LTC3701 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC3701' PDF : 20 Pages View PDF
LTC3701
APPLICATIO S I FOR ATIO
size for a fixed 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. 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 is more expensive than
ferrite. A reasonable compromise from the same manu-
facturer is Kool Mµ. Toroids are very space efficient,
especially when several layers of wire can be used, while
inductors wound on bobbins are generally easier to sur-
face mount. However, new designs for surface mount that
do not increase the height significantly are available from
Coiltronics, Coilcraft, Dale and Sumida.
Power MOSFET Selection
An external P-channel MOSFET must be selected for use
with each channel of the LTC3701. The main selection
criteria for the power MOSFET are the threshold voltage
VGS(TH), “on” resistance RDS(ON), reverse transfer capaci-
tance CRSS and the total gate charge.
Since the LTC3701 is designed for operation down to low
input voltages, a sublogic 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 LTC3701 is less than
the absolute maximum MOSFET VGS rating, typically 8V.
The required minimum RDS(ON) of the MOSFET is gov-
erned by its allowable power dissipation. For applications
that may operate the LTC3701 in dropout, i.e., 100% duty
cycle, the required RDS(ON) is given by:
( ) ( ) RDS(ON)DC=100% =
PP
IOUT(MAX) 2
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.
In applications where the maximum duty cycle is less than
100% and the LTC3701 is in continuous mode, the RDS(ON)
is governed by:
RDS(ON)
≅
PP
(DC)IOUT2(1+
δp)
where DC is the maximum operating duty cycle for that
channel of the LTC3701.
Output Diode Selection
The catch diode carries load current during the switch off-
time. The average diode current is therefore dependent on
the P-channel MOSFET duty cycle. At high input voltages,
the diode conducts most of the time. As VIN approaches
VOUT, the diode conducts for only a small fraction of the
time. The most stressful condition for the diode is when
the output is short-circuited. Under this condition, the
diode must safely handle IPEAK at close to 100% duty
cycle. Therefore, it is important to adequately specify the
diode peak current and average power dissipation so as
not to exceed the diode’s ratings.
Under normal load conditions, the average current con-
ducted by the diode is:
ID
=
VIN – VOUT
VIN + VD
IOUT
The allowable forward voltage drop in the diode is calcu-
lated from the maximum short-circuit current as:
VF
≈
PD
IPEAK
where PD is the allowable power dissipation and will be
determined by efficiency and/or thermal requirements.
A Schottky diode is a good choice for low forward drop and
fast switching time. Remember to keep lead length short
and observe proper grounding (see Board Layout Check-
list) to avoid ringing and increased dissipation.
3701fa
11
Share Link: 
