LTC3613 Datasheet, PDF (15/36 Pages) Linear Technology – 24V, 15A Monolithic Step Down Regulator

LTC3613
APPLICATIONS INFORMATION
The inductor value has a direct effect on ripple current.
The inductor ripple current, ΔI_L, decreases with higher
inductance or frequency and increases with higher V_IN:
ΔI_L =
V_OUT
f •L
⎛
• _⎝⎜ 1–
V_OUT
V_IN
⎞
⎠⎟
Accepting larger values of ΔI_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
ΔI_L = 0.4 • I_OUT(MAX) where I_OUT(MAX) is the maximum
output current for the application. The maximum ΔI_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
ΔI_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 Würth.
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. The maximum
output current is given by:
I_{O U T(M AX )}
=
V_{SE N SE(M AX )}
R_SENSE
+
1
2
ΔI_L
The V_SENSE(MAX) is shown in the figure “Maximum Current
Sense Voltage vs Temperature” in the Typical Performance
Characteristics. 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. When
setting current limit, ensure that the junction temperature
does not exceed the rating of 125°C.
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