LTC3839 Datasheet, PDF (18/50 Pages) Linear Technology – Fast, Accurate, 2-Phase, Single-Output Step-Down DC/DC Controller

LTC3839
APPLICATIONS INFORMATION
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.
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Current Limit Programming
The current sense comparators’ maximum trip voltage
between SENSE⁺ and SENSE^– (or “sense voltage”), when
ITH is clamped at its maximum 2.4V, is set by the voltage
applied 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. Note that ITH
is close to 2.4V when in 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.
Sufficient margin should be allowed to account for IC
and external component tolerances. The Electrical Char-
acteristics (EC) table gives the maximum valley current
sense threshold, V_{SENSE(MAX)1,2} , which is the guaranteed
specification over the operating junction temperature range
for either of the two channels of LTC3839. When designing
an application, the maximum value in the EC table should
always be used to assure that the maximum possible
current in a single channel does not exceed the rating of
the external components, such as power MOSFETs and
inductors, in a worse case fault condition.
To ensure a multiphase single-output application can deliver
its desired full load current, the minimum output current
capability of the application can be determined from the
lower limits of V_SENSE(MAX). For LTC3839, this can be done
using either worst-case or statistical tolerancing. Worst-
case tolerancing is the most conservative and is calculated
from the minimum value of the single channel V_SENSE(MAX)
in the EC table, multiplied by the number of phases (e.g.,
2x in a 2-phase application). Statistical tolerancing takes
into consideration the distribution of both current limit
channels to predict the effective statistical limits of the sum
of multiple channels’ V_SENSE(MAX). Based on distributions
over temperature of the 2-channel-sum [V_SENSE(MAX)1 +
V_SENSE(MAX)2 ] from the characterization LTC3839, the
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