LT8708
38
Rev 0
For more information
When low ESR ceramic capacitance is added in parallel
with the bulk capacitor, the V
IN
ripple is approximately:
∆V
(IN,BUCK,CERAM)
≅
I
OUT
•
V
OUT
V
IN
•ESR
CERAM
•
1
– exp
– V
OUT
V
IN
• ƒ •ESR
CERAM
• C
IN–CERAM
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
V
Add enough ceramic capacitance to make sure
∆V
(IN,BUCK,CERAM)
is adequate for the application. In a
properly designed application, ∆V
(IN,BUCK,CERAM)
should
be much smaller than ∆V
(IN,BUCK,BULK)
.
V
OUT
Capacitance: Discontinuous V
OUT
current is highest
in the boost region due to the M4 switch toggling on and
off. Make sure that the C
OUT
capacitor network has low
enough ESR and is sized to handle the maximum RMS
current. For boost operation, the V
OUT
RMS current is
given by:
I
(OUT,RMS)
≅
I
OUT
•
V
OUT
V
IN
– 1
A
This formula has a maximum when V
IN
is minimum and
V
OUT
is maximum.
C
OUT
is necessary to reduce the V
OUT
ripple caused by
discontinuities and ripple of I
OUT
. The effects of ESR and
the bulk capacitance must be considered when choosing
the right capacitor for a given V
OUT
ripple.
The V
OUT
ripple due to the voltage drop across the bulk
cap ESR without having any ceramic caps in parallel is
approximately:
∆V
(OUT,BOOST,BULK)
≅
V
OUT
•I
OUT
V
IN
•ESR
BULK
∆V
(OUT,BUCK,BULK)
≅
I
RIPPLE
•ESR
BULK
With enough ceramic caps added in parallel, the steady
state V
OUT
ripple due to charging and discharging the
ceramic C
OUT
is given by the following equations:
∆V
(OUT,BOOST,CERAM)
≅
I
OUT
•ESR
CERAM
•
1
– exp
V
IN
– V
OUT
V
OUT
• ƒ •ESR
CERAM
•C
OUT–CERAM
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
V
for V
OUT
> V
IN
, and
∆V
(OUT,BUCK,CERAM)
≅
V
OUT
•
1
–
V
OUT
V
IN
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
8
•L • ƒ
2
•C
OUT–CERAM
V
for V
OUT
< V
IN
Add enough ceramic caps to make sure ∆V
(OUT,BOOST,CERAM)
and ∆V
(OUT,BUCK,CERAM)
are adequate for the application.
In a properly designed application, ∆V
(OUT,BOOST,CERAM)
and ∆V
(OUT,BUCK,CERAM)
should be much smaller than
∆V
(OUT,BOOST,BULK)
and ∆V
(OUT,BUCK,BULK)
, respectively.
SCHOTTKY DIODE (D1, D2, D3, D4) SELECTION
During forward conduction the Schottky diodes, D2 and D4,
shown in Figure 1, conduct during the dead time between
the conduction of the power MOSFET switches. They help
to prevent the body diodes of synchronous switches M2
and M4 from turning on and storing charge. For example,
D4 can significantly reduce reverse-recovery current
when M3 turns on, which improves converter efficiency,
reduces switch M3 power dissipation, and reduces noise
in the inductor current sense resistor (R
SENSE
). Similarly,
during reverse conduction, D1 and D3 conduct during the
dead time between the conduction of the power MOSFET
switches. In order for the diodes to be effective, the induc-
tance between them and the synchronous switch must be
as small as possible, mandating that these components
be placed very close to the MOSFETs.
APPLICATIONS INFORMATION
