LTC3872 Linear Technology, LTC3872 Datasheet - Page 8

• Manufacturer Part Number: LTC3872
• Description: No RSENSE Current Mode Boost DC/DC Controller
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
LTC3872
mode gain and avoid subharmonic oscillation, it is recom-
mended that the ripple current in the inductor fall in the
range of 20% to 40% of the maximum average current.
For example, if the maximum average input current is
1A, choose an I
between 0.2 and 0.4.
Inductor Selection
Given an operating input voltage range, and having chosen
the operating frequency and ripple current in the inductor,
the inductor value can be determined using the following
equation:
Remember that boost converters are not short-circuit
protected. Under a shorted output condition, the inductor
current is limited only by the input supply capability.
The minimum required saturation current of the inductor
can be expressed as a function of the duty cycle and the
load current, as follows:
The saturation current rating for the inductor should be
checked at the minimum input voltage (which results in the
highest inductor current) and maximum output current.
Operating in Discontinuous Mode
Discontinuous mode operation occurs when the load cur-
rent is low enough to allow the inductor current to run
out during the off-time of the switch. Once the inductor
current is near zero, the switch and diode capacitances
resonate with the inductance to form damped ringing at
1MHz to 10MHz. If the off-time is long enough, the drain
voltage will settle to the input voltage.
Depending on the input voltage and the residual energy
in the inductor, this ringing can cause the drain of the
8
I
where
L SAT
(
L
Δ =
=
I
L
)
:
V
≥
Δ
IN MIN
χ
⎛
⎜
⎝
I
(
L
1
•
•
+
1
L
f
I
O MAX
)
–
χ
2
between 0.2A and 0.4A, and a value χ
(
•
D
⎞
⎟
⎠
U
D
M
•
MAX
A A X
1
)
I
O MAX
–
(
D
U
MAX
)
W
U
power MOSFET to go below ground where it is clamped
by the body diode. This ringing is not harmful to the IC
and it has been shown not to contribute signifi cantly to
EMI. Any attempt to damp it with a snubber will degrade
the effi ciency.
Inductor Core Selection
Once the value for L is known, the type of inductor must
be selected. Actual core loss is independent of core 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 in-
crease. Generally, there is a tradeoff between core losses
and copper losses that needs to be balanced.
Ferrite designs have very low core losses and are pre-
ferred at high switching frequencies, so design goals can
concentrate on copper losses and preventing saturation.
Ferrite core material saturates “hard,” meaning that the
inductance collapses rapidly when the peak design current
is exceeded. This results in an abrupt increase in inductor
ripple current and consequently, output voltage ripple. Do
not allow the core to saturate!
Different core materials and shapes will change the size/cur-
rent and price/current relationship of an inductor. Toroid
or shielded pot cores in ferrite or permalloy materials are
small and don’t radiate much energy, but generally cost
more than powdered iron core inductors with similar
characteristics. The choice of which style inductor to use
mainly depends on the price vs size requirements and any
radiated fi eld/EMI requirements. New designs for surface
mount inductors are available from Coiltronics, Coilcraft,
Toko and Sumida.
Power MOSFET Selection
The power MOSFET serves two purposes in the LTC3872:
it represents the main switching element in the power path
and its R
for the control loop. Important parameters for the power
MOSFET include the drain-to-source breakdown voltage
(BV
(R
and gate-to-drain charges (Q
DS(ON)
DSS
), the threshold voltage (V
) versus gate-to-source voltage, the gate-to-source
DS(ON)
represents the current sensing element
GS
GS(TH)
and Q
), the on-resistance
GD
, respectively),
3872fa