Datasheet

LTC3787
17
3787fc
APPLICATIONS INFORMATION
To scale the maximum inductor DCR to the desired sense
resistor value, use the divider ratio:
R
D
=
R
SENSE(EQUIV)
DCR
MAX
at T
L(MAX)
C1 is usually selected to be in the range of 0.1F to 0.47F.
This forces R1|| R2 to around 2k, reducing error that might
have been caused by the SENSE
–
pin’s ±1A current.
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
= 1/2V
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 conduc-
tion 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 in-
terrelated 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. Also, at
higher frequency the duty cycle of body diode conduction
is higher, which results in lower efficiency. 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
:
ΔI
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
= 1/2V
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
25% 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. Once the value of L is known, an
inductor with low DCR and low core losses should be
selected.