ltc3787gn Linear Technology Corporation, ltc3787gn Datasheet - Page 17

• Manufacturer Part Number: ltc3787gn
• Description: Ltc3787 - Polyphase Synchronous Boost Controller
• Manufacturer: Linear Technology Corporation

applicaTions inForMaTion
To scale the maximum inductor DCR to the desired sense
resistor value, use the divider ratio:
C1 is usually selected to be in the range of 0.1μF to 0.47μF .
This forces R1|| R2 to around 2k, reducing error that might
have been caused by the SENSE
The equivalent resistance R1|| R2 is scaled to the room
temperature inductance and maximum DCR:
The sense resistor values are:
The maximum power loss in R1 is related to duty cycle,
and will occur in continuous mode at V
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.
R
R1||R2 =
R1=
P
LOSS _ R1
D
=
R1||R2
DCR
R
R
D
(DCR at 20°C) • C1
SENSE(EQUIV)
MAX
=
; R2 =
(V
at T
OUT
L
L(MAX)
− V
R1• R
1− R
R1
IN
) • V
D
D
–
IN
pin’s ±1μA current.
IN
= 1/2V
OUT
:
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 ΔI
inductance or frequency and increases with higher V
Accepting larger values of ΔI
inductances, but results in higher output voltage ripple
and greater core losses. A reasonable starting point for
setting ripple current is ΔI
ΔI
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
inductor values (higher ΔI
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.
L
∆I
occurs at V
L
=
f • L
V
IN

 
IN
1−
= 1/2V
V
V
OUT
IN
OUT

 
L
L
.
) will cause this to occur at
= 0.3(I
L
L
allows the use of low
decreases with higher
MAX
LTC3787
). The maximum
SENSE
. Lower
17
3787fa
IN
: