Datasheet

LTC3833
15
3833f
APPLICATIONS INFORMATION
ripple current does not exceed a specified maximum, the
inductance should be chosen according to:
L =
V
OUT
f • ∆I
L(MAX)
• 1−
V
OUT
V
IN(MAX)






Once the value for L is known, the type of inductor must
be selected. High efficiency converters generally cannot
tolerate the core loss of low cost powdered iron cores,
forcing the use of more expensive ferrite, molypermalloy
or Kool Mμ cores. Ferrite core material saturates hard,
meaning that inductance collapses abruptly when the
peak design current is exceeded. This results in 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 Wurth.
Current Sense Pins and Current Limit Programming
Inductor current is sensed through the SENSE
+
and
SENSE
–
pins and fed into the internal current compara-
tors. The common mode input voltage range of the cur-
rent comparators is –0.5V to 5.5V. Both SENSE pins are
high impedance inputs. When the common mode range
is between –0.5V to 1.1V, there is no input bias current,
and when between 1.4V and 5.5V, there is less than 1μA
of current flowing into the pins. Between 1.1V and 1.4V,
the input bias current will be zero if the common mode
voltage is ramped up from 1.1V and less than 1μA if the
common mode voltage is ramped down from 1.4V. The
high impedance inputs to the current comparator allow
accurate DCR sensing. However, care must be taken not
to float these pins during normal operation.
The maximum allowed sense voltage V
SENSE(MAX)
between
SENSE
+
and SENSE
–
is set by the voltage applied to the
V
RNG
pin and is given by:
V
SENSE(MAX)
= 0.05 • V
RNG
The current mode control loop does not allow the induc-
tor current valleys to exceed 0.05 • V
RNG
. In practice, one
should allow sufficient margin to account for variations
in the LTC3833 and external component values. 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 the V
RNG
pin between 0.6V and 2V,
resulting in maximum sense voltages between 30mV and
100mV. The wide voltage sense range allows for a 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 operates with a maximum sense
voltage of 30mV. When the V
RNG
pin is tied to INTV
CC
, the
device operates with a maximum sense voltage of 50mV.
R
SENSE
Inductor Current Sensing
A typical R
SENSE
inductor current sensing scheme is
shown in Figure 3. R
SENSE
is chosen based on the required
maximum output current. Given the maximum current,
I
OUT(MAX)
, maximum sense voltage, V
SENSE(MAX)
, set by the
V
RNG
pin, and maximum inductor ripple current, ∆I
L(MAX)
,
the value of R
SENSE
can be chosen as:
R
SENSE
=
V
SENSE(MAX)
I
OUT(MAX)
–
∆I
L(MAX)
2
Conversely, given R
SENSE
and I
OUT(MAX)
, V
SENSE(MAX)
and thus the V
RNG
voltage could be determined from the
above equation. To assure that the maximum rated output
current can be supplied for different operating conditions
and component variations, sufficient design margin should
be built into these calculations.
Figure 3. R
SENSE
Current Sensing
R
F
R ESL
R
SENSE
RESISTOR
AND
PARASITIC INDUCTANCE
FILTER COMPONENTS
PLACED NEAR SENSE PINS
R
F
SENSE
+
LTC3833
SENSE
–
C
F
3833 F03
V
OUT