustain the output and the input voltage variation
to maintain the ripple current ∆ I
value can be calculated with this relationship:
Where F
Increasing the value of the inductance reduces the ripple current but, at the same time, reduces the con-
verter response time to a load transient. The response time is the time required by the inductor to change
its current from initial to final value. Since the inductor has not finished its charging time, the output current
is supplied by the output capacitors. Minimizing the response time can minimize the output capacitance
required.
The response time to a load transient is different for the application or the removal of the load: if during
the application of the load the inductor is charged by a voltage equal to the difference between the input
and the output voltage, during the removal it is discharged only by the output voltage. The following ex-
pressions give approximate response time for ∆ I load transient in case of enough fast compensation net-
work response:
The worst condition depends on the input voltage available and the output voltage selected. Anyway the
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SW
MAX
is the switching frequency, V
is the maximum duty cycle value. The lower is the ESR, the lower is the output drop during
t
application
∆ V
L
between 20% and 30% of the maximum output current. The inductance
OUT
=
=
∆ V
L
----------------------------- -
V
IN
IN
--------------------------------------------------------------------------------
4 C
=
OUT
is the input voltage and V
L ∆ I
⋅
–
V
----------------------------- -
⋅
V
IN
OUT
fs ∆ I
OUT
=
–
⋅
V
∆ I
⋅
∆ I
OUT
(
L
OUT
V
2
In
OUT
⋅
⋅
⋅
V
-------------- -
d
ESR
V
t
OUT
max
removal
⋅
IN
L
–
V
OUT
=
OUT
-------------- -
V
L ∆ I
OUT
)
⋅
is the output voltage.