LTC3737 Datasheet, PDF (14/24 Pages) Linear Technology – Dual 2-Phase, No RSENSE DC/DC Controller with Output Tracking

LTC3737
APPLICATIO S I FOR ATIO
that ripple current does not exceed a specified maximum,
the inductor should be chosen according to:
L ≥ ^{VIN – VOUT} • ^{VOUT + VD}
f_OSC • I_RIPPLE V_IN + V_D
Burst Mode Operation Considerations
The choice of R_DS(ON) and inductor value also determines
the load current at which the LTC3737 enters Burst Mode
operation. When bursting, the controller clamps the peak
inductor current to approximately:
I_BURST(PEAK)
=
1
4
•
∆V_SENSE(MAX)
R_DS(ON)
The corresponding average current depends on the amount
of ripple current. Lower inductor values (higher I_RIPPLE)
will reduce the load current at which Burst Mode operation
begins.
The ripple current is normally set so that the inductor
current is continuous during the burst periods. Therefore,
I_RIPPLE ≤ I_BURST(PEAK)
This implies a minimum inductance of:
L_MIN
≥
V_IN – V_OUT
f_OSC • I_BURST(PEAK)
•
V_OUT + V_D
V_IN + V_D
A smaller value than L_MIN could be used in the circuit,
although the inductor current will not be continuous
during burst periods, which will result in slightly lower
efficiency. In general, though, it is a good idea to keep
I_RIPPLE comparable to I_BURST(PEAK).
Inductor Core Selection
Once the value of 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 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. Core saturation 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, designs for surface mount that do
not increase the height significantly are available from
Coiltronics, Coilcraft, Dale and Sumida.
Output Diode Selection
The catch diode carries load current during the switch off
time of the power MOSFETs . 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 V_IN approaches V_OUT, 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 I_PEAK at close
to 100% duty cycle. Therefore, it is important to ad-
equately specify the diode peak current and average power
dissipation so as not to exceed the diode’s ratings.
Under normal conditions, the average current conducted
by the diode is:
I_D
=
V_IN – V_OUT
V_IN + V_D
• I_OUT
The allowable forward voltage drop in the diode is calcu-
lated from the maximum short-circuit current as:
V_F
≈
P_D
I_PEAK
Kool Mµ is a registered trademark of Magnetics, Inc.
3737f
14