Datasheet

LTC3876
20
3876f
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
Sumida, Panasonic, Coiltronics, Coilcraft, Toko, Vishay,
Pulse and Würth.
Current Sense Pins
Inductor current is sensed through voltage between
SENSE
+
and SENSE
–
pins, the inputs of the internal current
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
+
pins are quasi-high
impedance inputs. There is no bias current into a SENSE
+
pin when its corresponding channel’s SENSE
–
pin ramps
up from below 1.1V and stays below 1.4V. But there is a
small (~1µA) current flowing into a SENSE
+
pin when its
corresponding SENSE
–
pin ramps down from 1.4V and
stays above 1.1V. Such currents also exist on SENSE
–
pins.
But in addition, each SENSE
–
pin has an internal 500k
resistor to SGND. The resulted current (V
OUT
/500k) will
dominate the total current flowing into the SENSE
–
pins.
SENSE
+
and SENSE
–
pin currents have to be taken into
account when designing either R
SENSE
or DCR inductor
current sensing.
Current Limit Programming
The current sense comparators’ maximum trip voltage
between SENSE
+
and SENSE
–
(or “sense voltage”), when
ITH is clamped at its maximum, is set by the voltage ap-
plied to the V
RNG
pin and is given by:
V
SENSE(MAX)
= 0.05V
RNG
The valley current mode control loop does not allow the
inductor current valley to exceed 0.05V
RNG
. In practice, one
should allow sufficient margin, to account for tolerance of
the parts and external component values. Note that ITH is
close to 2.4V for channel 1 and 2.2V for channel 2 when
in positive current limit.
An external resistive divider from INTV
CC
can be used to set
the voltage on a V
RNG
pin between 0.6V and 2V, resulting
in a maximum sense voltage between 30mV and 100mV.
Such wide voltage range allows for variety of applications.
The V
RNG
pin can also be tied to either SGND or INTV
CC
to force internal defaults. When V
RNG
is tied to SGND, the
device has an equivalent V
RNG
of 0.6V. When the V
RNG
pin
is tied to INTV
CC
, the device has an equivalent V
RNG
of 2V.
R
SENSE
Inductor Current Sensing
The LTC3876 can be configured to sense the inductor
currents through either low value series current sensing
resistors (R
SENSE
) or inductor DC resistance (DCR). The
choice between the two current sensing schemes is largely
a design trade-off between cost, power consumption and
accuracy. DCR sensing is becoming popular because it
saves expensive current sensing resistors and is more
power efficient, especially in high current applications.
However, current sensing resistors provide the most ac-
curate current limits for the controller.
A typical R
SENSE
inductor current sensing scheme is shown
in Figure 3a. The filter components (R
F
, C
F
) need to be
placed close to the IC. The positive and negative sense
traces need to be routed as a differential pair close to-
gether and (4-wire) Kelvin connected underneath the sense
resistor, as shown in Figure 3b. Sensing current elsewhere
can effectively add parasitic inductance to the current sense
element, degrading the information at the sense terminals
and making the programmed current limit unpredictable.
APPLICATIONS INFORMATION