LTC3801B_15 Datasheet, PDF (8/12 Pages) Linear Technology – Micropower Constant Frequency Step-Down DC/DC Controllers in ThinSOT

LTC3801/LTC3801B
APPLICATIO S I FOR ATIO
I_RIPPLE = 0.4(I_OUT(MAX)). Remember, the maximum I_RIPPLE
occurs at the maximum input voltage.
In Burst Mode operation on the LTC3801, the ripple
current is normally set such that the inductor current is
continuous during the burst periods. Therefore, the peak-
to-peak ripple current must not exceed:
I_RIPPLE ≤
0.03
R_SENSE
This implies a minimum inductance of:
L_MIN
=
V_IN − V_OUT
f


0.03
R_SENSE




V_OUT + V_D
V_IN + V_D


(Use V_IN(MAX) = V_IN)
A smaller value than L_MIN could be used in the circuit;
however, the inductor current will not be continuous
during burst periods.
Inductor Core Selection
Once the value for L is known, the type of inductor must be
selected. High efficiency converters generally cannot af-
ford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite, molypermalloy
or Kool Mµ^® cores. Actual core loss is independent of core
size for a fixed inductor value, but it is very dependent on
inductance selected. As inductance increases, core losses
go down. Unfortunately, increased inductance requires
more turns of wire and therefore copper losses will in-
crease. Ferrite designs have very low core losses and are
preferred at high switching frequencies, so design goals
can concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard,” which means that
inductance collapses abruptly when the peak design cur-
rent is exceeded. This 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 it is more expensive
than ferrite. A reasonable compromise from the same
manufacturer is Kool Mµ. Toroids are very space efficient,
especially when you can use several layers of wire.
Because they generally lack a bobbin, mounting is more
difficult. However, new designs for surface mount that do
not increase the height significantly are available.
Power MOSFET Selection
An external P-channel power MOSFET must be selected
for use with the LTC3801/LTC3801B. The main selection
criteria for the power MOSFET are the threshold voltage
V_GS(TH) and the “on” resistance R_DS(ON), reverse transfer
capacitance C_RSS and total gate charge.
Since the LTC3801/LTC3801B are designed for operation
down to low input voltages, a sublogic 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 LTC3801/LTC3801B is less than the absolute maxi-
mum V_GS rating, typically 8V.
The required minimum R_DS(ON) of the MOSFET is governed
by its allowable power dissipation. For applications that may
operate the LTC3801/LTC3801B in dropout, i.e., 100% duty
cycle, at its worst case the required R_DS(ON) is given by:
( ) ( ) ^{RDS(ON)DC=100% =}
P_P
I_OUT(MAX) ² 1+ δp
where P_P is the allowable power dissipation and δp is the
temperature dependency of R_DS(ON). (1 + δp) is generally
given for a MOSFET in the form of a normalized R_DS(ON) vs
temperature curve, but δp = 0.005/°C can be used as an
approximation for low voltage MOSFETs.
In applications where the maximum duty cycle is less than
100% and the LTC3801/LTC3801B are in continuous
mode, the R_DS(ON) is governed by:
R_DS(ON)
≅
P_P
(^DC)^I_OUT2(¹⁺
δp₎
where DC is the maximum operating duty cycle of the
LTC3801/LTC3801B.
Kool Mµ is a registered trademark of Magnetics, Inc.
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sn3801 3801fs