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LTM4661 Fiches technique(PDF) 8 Page - Linear Technology

No de pièce LTM4661
Description  15V, 4A Step-Up 關Module Regulator
Download  20 Pages
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Fabricant  LINER [Linear Technology]
Site Internet  http://www.linear.com
Logo LINER - Linear Technology

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LTM4661
8
4661f
For more information www.linear.com/LTM4661
The typical LTM4661 application circuit is shown in
Figure 17. External component selection is primarily de-
termined by the input voltage, the output voltage and the
maximum load current.
Minimum Input Voltage
The LTM4661 is designed to allow start-up from input
voltages as low as 1.8V. When VOUT exceeds 2.5V, the
LTM4661 continues to regulate its output, even when VIN
falls as low as 0.7V. This feature extends operating times
by maximizing the amount of energy that can be extracted
from the input source. The limiting factors for the applica-
tion become the availability of the power source to supply
sufficient power to the output at the low input voltage,
and the maximum duty cycle, which is clamped at 94%.
In a step-up boost converter, the duty cycle can be cal-
culated at:
D = 1–
VIN • η
VOUT
where ηistheconverterefficiency.85%isagoodestimate
to start with.
Note that at low input voltages, voltage drops due to series
resistance become critical and greatly limit the power
delivery capability of the converter.
Output Current Capability
The LTM4661 is designed to provide up to 4A switch cur-
rent. Due to the nature of the boost converter, the actually
output current capability depends highly on the input/
output voltage ratio. The peak inductor current, same as
switch current, in a boost converter can be calculated as:
ISW =
VIN • D
2 • fS • L
+
IOUT
1– D
where D is the duty cycle showing above and fS = 1MHz
and L = 2.2µH/2 = 1.1µH.
APPLICATIONS INFORMATION
Based on common input and output values, Table 1 lists
differentoutputcurrentcapabilityoftheLTM4661module.
Table 1. Output Current Capability vs Input Voltage
VIN (V)
3.3
5
VOUT (V)
5
8
12
15
8
12
15
Output Current (A)
1.9
1
0.7
0.5
1.7
1
0.7
Efficiency (%)
82
84
83
81
88
87
88
Peak Switch Current (A)
4
3.8
3.9
4
3.94 4.1
4.1
Output Voltage Programming
The PWM controller has an internal 1.2V reference volt-
age. As shown in the Block Diagram, a 100k 0.5% internal
feedback resistor connects VOUT and FB pins together.
Adding a resistor RFB from FB pin to GND, programs the
output voltage:
RFB =
1.2V
VOUT – 1.2V
• 100k
Table 2. VFB Resistor Value vs Various Output Voltages
VOUT (V)
1.2
2.5
3.3
5
8
12
15
RFB (k)
OPEN
93.1
57.6
31.6
17.8
11.0
8.66
For parallel operation of N-piece of LTM4661 modules, the
following equation can be used to solve for RFB:
RFB =
1.2V
VOUT – 1.2V
100k
N
Multiphase Operation
The LTM4661 uses a unique dual-phase single-output
architecture, rather than the conventional single phase of
otherboostconverters.Byinterleavingtwophasesequally
spaced 180° apart, both input and output current ripple
get significantly reduced as well as the amount of input
and output decoupling capacitor required.


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