LT1946EMS8 Ver la hoja de datos (PDF) - Linear Technology
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LT1946EMS8 Datasheet PDF : 12 Pages
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OPERATIO
The LT1946 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 LT1946. Frequency foldback is used to reduce the
oscillator frequency by a factor of 3 when the FB pin is
LT1946
below a nominal value of 0.5V. This is accomplished via
comparator 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 exceeds 0.5V, the oscillator returns to the normal
frequency of 1.2MHz. A soft-start function is also provided
by the LT1946. When the part is brought out of shutdown,
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 al-
lowed to exceed the nominal value of 2.1A. If the switch
current reaches 2.1A, the SR latch is reset regardless of
the output of comparator A2. This current limit helps
protect the power switch as well as the external compo-
nents connected to the LT1946.
APPLICATIO S I FOR ATIO
Inductor Selection
Several inductors that work well with the LT1946 are listed
in Table 1. This table is not exclusive; there are many other
manufacturers and inductors 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 inductors
should be used to obtain the best efficiency, as core losses
at 1.2MHz 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 mini-
mize I2R power losses. A 4.7µH to 10µH inductor will be
the best choice for most LT1946 designs. Note that in
some applications, the current handling requirements 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.
Table 1. Recommended Inductors
PART
MAX
L DCR
(µH) (mΩ)
SIZE
L×W×H
(mm)
CDRH5D18-4R1 4.1
CDRH5D18-5R4 5.4
CDRH5D28-5R3 5.3
CDRH5D28-6R2 6.2
CDRH5D28-8R2 8.2
57 5.7 × 5.7 × 2
76
38 5.7 × 5.7 × 3
45
53
ELL6SH-4R7M 4.7 50 6.4 × 6 × 3
ELL6SH-5R6M 5.6 59
ELL6SH-6R8M 6.8 62
RLF5018T-
4R7M1R4
4.7 45 5.6 × 5.2 × 1.8
VENDOR
Sumida
(847) 956-0666
www.sumida.com
Panasonic
(408) 945-5660
www.panasonic.com
TDK
(847) 803-6100
www.tdk.com
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