lt3724 Linear Technology Corporation, lt3724 Datasheet - Page 12

• Manufacturer Part Number: lt3724
• Description: High Voltage, Current Mode Switching Regulator Controller
• Manufacturer: Linear Technology Corporation

LT3724
APPLICATIO S I FOR ATIO
The basic LT3724 step-down (buck) application, shown in
the Typical Application on the front page, converts a larger
positive input voltage to a lower positive or negative
output voltage. This Application Information section as-
sists selection of external components for the require-
ments of the power supply.
R
The current sense resistor, R
current of the supply (See Typical Application on front
page). Its value is chosen based on the maximum required
output load current. The LT3724 current sense amplifier
has a maximum voltage threshold of, typically, 150mV.
Therefore, the peak inductor current is 150mV/R
The maximum output load current, I
inductor current minus half the peak-to-peak ripple cur-
rent, ΔI
Allowing adequate margin for ripple current and external
component tolerances, R
follows:
Typical values for R
to 0.05Ω.
Inductor Selection
The critical parameters for selection of an inductor are
minimum inductance value, volt-second product, satura-
tion current and/or RMS current.
The minimum inductance value is calculated as follows:
f
The typical range of values for ΔI
(0.5 • I
current of the supply. Using ΔI
good design compromise between inductor performance
versus inductor size and cost. Higher values of ΔI
increase the peak currents, requiring more filtering on the
input and output of the supply. If ΔI
12
SW
SENSE
R
L V
is the switch frequency (200kHz).
SENSE
≥
OUT(MAX)
L
Selection
OUT
.
=
•
I
OUT MAX
f
100
SW
V
), where I
IN MAX
(
•
mV
(
U
V
IN MAX
SENSE
)
(
)
–
U
OUT(MAX)
V
SENSE
)
are in the range of 0.005Ω
OUT
SENSE
•
Δ
L
I
= 0.3 • I
L
L
, monitors the inductor
can be calculated as
L
W
is (0.2 • I
is the maximum load
OUT(MAX)
is too high, the slope
OUT(MAX)
OUT(MAX)
, is the peak
U
yields a
SENSE
L
) to
will
.
compensation circuit is ineffective and current mode
instability may occur at duty cycles greater than 50%.
Lower values of ΔI
netics. A value of ΔI
of I
the supply.
Some magnetics vendors specify a volt-second product in
their datasheet. If they do not, consult the magnetics
vendor to make sure the specification is not being ex-
ceeded by your design. The volt-second product is calcu-
lated as follows:
The magnetics vendors specify either the saturation cur-
rent, the RMS current or both. When selecting an inductor
based on inductor saturation current, use the peak current
through the inductor, I
saturation current specification is the current at which the
inductance, measured at zero current, decreases by a
specified amount, typically 30%.
When selecting an inductor based on RMS current rating,
use the average current through the inductor, I
The RMS current specification is the RMS current at which
the part has a specific temperature rise, typically 40°C,
above 25°C ambient.
After calculating the minimum inductance value, the volt-
second product, the saturation current and the RMS
current for your design, select an off-the-shelf inductor. A
list of magnetics vendors can be found at www.linear.com,
or contact the Linear Technology Application Department.
For more detailed information on selecting an inductor,
please see the “Inductor Selection” section of Linear
Technology Application Note 44.
Step-Down Converter: MOSFET Selection
The selection criteria of the external N-channel standard
level power MOSFET include on resistance(R
verse transfer capacitance (C
voltage (V
continuous drain current.
Volt-second (μsec) =
OUT(MAX)
DSS
ripple current around the DC output current of
), total gate charge (Q
L
L
require larger and more costly mag-
= 0.3 • I
OUT(MAX)
(
V
IN MAX
RSS
(
OUT(MAX)
V
IN MAX
), maximum drain source
(
)
+ ΔI
–
V
G
OUT
)
L
produces a ±15%
), and maximum
•
/2. The inductor
f
SW
)•
V
DS(ON)
OUT
OUT(MAX)
), re-
3724fb
.