LTC3838-2_15 Datasheet, PDF (22/56 Pages) Linear Technology – Dual, Fast, Accurate Step-Down DC/DC Controller with xternal Reference Voltage and Dual Differential Output Sensing

LTC3838-2
APPLICATIONS INFORMATION
The inductor value has a direct effect on ripple current.
The inductor ripple current ∆I_L decreases with higher
inductance or frequency and increases with higher V_IN:
∆I_L
=


V_OUT
f •L
_1–
V_OUT
V_IN



Accepting larger values of ∆I_L allows the use of low induc-
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_L
= 0.4 • I_MAX. The maximum ∆I_L occurs at the maximum
input voltage. To guarantee that ripple current does not
exceed a specified maximum, the inductance should be
chosen according to:
L
=


f
•
V_OUT
∆I_{L(MAX )}

^1–
V_OUT
V_{IN(MAX )}


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
voltage applications are available from manufacturers
such as Sumida, Panasonic, Coiltronics, Coilcraft, Toko,
Vishay, Pulse and Würth. In designs of higher switching
frequency, especially in the MHz range, core loss can be
very significant. Be sure to check with the manufacturer
on the frequency characteristics of the core material.
Current Sense Pins
Inductor current is sensed through voltage between
SENSE⁺ and SENSE^– pins, the inputs of the internal current
comparators. 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 V_SENSE(MAX)), when ITH
is clamped at its maximum 2.4V, is 30mV typical.
The valley current mode control loop does not allow the
inductor current valley to exceed V_SENSE(MAX). But note
that the peak inductor current is higher than this valley
current limit by the amount of the inductor ripple current.
Also when calculating the peak current limit, allow sufficient
margin to account for the tolerance of V_SENSE(MAX) as given
in the Electrical Characteristics table, and variations in
values of external components (such as the inductor), as
well as the range of the input voltage(since ripple current
is a function of input voltage).
Either low value series current sensing resistor (R_SENSE)
or the DC resistance of the inductor (DCR) can be used
to monitor the inductor current. The choice between the
two current sensing schemes is largely a design trade-
off among accuracy, power consumption, and cost. The
R_SENSE method offers more precise control of the current
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