LTC1775 LINER [Linear Technology], LTC1775 Datasheet - Page 12

• Manufacturer Part Number: LTC1775
• Description: High Power No RSENSE TM Current Mode Synchronous Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
Kool M is a registered trademark of Magnetics, Inc.
LTC1775
Normally, the top and bottom MOSFETs will be of the same
type. A bottom MOSFET with lower R
may be chosen if the resulting increase in short-circuit
current is tolerable. However, the bottom MOSFET should
never be chosen to have a higher nominal R
top MOSFET.
Inductor Core Selection
Once the value for L is known, the type of inductor must be
selected. High efficiency converters generally cannot
afford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite, molypermalloy
or Kool M
size for a fixed inductor value, but it 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 increase.
Ferrite designs have very low core loss and are preferred
at high switching frequencies, so design goals can con-
centrate on copper loss and preventing saturation. Ferrite
core material saturates “hard,” which means that induc-
tance collapses rapidly when the peak design current 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 manu-
facturer is Kool M . 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, designs for surface mount are available
which do not increase the height significantly.
Schottky Diode Selection
The Schottky diode D1 shown in Figure 1 conducts during
the dead time between the conduction of the power
MOSFETs. This prevents the body diode of the bottom
MOSFET from turning on and storing charge during the
dead time, which could cost as much as 1% in efficiency.
A 1A Schottky diode is generally a good size for 3A to 5A
12
®
cores. Actual core loss is independent of core
U
U
W
DS(ON)
DS(ON)
than the top
U
than the
regulators. The diode may be omitted if the efficiency loss
can be tolerated.
Parasitic Lead Inductance Effects
Because the LTC1775 is designed to operate with rela-
tively large currents through single (or multiple) MOSFET
switches, the lead inductance of these power switches can
become a significant concern. The table below shows
typical values of lead inductance for some common pack-
ages:
Of particular concern are switches in TO-220 packages
which can have a series inductance of between 4nH and
12nH depending upon the depth of insertion into the
circuit board. When the main (top) switch is turned on, the
lead inductance LP forms a voltage divider with the power
inductor L1. The voltage V
the voltage from the switch on-resistance and increases
the current sense voltage.
The result is lower value of current limit than would have
been expected otherwise. For example, a 10nH lead induc-
tance with a 5 H power inductor has 50mV across it when
V
will be reached when the switch voltage due to on-
resistance is only 250mV, a 17% reduction. This effect is
most noticeable at higher input voltages.
Lead inductance also reduces the benefit of the Schottky
diode D1 by delaying commutation of the inductor current
from the diode over to the synchronous (bottom) switch.
With the diode forward biased when the synchronous
switch turns on, there is only about 500mV applied across
the lead and trace inductance between the switch and the
diode. It takes about 400ns to commutate a 20A current in
this case. This delay reduces efficiency and can also
increase the foldback current limit of the LTC1775. The
IN
V
= 30V and V
LP
= (V
MOSFET Package
IN
TO-220
DDPAK
DPAK
SO-8
– V
OUT
OUT
)LP/L1
= 5V. Thus, the 300mV current limit
LP
across this parasitic adds to
Lead Inductance
4nH to 12nH
1.5nH
4nH
1nH