LTC1872BES6 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC1872BES6 Datasheet PDF : 12 Pages
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LTC1872B
APPLICATIONS INFORMATION
This implies a minimum inductance of:
LMIN
=
VIN
f
0.03
RSENSE
VOUT + VD − VIN
VOUT + VD
A smaller value than LMIN could be used in the circuit;
however, the inductor current will not be continuous
during burst periods.
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 VGS(TH), the “on” resistance RDS(ON),
reverse transfer capacitance CRSS and total gate charge.
Since the LTC1872B is designed for operation down to low
input voltages, a logic level threshold MOSFET (RDS(ON)
guaranteed at VGS = 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 LTC1872B is less
than the absolute maximum VGS rating, typically 8V.
The required minimum RDS(ON) of the MOSFET is gov-
erned by its allowable power dissipation given by:
RDS(ON)
≅
(DC
PP
)IIN2(1+
δp)
where PP is the allowable power dissipation and δp is the
temperature dependency of RDS(ON). (1 + δp) is generally
given for a MOSFET in the form of a normalized RDS(ON) vs
temperature curve, but δp = 0.005/°C can be used as an
approximation for low voltage MOSFETs. DC is the maxi-
mum operating duty cycle of the LTC1872B.
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:
ID(avg) = IOUT
It is important to adequately specify the diode peak current
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.
CIN and COUT 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:
( ) CIN Required IRMS ≈ 0.3 IRIPPLE
where IRIPPLE 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 operat-
ing frequency of the LTC1872B, ceramic capacitors can
also be used for CIN. Always consult the manufacturer if
there is any question.
The selection of COUT 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 (∆VOUT) is approximated by:
7
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