LT1943 데이터 시트보기 (PDF) - Linear Technology
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LT1943 Datasheet PDF : 20 Pages
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U
OPERATIO
DC = | VOUT |
VIN+ | VOUT |
The LT1943 can still be used in applications where the duty
cycle, as calculated above, is above the maximum. How-
ever, the part must be operated in discontinuous mode so
that the actual duty cycle is reduced.
Inductor Selection
Several inductors that work well with the LT1943 regulator
are listed in Table 4. Besides these, many other inductors
will work. Consult each manufacturer for detailed informa-
tion and for their entire selection of related parts. Use
ferrite core inductors to obtain the best efficiency, as core
losses at frequencies above 1MHz are much lower for
ferrite cores than for powdered-iron units. A 10µH to 22µH
inductor will be the best choice for most LT1943 step-up
and charge pump designs. Choose an inductor that can
carry the entire switch current without saturating. For
inverting and SEPIC regulators, a coupled inductor, or two
separate inductors is an option. When using coupled
inductors, choose one that can handle at least the switch
current without saturating. If using uncoupled inductors,
each inductor need only handle approximately one-half of
the total switch current. A 4.7µH to 15µH coupled inductor
or two 10µH to 22µH uncoupled inductors will usually be
the best choice for most LT1943 inverting and SEPIC
designs.
Table 4. Inductors.
Part Number
Value (µH)
Coiltronics
TP3-4R7
4.7
TP4-100
10
Sumida
CD73-100
10
CDRH5D18-6R2
6.2
CDRH4D28-100
10
CDRH4D28-100
10
Coilcraft
DO3314-103
10
1008PS-103
10
IRMS (A)
1.5
1.5
1.44
1.4
1.3
1.0
0.8
0.78
DCR (Ω)
0.181
0.146
0.080
0.071
0.048
0.095
0.520
0.920
Height (mm)
2.2
3.0
3.5
2.0
3.0
3.0
1.4
2.8
LT1943
Output Capacitor Selection
Use low ESR (equivalent series resistance) capacitors 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 pack-
ages. X7R dielectrics are preferred, followed by X5R, as
these materials retain their capacitance over wide voltage
and temperature ranges. A 10µF to 22µF output capacitor
is sufficient for most LT1943 applications. Even less
capacitance is required for outputs with |VOUT| > 20V or
|IOUT| < 100mA. Solid tantalum or OS-CON capacitors will
also work, but they will occupy more board area and will
have a higher ESR than a ceramic capacitor. Always use a
capacitor with a sufficient voltage rating.
Diode Selection
A Schottky diode is recommended for use with the LT1943
switcher 2 and switcher 4. The Schottky diode for switcher
3 is integrated inside the LT1943. Choose diodes for
switcher 2 and switcher 4 rated to handle an average
current greater than the load current and rated to handle
the maximum diode voltage. The average diode current in
the step-up, SEPIC, and inverting configurations is equal
to the load current. Each of the two diodes in the charge
pump configurations carries an average diode current
equal to the load current. The maximum diode voltage in
the step-up and charge pump configurations is equal to
|VOUT|. The maximum diode voltage in the SEPIC and
inverting configurations is VIN + |VOUT|.
Input Capacitor Selection
Bypass the input of the LT1943 circuit with a 4.7µF or
higher ceramic capacitor of X7R or X5R type. A lower value
or a less expensive Y5V type will work if there is additional
bypassing provided by bulk electrolytic capacitors or if the
input source impedance is low. The following paragraphs
describe the input capacitor considerations in more detail.
Step-down regulators draw current from the input supply
in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage ripple
at the LT1943 input and to force this switching current into
a tight local loop, minimizing EMI. The input capacitor
1943fa
15
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