Datasheet

LTC3788
17
3788fc
applicaTions inForMaTion
The equivalent resistance R1|| R2 is scaled to the room
temperature inductance and maximum DCR:
R1||R2 =
L
(DCR at 20°C) • C1
The sense resistor values are:
R1=
R1||R2
R
D
; R2 =
R1• R
D
1−R
D
The maximum power loss in R1 is related to duty cycle,
and will occur in continuous mode at V
IN
= 1/2 V
OUT
:
P
LOSS
R1=
(V
OUT
− V
IN
) • V
IN
R1
Ensure that R1 has a power rating higher than this value.
If high efficiency is necessary at light loads, consider this
power loss when deciding whether to use DCR sensing or
sense resistors. Light load power loss can be modestly
higher with a DCR network than with a sense resistor,
due to the extra switching losses incurred through R1.
However, DCR sensing eliminates a sense resistor, reduces
conduction losses and provides higher efficiency at heavy
loads. Peak efficiency is about the same with either method.
Inductor Value Calculation
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use of
smaller inductor and capacitor values. Why would anyone
ever choose to operate at lower frequencies with larger
components? The answer is efficiency. A higher frequency
generally results in lower efficiency because of MOSFET
gate charge and switching losses. In addition to this basic
trade-off, the effect of inductor value on ripple current and
low current operation must also be considered.
The inductor value has a direct effect on ripple current.
The inductor ripple current ∆I
L
decreases with higher
inductance or frequency and increases with higher V
IN
:
∆I
L
=
V
IN
f • L
1−
V
IN
V
OUT






Accepting larger values of ∆I
L
allows the use of low
inductances, but results in higher output voltage ripple
and greater core losses. A reasonable starting point for
setting ripple current is ∆I
L
= 0.3(I
MAX
). The maximum
∆I
L
occurs at V
IN
= 1/2 V
OUT
.
The inductor value also has secondary effects. The tran-
sition to Burst Mode operation begins when the average
inductor current required results in a peak current below
10% of the current limit determined by R
SENSE
. Lower
inductor values (higher ∆I
L
) will cause this to occur at
lower load currents, which can cause a dip in efficiency in
the upper range of low current operation. In Burst Mode
operation, lower inductance values will cause the burst
frequency to decrease.
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 or molypermalloy
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, because increased inductance requires
more turns of wire, copper losses will increase.
Ferrite
core inductors have very low core loss and are
preferred at high switching frequencies, so design goals
can concentrate on copper loss and preventing satura-
tion. Ferrite core material saturates “hard,” which means
that inductance collapses abruptly 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!
Power MOSFET Selection
Tw o external power MOSFETs must be selected for each
controller in the LTC3788: one N-channel MOSFET for the
bottom (main) switch, and one N-channel MOSFET for the
top (synchronous) switch.
The peak-to-peak gate drive levels are set by the INTV
CC
voltage. This voltage is typically 5.4V during start-up
(see EXTV
CC
pin connection). Consequently, logic-level
threshold MOSFETs must be used in most applications.
The only exception is if low input voltage is expected (V
IN
< 5V); then, sub-logic level threshold MOSFETs (V
GS(TH)
< 3V) should be used. Pay close attention to the BV
DSS