Datasheet

LTC3809
15
3809fc
size for a fi xed inductor value, but is very dependent on
the inductance selected. As inductance increases, core
losses go down. Unfortunately, increased inductance
requires more turns of wire and therefore copper losses
will increase.
Ferrite designs have very low core losses and are pre-
ferred at high switching frequencies, so design goals can
concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard”, which means that
i n d u c t a n c e c o l l a p s e s a b r u p t l y w h e n t h e p e a k d e s i g n c u r r e n t
is exceeded. Core saturation results in an abrupt increase
in inductor ripple current and consequent output voltage
ripple. Do not allow the core to saturate!
Molypermalloy (from Magnetics, Inc.) is a very good,
low loss core material for toroids, but is more expensive
than ferrite. A reasonable compromise from the same
manufacturer is Kool Mμ. Toroids are very space effi cient,
especially when several layers of wire can be used, while
inductors wound on bobbins are generally easier to sur-
face mount. However, designs for surface mount that do
not increase the height signifi cantly are available from
Coiltronics, Coilcraft, Dale and Sumida.
Schottky Diode Selection (Optional)
The schottky diode D in Figure 11 conducts current dur-
ing the dead time between the conduction of the power
MOSFETs. This prevents the body diode of the bottom
N-channel MOSFET from turning on and storing charge
during the dead time, which could cost as much as 1%
in effi ciency. A 1A Schottky diode is generally a good
size for most LTC3809 applications, since it conducts
a relatively small average current. Larger diode results
in additional transition losses due to its larger junction
capacitance. This diode may be omitted if the effi ciency
loss can be tolerated.
C
IN
and C
OUT
Selection
In continuous mode, the source current of the P-channel
MOSFET is a square wave of duty cycle (V
OUT
/V
IN
). To
prevent large voltage transients, a low ESR input capacitor
sized for the maximum RMS current must be used. The
maximum RMS capacitor current is given by:
C
IN
Re •
•–
/
quiredI I
VVV
V
RMS MAX
OUT IN OUT
IN
≈
()
12
This formula has a maximum value at V
IN
= 2V
OUT
,
where I
RMS
= I
OUT
/2. This simple worst-case condition
is commonly used for design because even signifi cant
deviations do not offer much relief. Note that capacitor
manufacturer’s ripple current ratings are often based on
2000 hours of life. This makes it advisable to further derate
the capacitor or to choose a capacitor rated at a higher
temperature than required. Several capacitors may be
paralleled to meet the size or height requirements in the
design. Due to the high operating frequency of the LTC3809,
ceramic capacitors can also be used for C
IN
. Always consult
the manufacturer if there is any question.
The selection of C
OUT
is driven by the effective series
resistance (ESR). Typically, once the ESR requirement
is satisfi ed, the capacitance is adequate for fi ltering. The
output ripple (ΔV
OUT
) is approximated by:
Δ≈ +
⎛
⎝
⎜
⎞
⎠
⎟
V I ESR
fC
OUT RIPPLE
OUT
•
••
1
8
where f is the operating frequency, C
OUT
is the output
capacitance and I
RIPPLE
is the ripple current in the induc-
tor. The output ripple is highest at maximum input voltage
since I
RIPPLE
increase with input voltage.
Setting Output Voltage
The LTC3809 output voltage is set by an external feed-
back resistor divider carefully placed across the output,
as shown in Figure 3. The regulated output voltage is
determined by:
VV
R
R
OUT
B
A
=+
⎛
⎝
⎜
⎞
⎠
⎟
06 1.•
APPLICATIONS INFORMATION