Datasheet

LTC3127
14
3127f
applicaTions inForMaTion
Capacitor Selection Example
In this example, a pulsed load application requires that
V
OUT
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 V
IN
to V
OUT
step-up ratio. All of the following calculations assume a
minimum V
IN
of 3V and an efficiency of 90%.
Given the application, the following is known:
V
IN
= 3V
V
OUT
= 3.6V
I
IN(MAX)
= 500mA
I
PULSE
= 1.5A
I
STANDBY
= 0A
h = 0.9
D = 0.125
T = 1/217Hz = 4.6ms
V
DROOP
= 300mV
Step 1: Check to make sure the application can provide
enough current to recover from the pulsed load using the
I
LOAD(MAX)
equation:
I
V m A
V
A
LOAD MAX( )
• • .
. • .
= =
3 500 0 9
0 125 3 6
3
The maximum load that can be pulsed at this V
IN
to V
OUT
combination is 3A.
Step 2: Calculate the minimum output capacitance re-
quired.
C A
V m A
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, 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:
V
A
V m A
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
V
DROOP ESR
55
3 500 0 9
3 6
0 0 04
0
A
V m A
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.