LTC1872
7
1872fa
For more information
www.linear.com/LTC1872
applicaTions inForMaTion
Inductor Selection
When selecting the inductor, keep in mind that inductor
saturation current has to be greater than the current limit
set by the current sense resistor. Also, keep in mind that
the DC resistance of the inductor will affect the efficiency.
Off the shelf inductors are available from Murata, Coilcraft,
Toko, Panasonic, Coiltronics and many other suppliers.
Power MOSFET Selection
The main selection criteria for the power MOSFET are the
threshold voltage V
GS(TH)
, the “on” resistance R
DS(ON)
,
reverse transfer capacitance C
RSS
and total gate charge.
Since the LTC1872 is designed for operation down to low
input voltages, a logic level threshold MOSFET (R
DS(ON)
guaranteed at V
GS
= 2.5V) is required for applications
that work close to this voltage. When these MOSFETs are
used, make sure that the input supply to the LTC1872 is
less than the absolute maximum V
GS
rating, typically 8V.
The required minimum R
DS(ON)
of the MOSFET is governed
by its allowable power dissipation given by:
R
DS(ON)
≅
P
P
DC
( )
I
IN
2
1
+δ
p
(
)
where P
P
is the allowable power dissipation and
δ
p is the
temperature dependency of R
DS(ON)
. (1 +
δ
p) is generally
given for a MOSFET in the form of a normalized R
DS(ON)
vs temperature curve, but
δ
p = 0.005/°C can be used as
an approximation for low voltage MOSFETs. DC is the
maximum operating duty cycle of the LTC1872.
Output Diode Selection
Under normal load conditions, the average current con-
ducted by the diode in a boost converter is equal to the
output load current:
I
D(avg)
=
I
OUT
It is important to adequately specify the diode peak cur-
rent and average power dissipation so as not to exceed
the diode ratings.
Schottky diodes are recommended for low forward drop
and fast switching times. Remember to keep lead length
short and observe proper grounding (see Board Layout
Checklist) to avoid ringing and increased dissipation.
C
IN
and C
OUT
Selection
To prevent large input voltage ripple, a low ESR input
capacitor sized for the maximum RMS current must be
used. The maximum RMS capacitor current for a boost
converter is approximately equal to:
C
IN
Required I
RMS
≈
0.3
( )
I
RIPPLE
where I
RIPPLE
is as defined in the Inductor Value Calcula-
tion section.
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
LTC1872, 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 required effective
series resistance (ESR). Typically, once the ESR require-
ment is satisfied, the capacitance is adequate for filtering.
The output ripple (∆V
OUT
) is approximated by:
Δ
V
OUT
≈
I
O
•
V
OUT
+
V
D
V
IN
+
I
RIPPLE
2
⎛
⎝
⎜
⎞
⎠
⎟•
ESR
2
+
1
2
π
fC
OUT
⎛
⎝
⎜⎜
⎞
⎠
⎟⎟
2
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
1
2
