ltc3827ig-1 Linear Technology Corporation, ltc3827ig-1 Datasheet - Page 13

• Manufacturer Part Number: ltc3827ig-1
• Description: Low Iq, Dual, 2-phase Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
R
R
The current comparator has a maximum threshold of
100mV/R
SGND to 10V. The current comparator threshold sets the
peak of the inductor current, yielding a maximum average
output current I
peak-to-peak ripple current, ΔI
Allowing a margin for variations in the IC and external
component values yields:
When using the controller in very low dropout conditions,
the maximum output current level will be reduced due to
the internal compensation required to meet stability
criterion for buck regulators operating at greater than
50% duty factor. A curve is provided in the Typical
Performance Characteristics section to estimate this re-
duction in peak output current level depending upon the
operating duty factor.
Operating Frequency and Synchronization
The choice of operating frequency, is a trade-off between
efficiency and component size. Low frequency operation
improves efficiency by reducing MOSFET switching losses,
both gate charge loss and transition loss. However, lower
frequency operation requires more inductance for a given
amount of ripple current.
The internal oscillator for each of the LTC3827-1’s controllers
runs at a nominal 400kHz frequency when the PLLLPF pin
is left floating and the PLLIN/MODE pin is a DC low or high.
Pulling the PLLLPF to INTV
pulling the PLLLPF to SGND selects 250kHz operation.
Alternatively, the LTC3827-1 will phase-lock to a clock
signal applied to the PLLIN/MODE pin with a frequency
between 140kHz and 650kHz (see Phase-Locked Loop
and Frequency Synchronization).
Inductor Value Calculation
The operating frequency and inductor selection are inter-
related in that higher operating frequencies allow the use
SENSE
SENSE
R
SENSE
Selection For Output Current
is chosen based on the required output current.
SENSE
=
80
I
MAX
MAX
mV
and an input common mode range of
U
equal to the peak value less half the
U
CC
selects 530kHz operation;
L
.
W
U
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 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 ΔI
tance 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
occurs at the maximum input voltage.
The inductor value also has secondary effects. The transi-
tion 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
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.
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, increased inductance
requires more turns of wire and therefore copper losses
will increase.
Ferrite designs have very low core loss and are preferred
at high switching frequencies, so design goals can con-
centrate on copper loss and preventing saturation. Ferrite
ΔI
L
=
( )( )
f L
1
V
OUT
⎛
⎜
⎝
1
–
V
L
L
V
=0.3(I
OUT
L
decreases with higher induc-
IN
) will cause this to occur at
⎞
⎟
⎠
L
MAX
allows the use of low
LTC3827-1
). The maximum ΔI
SENSE
IN
. Lower
:
13
38271fd
L