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LTC3127 Datasheet(PDF) 14 Page - Linear Technology

Part # LTC3127
Description  1A Buck-Boost DC/DC Converter with Programmable Input Current Limit
PDF  20 Pages
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LTC3127 Datasheet(HTML) 14 Page - Linear Technology

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LTC3127

3127f
applicaTions inForMaTion
Capacitor Selection Example
In this example, a pulsed load application requires that
VOUT droops less than 300mV. The application is a Li-Ion
battery input to a 3.6V output. The pulsed load is a no-load
to a 1.5A step with a frequency of 217Hz and a duty cycle
of 12.5%. The input current limit is set to 500mA. In order
to meet the 300mV droop requirement, the amount of
capacitance must be calculated at the highest VIN to VOUT
step-up ratio. All of the following calculations assume a
minimum VIN of 3V and an efficiency of 90%.
Given the application, the following is known:
VIN = 3V
VOUT = 3.6V
IIN(MAX) = 500mA
IPULSE = 1.5A
ISTANDBY = 0A
h = 0.9
D = 0.125
T = 1/217Hz = 4.6ms
VDROOP = 300mV
Step 1: Check to make sure the application can provide
enough current to recover from the pulsed load using the
ILOAD(MAX) equation:
I
V
mA
V
A
LOAD MAX
(
)
•
• .
.
• .
=
=
3
500
0 9
0 125 3 6
3
The maximum load that can be pulsed at this VIN to VOUT
combination is 3A.
Step 2: Calculate the minimum output capacitance re-
quired.
C
A
V
mA
V
OUT MIN
(
)
.
•
• .
.
•
.
≥
−


1 5
3
500
0 9
3 6
0 125 •• .
.
4 6
300
2 15
ms
mV
mF
=
Step 3: For this application a 2.2mF Vishay Tantamount
tantalum,lowESRcapacitorisselected.Thiscapacitorhas
a maximum ESR of 0.04Ω. With the selected capacitor,
the amount of droop must be calculated:
V
A
V
mA
V
A
DROOP LOAD
_
.
•
• .
.
=
−
−


1 5
3
500
0 9
3 6
0

=
=
• .
• .
.
.
.
_
0 125 4 6
2 2
0 294
1
ms
mF
V
VDROOP ESR
55
3
500
0 9
3 6
0
0 04
0
A
V
mA
V
A
−
−


=
•
• .
.
• .
Ω
..
.
_
_
045
0 339
V
V
V
V
V
DROOP
DROOP LOAD
DROOP ESR
=
+
=
Due to the ESR of the capacitor, the total droop is greater
than 300mV. In this case, if the higher droop cannot be
accepted, a larger valued, lower ESR capacitor can be
selected.



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