LTC3780EG Linear Technology, LTC3780EG Datasheet - Page 17

• Manufacturer Part Number: LTC3780EG
• Description: IC,SMPS CONTROLLER,CURRENT-MODE,CMOS,SSOP,24PIN,PLASTIC
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Inductor Selection
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use
of smaller inductor and capacitor values. The inductor
value has a direct effect on ripple current. The inductor
current ripple ∆I
maximum inductor current. For a given ripple the induc-
tance terms are as follows:
where:
For high efficiency, choose an inductor with low core loss,
such as ferrite and molypermalloy (from Magnetics, Inc.).
Also, the inductor should have low DC resistance to reduce
the I
inductor current without saturating. To minimize radiated
noise, use a toroid, pot core or shielded bobbin inductor.
C
In Boost mode, input current is continuous. In Buck mode,
input current is discontinuous. In Buck mode, the selec-
tion of input capacitor C
input square wave current. Use a low ESR capacitor sized
to handle the maximum RMS current. For Buck operation,
the input RMS current is given by:
IN
f is operating frequency, Hz
% Ripple is allowable inductor current ripple, %
V
V
V
I
L
L
I
OUT(MAX)
RMS
and C
IN(MIN)
IN(MAX)
OUT
BOOST
BUCK
2
R losses, and must be able to handle the peak
is output voltage, V
≈
OUT
>
I
OUT MAX
>
is minimum input voltage, V
is maximum input voltage, V
ƒ
is maximum output load current
V
Selection
V
•
ƒ
IN MIN
(
OUT
I
OUT MAX
(
•
L
I
OUT MAX
is typically set to 20% to 40% of the
U
)
(
•
)
•
2
(
(
V
V
•
IN MAX
IN
V
OUT
(
)
IN
(
V
U
is driven by the need to filter the
• %
OUT
)
• %
•
Ripple V
)
–
–
Ripple V
V
V
V
OUT
V
IN MIN
IN
OUT
W
(
•
– 1
)
•
IN MAX
•
)
)
100
(
OUT
•
100
2
U
)
H
H
,
This formula has a maximum at V
I
commonly used for design because even significant
deviations do not offer much relief. Note that ripple
current ratings from capacitor manufacturers are often
based on only 2000 hours of life which makes it advisable
to derate the capacitor.
In Boost mode, the discontinuous current shifts from the
input to the output, so C
the output voltage ripple. The effects of ESR (equivalent
series resistance) and the bulk capacitance must be con-
sidered when choosing the right capacitor for a given
output ripple voltage. The steady ripple due to charging
and discharging the bulk capacitance is given by:
where C
The steady ripple due to the voltage drop across the ESR
is given by:
Multiple capacitors placed in parallel may be needed to
meet the ESR and RMS current handling requirements.
Dry tantalum, special polymer, aluminum electrolytic and
ceramic capacitors are all available in surface mount
packages. Ceramic capacitors have excellent low ESR
characteristics but can have a high voltage coefficient.
Capacitors are now available with low ESR and high ripple
current ratings such as OS-CON and POSCAP.
Power MOSFET Selection and
Efficiency Considerations
The LTC3780 requires four external N-channel power
MOSFETs, two for the top switches (Switch A and D,
shown in Figure 1) and two for the bottom switches
RMS
∆V
∆V
Ripple Boost Cap
Ripple Buck Cap
= I
BOOST,ESR
BUCK,ESR
OUT(MAX)
OUT
(
(
is the output filter capacitor.
= I
,
= I
,
/2. This simple worst-case condition is
L(MAX,BUCK)
L(MAX,BOOST)
)
)
=
=
OUT
I
OUT MAX
I
OUT MAX
must be capable of reducing
(
(
• ESR
C
OUT
• ESR
C
)
OUT
)
•
•
•
(
(
V
IN
V
V
•
IN MAX
IN MAX
OUT
V
= 2V
(
(
OUT
LTC3780
–
)
OUT
)
•
V
–
•
IN MIN
f
f
V
(
, where
17
OUT
)
)
3780f
)
V
V