ltc3891 Linear Technology Corporation, ltc3891 Datasheet - Page 16

• Manufacturer Part Number: ltc3891
• Description: Ltc3891 - Low Iq, 60v Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

LTC3891
APPLICATIONS INFORMATION
The equivalent resistance R1 || R2 is scaled to the tem-
perature 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 the maximum input
voltage:
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 inter-
related n that higher operating frequencies allow the use
of smaller inductor and capacitor values. So 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 switching and gate charge losses. In addi-
tion to this basic trade-off, the effect of inductor value
on ripple current and low current operation must also be
considered.
16
R1|| R2 =
R1=
P
LOSS
R1|| R2
R1=
R
D
(
DCR at 20°C
(
; R2 =
V
IN(MAX)
L
R1• R
1– R
– V
R1
)
OUT
D
• C1
D
)
• V
OUT
The inductor value has a direct effect on ripple current.
The inductor ripple current, ∆I
inductance or higher frequency and increases with higher
V
Accepting larger values of ∆I
ductances, but results in higher output voltage ripple and
greater core losses. A reasonable starting point for setting
ripple current is ∆I
at the maximum input voltage.
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 ∆I
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
value 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
for 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 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!
IN
ΔI
:
L
=
( )
f
1
( )
L
V
OUT
L
= 0.3(I



1–
MAX
V
L
V
OUT
) will cause this to occur at
IN
L
). The maximum ∆I
allows the use of low in-
L



, decreases with higher
SENSE
L
. Lower
occurs
3891f