LTC3855 LINER [Linear Technology], LTC3855 Datasheet - Page 20

• Manufacturer Part Number: LTC3855
• Description: Dual, Fast, Accurate Step-Down DC/DC Controller
• Manufacturer: LINER [Linear Technology]

LTC3838
APPLICATIONS INFORMATION
The inductor value has a direct effect on ripple current.
The inductor ripple current ∆I
inductance or frequency and increases with higher V
Accepting larger values of ∆I
tances, but results in higher output voltage ripple, higher
ESR losses in the output capacitor, and greater core losses.
A reasonable starting point for setting ripple current is ∆I
= 0.4 • I
input voltage. To guarantee that ripple current does not
exceed a specified maximum, the inductance should be
chosen according to:
Inductor Core Selection
Once the value for L is known, the type of inductor must
be selected. The two basic types are iron powder and fer-
rite. The iron powder types have a soft saturation curve
which means they do not saturate hard like ferrites do.
However, iron powder type inductors have higher core
losses. Ferrite designs have very low core loss and are
preferred at high switching frequencies, so design goals
can concentrate on copper loss and preventing saturation.
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 core material saturates hard, which means that in-
ductance collapses abruptly when the peak design current
is exceeded. This results an abrupt increase in inductor
ripple current and consequent output voltage ripple. Do
not allow the core to saturate!
A variety of inductors designed for high current, low volt-
age applications are available from manufacturers such as
20
L =
ΔI
L
⎛
⎜
⎝
=
MAX
f • ΔI
⎛
⎝ ⎜
V
f •L
V
OUT
. The maximum ∆I
OUT
L(MAX)
⎞
⎠ ⎟
⎛
⎝ ⎜
1–
⎞
⎟ 1–
⎠
⎛
⎜
⎝
V
V
OUT
IN
V
IN(MAX)
⎞
⎠ ⎟
L
V
OUT
allows the use of low induc-
L
L
occurs at the maximum
decreases with higher
⎞
⎟
⎠
IN
:
L
Sumida, Panasonic, Coiltronics, Coilcraft, Toko, Vishay,
Pulse and Würth.
Current Sense Pins
Inductor current is sensed through voltage between
SENSE
comparators. The input voltage range of the SENSE pins is
–0.5V to 5.5V. Care must be taken not to float these pins
during normal operation. The SENSE
impedance inputs. There is no bias current into a SENSE
pin when its corresponding channel’s SENSE
up from below 1.1V and stays below 1.4V. But there is a
small (~1μA) current flowing into a SENSE
corresponding SENSE
stays above 1.1V. Such currents also exist on SENSE
But in addition, each SENSE
resistor to SGND. The resulted current (V
dominate the total current flowing into the SENSE
SENSE
account when designing either R
current sensing.
Current Limit Programming
The current sense comparators’ maximum trip voltage
between SENSE
ITH is clamped at its maximum 2.4V, is set by the voltage
applied to the V
The valley current mode control loop does not allow the
inductor current valley to exceed 0.05V
one should allow sufficient margin, to account for tolerance
of the parts and external component values. Note that ITH
is close to 2.4V when in current limit.
An external resistive divider from INTV
the voltage on a V
in a maximum sense voltage between 30mV and 100mV.
Such wide voltage range allows for variety of applications.
The V
to force internal defaults. When V
device has an equivalent V
is tied to INTV
V
SENSE(MAX)
RNG
+
+
and SENSE
and SENSE
pin can also be tied to either SGND or INTV
CC
= 0.05V
RNG
+
, the device has an equivalent V
and SENSE
RNG
–
pin and is given by:
–
pins, the inputs of the internal current
pin currents have to be taken into
pin between 0.6V and 2V, resulting
–
RNG
pin ramps down from 1.4V and
RNG
–
–
(or “sense voltage”), when
of 0.6V. When the V
pin has an internal 500k
RNG
SENSE
+
CC
is tied to SGND, the
pins are quasi-high
can be used to set
or DCR inductor
RNG
OUT
+
. In practice,
pin when its
–
/500k) will
pin ramps
RNG
RNG
–
–
of 2V.
pins.
pins.
3838fa
pin
CC
+