LT1945EMS Linear Technology, LT1945EMS Datasheet - Page 5

• Manufacturer Part Number: LT1945EMS
• Description: IC DC/DC CONV MICRPWR DUAL10MSOP
• Manufacturer: Linear Technology
• Type: Step-Up (Boost), Inverting, Flyback, Sepicr

Specifications of LT1945EMS

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 2
• Voltage - Output: 1.23 ~ 36 V
• Current - Output: 350mA
• Voltage - Input: 1.2 ~ 15 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 10-MSOP, Micro10™, 10-uMAX, 10-uSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -
• Frequency - Switching: -

OPERATION
current in inductors L1 and L2 begins ramping up. Once
the switch current reaches 350mA, comparator A2 resets
the one-shot, which turns off Q3 for 400ns. L2 continues
to deliver current to the output while Q3 is off. Q3 turns on
again and the inductor currents ramp back up to 350mA,
then A2 again resets the one-shot. This switching action
continues until the output voltage is charged up (until the
NFB1 pin reaches –1.23V), then A1 turns off the internal
circuitry and the cycle repeats.
APPLICATIONS INFORMATION
Choosing an Inductor
Several recommended inductors that work well with the
LT1945 are listed in Table 1, although 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. Many different sizes and
shapes are available. Use the equations and recommenda-
tions in the next few sections to fi nd the correct inductance
value for your design.
Table 1. Recommended Inductors
PART
LQH3C4R7
LQH3C100
LQH3C220
CD43-4R7
CD43-100
CDRH4D18-4R7
CDRH4D18-100
DO1608-472
DO1608-103
DO1608-223
Inductor Selection—Boost Regulator
The formula below calculates the appropriate inductor value
to be used for a boost regulator using the LT1945 (or at
least provides a good starting point). This value provides
a good tradeoff in inductor size and system performance.
Pick a standard inductor close to this value. A larger value
can be used to slightly increase the available output current,
but limit it to around twice the value calculated below, as
too large of an inductance will increase the output voltage
ripple without providing much additional output current.
VALUE (μH)
4.7
4.7
4.7
4.7
10
22
10
10
10
22
MAX DCR (Ω)
0.26
0.30
0.92
0.11
0.18
0.16
0.20
0.09
0.16
0.37
VENDOR
Murata
[phone]
www.murata.com
Sumida
[phone]
www.sumida.com
Coilcraft
[phone]
www.coilcraft.com
The second switching regulator is a step-up converter
(which generates a positive output) but the basic operation
is the same.The LT1945 contains additional circuitry to
provide protection during start-up and under short-circuit
conditions. When the FB2 pin voltage is less than approxi-
mately 600mV, the switch off-time is increased to 1.5μs
and the current limit is reduced to around 250mA (70%
of its normal value). This reduces the average inductor
current and helps minimize the power dissipation in the
power switch and in the external inductor and diode.
A smaller value can be used (especially for systems with
output voltages greater than 12V) to give a smaller physical
size. Inductance can be calculated as:
where V
and t
battery powered applications, use the minimum V
in the above equation. For most regulators with output
voltages below 7V, a 4.7μH inductor is the best choice,
even though the equation above might specify a smaller
value. This is due to the inductor current overshoot that
occurs when very small inductor values are used (see
Current Limit Overshoot section).
For higher output voltages, the formula above will give large
inductance values. For a 2V to 20V converter (typical LCD
Bias application), a 21μH inductor is called for with the
above equation, but a 10μH inductor could be used without
excessive reduction in maximum output current.
Inductor Selection—SEPIC Regulator
The formula below calculates the approximate inductor
value to be used for a SEPIC regulator using the LT1945.
As for the boost inductor selection, a larger or smaller
value can be used.
L
L
=
=
OFF
2
V
⎛
⎝ ⎜
D
OUT
= 400ns; for designs with varying V
V
= 0.4V (Schottky diode voltage), I
OUT
−
I
LIM
V
I
+
IN MIN
LIM
V
(
D
⎞
⎠ ⎟
)
t
+
OFF
V
D
t
OFF
LT1945
LIM
IN
= 350mA
such as
IN
value
1945fa
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