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LT1946A Datasheet PDF : 12 Pages
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LT1946A
U
OPERATIO
The LT1946A uses a constant frequency, current mode
control scheme to provide excellent line and load regula-
tion. Please refer to Figure 2 for the following description
of the part’s operation. At the start of the oscillator cycle,
the SR latch is set, turning on the power switch Q1. The
switch current flows through the internal current sense
resistor generating a voltage. This voltage is added to a
stabilizing ramp and the resulting sum is fed into the
positive terminal of the PWM comparator A2. When this
voltage exceeds the level at the negative input of A2, the SR
latch is reset, turning off the power switch. The level at the
negative input of A2 (VC pin) is set by the error amplifier
(A1) and is simply an amplified version of the difference
between the feedback voltage and the reference voltage of
1.250V. In this manner, the error amplifier sets the correct
peak current level to keep the output in regulation.
Two functions are provided to enable a very clean start-up
for the LT1946A. Frequency foldback is used to reduce the
oscillator frequency by one-third when the FB pin is below
a nominal value of 0.5V. This is accomplished via com-
parator A3. This feature reduces the minimum duty cycle
that the part can achieve thus allowing better control of the
switch current during start-up. When the FB pin voltage
goes above 0.5V, the oscillator returns to the normal
frequency of 2.7MHz. A soft-start function is also provided
by the LT1946A. When the part is brought out of shut-
down, 4µA of current is sourced out of the SS pin. By
connecting an external capacitor to the SS pin, the rate of
voltage rise on the pin can be set. Typical values for the
soft-start capacitor range from 10nF to 200nF. The SS pin
directly limits the rate of rise on the VC pin, which in turn
limits the peak switch current. Current limit is not shown
in Figure 2. The switch current is constantly monitored and
not allowed to exceed the nominal value of 2.1A. If the
switch current reaches 2.1A, the SR latch is reset regard-
less of the output of comparator A2. This current limit
protects the power switch as well as various external
components connected to the LT1946A.
APPLICATIO S I FOR ATIO
Inductor Selection
Several inductors that work well with the LT1946A are
listed in Table 1. This table is not complete, and there are
many other manufacturers and devices that can be used.
Consult each manufacturer for more detailed information
and for their entire selection of related parts, as many
different sizes and shapes are available. Ferrite core induc-
tors should be used to obtain the best efficiency, as core
losses at 2.7MHz are much lower for ferrite cores than for
the cheaper powdered-iron ones. Choose an inductor that
can handle at least 1.5A without saturating, and ensure
that the inductor has a low DCR (copper-wire resistance)
to minimize I2R power losses. A 1.5µH to 4.7µH inductor
will be the best choice for most LT1946A designs. Note
that in some applications, the current handling require-
ments of the inductor can be lower, such as in the SEPIC
topology where each inductor only carries one-half of the
total switch current.
The inductors shown in Table 1 were chosen for small size.
For better efficiency, use similar valued inductors with a
larger volume.
6
Table 1. Recommended Inductors - LT1946A
PART
MAX
L DCR
(µH) (mΩ)
Size
LxWxH
(mm)
RLF5018-1R5M2R1 1.5 25 5.2x5.6x1.8
RLF5018-2R7M1R8 2.7 33
RLF5018-4R7M1R4 4.7 45
RLF5018-100MR94 10.0 67
LPO1704-122MC
LPO1704-222MC
1.2 80 5.5x6.6x1.0
2.2 120
CR43-2R2
CR43-3R3
2.2 71 4.5x4.0x3.2
3.3 86
VENDOR
TDK
(847) 803-6100
www.tdk.com
Coilcraft
(800) 322-2645
www.coilcraft.com
Sumida
(847) 956-0666
www.sumida.com
Capacitor Selection
Low ESR (equivalent series resistance) capacitors should
be used at the output to minimize the output ripple voltage.
Multilayer ceramic capacitors are an excellent choice, as
they have an extremely low ESR and are available in very
small packages. X5R dielectrics are preferred, followed by
X7R, as these materials retain the capacitance over wide
voltage and temperature ranges. A 2.2µF to 20µF output
sn1946a 1946afs
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