LTC3899_15 Datasheet, PDF (21/38 Pages) Linear Technology – 60V Low IQ, Triple Output, Buck/Buck/Boost Synchronous Controller

LTC3899
Applications Information
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⁺ pin’s ±1μA current.
The equivalent resistance R1||R2 is scaled to the tempera-
ture inductance and maximum DCR:
R1R2
=
(DCR
at
L
20°C)
•
C1

The sense resistor values are:
R1=
R1

R2
;
R2 = ^R1•RD
R_D
1− R_D

The maximum power loss in R1 is related to duty cycle,
and will occur in continuous mode at the maximum input
voltage:
( ) _{PLOSS R1=}
V_IN(MAX) − V_OUT
R1
• V_OUT
For the boost controller, the maximum power loss in R1
will occur in continuous mode at V_IN = 1/2 • V_OUT:
( ) _{PLOSS R1=}
V_OUT(MAX) − V_IN
R1
• V_IN
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. 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 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 higher frequency. For the buck controllers,
∆I_L increases with higher V_IN:
∆I_L
=
(
1
f)(L
)
V_OUT


1−
V_OUT
V_IN


For the boost controller, ∆I_L increases with higher V_OUT:
∆I_L
=
(
1
f)(L
)
V_IN


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 the maximum input voltage for the bucks
and V_IN = 1/2 • V_OUT for the boost.
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 (30% for the boost) 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
value selected. As inductance increases, core losses go
down. Unfortunately, increased inductance requires more
turns of wire and therefore copper losses will increase.
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3899f
21