LTC3127 Linear Technology, LTC3127 Datasheet - Page 14

• Manufacturer Part Number: LTC3127
• Description: 1A Buck-Boost DC/DC Converter
• Manufacturer: Linear Technology

LTC3127
applicaTions inForMaTion
Capacitor Selection Example
In this example, a pulsed load application requires that
V
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 V
step-up ratio. All of the following calculations assume a
minimum V
Given the application, the following is known:
V
V
I
I
I
h = 0.9
D = 0.125
T = 1/217Hz = 4.6ms
V
Step 1: Check to make sure the application can provide
enough current to recover from the pulsed load using the
I
The maximum load that can be pulsed at this V
combination is 3A.

IN(MAX)
PULSE
STANDBY
LOAD(MAX)
OUT
IN
OUT
DROOP
I
LOAD MAX
= 3V
droops less than 300mV. The application is a Li-Ion
= 3.6V
= 1.5A
= 300mV
= 500mA
(
= 0A
equation:
IN
)
of 3V and an efficiency of 90%.
=
3
V
0 125 3 6
•
.
500
mA
• .
• .
V
0 9
=
3
A
IN
IN
to V
to V
OUT
OUT
Step 2: Calculate the minimum output capacitance re-
quired.
Step 3: For this application a 2.2mF Vishay Tantamount
tantalum, low ESR capacitor is selected. This capacitor has
a maximum ESR of 0.04Ω. With the selected capacitor,
the amount of droop must be calculated:
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.
V
=
V



=
V


 
1
DROOP LOAD
1 5
DROOP ESR
DROOP
0 294
0
C
.
.
5 5
.
. .
OUT MIN
045
A
A
−
−
(
_
_
V
V
=

 

 
3
3
V
V
V
)
DROOP LOAD
≥
=
•
•
=
500
500

 
•
3 6
1 5
3 6
.
.
0 125 • • .
.
_
mA
mA
.
V
A
V
300
−
• .
• .
2 2
3
0 9
0 9
.
+
V
mV
4 6
mF
V
•
−
DROOP ESR
−
500
ms
3 6
0
0
www.DataSheet4U.com
.
A
A
mA

 

 
V
=






_
• .
• .
• .
2 15
0 04
0 125 4 6
0 9
.
=

 
mF
Ω
0 339
.
• .
V
ms
3127f