lt1941 Linear Technology Corporation, lt1941 Datasheet - Page 15

• Manufacturer Part Number: lt1941
• Description: Triple Monolithic Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
can handle at least 1.5A of current without saturating and
ensure that the inductor has a low DCR (copper-wire re-
sistance) to minimize I
inductors, each inductor need only handle one-half of the
total switch current so that 0.75A per inductor is sufficient.
A 4.7µH to 15µH coupled inductor or two 15µH to 20µH
uncoupled inductors will usually be the best choice for most
LT1941 inverter designs. A 4.7µH to 15µH inductor will be
the best choice for most LT1941 boost designs. In this case,
the single inductor must carry the entire 1.5A peak switch
current.
Table 4. Inductors
PART NUMBER
Coiltronics
TP3-4R7
TP4-100
Sumida
CD73-100
CDRH5D18-6R2
CDRH5D28-100
CDRH4D28-100
Coilcraft
D03314-103
1008PS-103
Output Capacitor Selection
Use low ESR (equivalent series resistance) capacitors at
the output to minimize the output ripple voltage. Multi-
layer ceramic capacitors are an excellent choice; they
have an extremely low ESR and are available in very small
packages. X7R dielectrics are preferred, followed by X5R,
as these materials retain their capacitance over wide
voltage and temperature ranges. A 4.7µF to 20µF output
capacitor is sufficient for most LT1941 applications. Solid
tantalum or OS-CON capacitors will work but they will
occupy more board area and will have a higher ESR than
a ceramic capacitor. Always use a capacitor with a suffi-
cient voltage rating.
VALUE
U
(µH)
4.7
6.2
10
10
10
10
10
10
2
R power losses. If using uncoupled
U
I
SAT(DC)
1.44
0.78
1.5
1.5
1.4
1.3
1.0
0.8
(A)
W
0.181
0.146
0.080
0.071
0.048
0.095
0.520
0.920
DCR
(Ω)
U
HEIGHT
(mm)
2.2
3.0
3.5
2.0
3.0
3.0
1.4
2.8
Diode Selection
A Schottky diode is recommended for use with the LT1941
inverter/boost regulator. The Microsemi UPS120 is a very
good choice. Where the input to output voltage differential
exceeds 20V, use the UPS140 (a 40V diode). These diodes
are rated to handle an average forward current of 1A. For
applications where the average forward current of the
diode is less than 0.5A, use an ON Semiconductor
MBR0520L diode. The load current for boost, SEPIC and
inverting configurations is equal to the average diode
current.
BIAS2 Pin Considerations
The BIAS2 pin provides the drive current for the inverter/
boost switch. The voltage source on the BIAS2 line should
be able to supply the rated current and be at a minimum
of 2.5V. For highest efficiency, use the lowest voltage
source possible (V
the V
INPUT CAPACITOR SELECTION
Bypass the input of the LT1941 circuit with a 10µF 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 ca-
pacitor is required to reduce the resulting voltage ripple at
the LT1941 input and to force this switching current into
a tight local loop, minimizing EMI. The input capacitor
must have low impedance at the switching frequency to do
this effectively and it must have an adequate ripple current
rating. With two switchers operating at the same frequency
but with different phases and duty cycles, calculating the
input capacitor RMS current is not simple; however, a
conservative value is the RMS input current for the chan-
nel that is delivering the most power (V
C
IN RMS
BIAS2
(
)
• I
=
BIAS2
I
OUT
power loss inside the part.
OUT
•
V
= 3.3V, for example) to minimize
OUT
(
V
V
IN
IN
–
V
OUT
OUT
)
<
times I
I
LT1941
OUT
2
15
OUT
1941fa
):