Texas Instruments LMP8646 Evaluation Board LMP8646EB/NOPB LMP8646EB/NOPB Datenbogen
Produktcode
LMP8646EB/NOPB
LMP8646EB User’s Guide
November 2011
Page 4 of 20
SNOU010
2.
Quick Start: Supercap Application
A supercap application requires a very high capacitive load to be charged. This example assumes the
output capacitor is 3.3F with a limited load current at 1.5A. The LM3102 will provide the current to
charge the supercap, and the LMP8646 will monitor this current to make sure it does not exceed the
desired 1.5A value.
output capacitor is 3.3F with a limited load current at 1.5A. The LM3102 will provide the current to
charge the supercap, and the LMP8646 will monitor this current to make sure it does not exceed the
desired 1.5A value.
This is done by connecting the LMP8646 output to the feedback pin of the LM3102, as shown in
Figure 1. This feedback voltage at the FB pin is compared to a 0.8V internal reference. Any voltage
above this 0.8V means the output current is above the desired value of 1.5A, and the LM3102 will
reduce its output current to maintain the desired 0.8V at the FB pin.
Figure 1. This feedback voltage at the FB pin is compared to a 0.8V internal reference. Any voltage
above this 0.8V means the output current is above the desired value of 1.5A, and the LM3102 will
reduce its output current to maintain the desired 0.8V at the FB pin.
The following steps show the design procedures for this supercap application. The first step is to
select the discrete components for the LM3102 by following the design steps described in its
application note. Next, integrate the LMP8646 into the system and select the proper values for
its gain, bandwidth, and output resistor. Lastly, capture the results and adjust the components to
yield the desired performance.
select the discrete components for the LM3102 by following the design steps described in its
application note. Next, integrate the LMP8646 into the system and select the proper values for
its gain, bandwidth, and output resistor. Lastly, capture the results and adjust the components to
yield the desired performance.
-
RON
RON
51.1 k
CSS
10 nF
10 nF
VIN = 18V
LM3102
COUT
47 F
47 F
LOUT
10 H
10 H
10 nF
V+ = 6V
FB
0.8V
R
G
50 k
C
G
1.8 nF
LMP_V
OUT
ROUT
160
RFBT
10 k
10 k
RFBB
2 k
0.1 F
& 10 F
& 10 F
V
+
+IN
-IN
R
SENSE
55 m
LMP8646
VCC
I
LIMIT
= I
CLOSE_LOOP
= 1.5A
I
OPEN_LOOP
= 2.5A
CIN
0.1 F
CIN
2x10 F
CBST
33 nF
BST
SW
VIN
SS
R
G
5V
V
-
SUPERCAP
3.3F
3.3F
V
O_LOAD
= 4.8V
Figure 2 – Schematic for the SuperCap Application
1. Step 1: Choose Components for the LM3102
Refer to the LM3102 evaluation board app note (AN-1646) located at
http://www.ti.com/lit/snva248
to
choose the appropriate components for the LM3102.
2. Step 2: Choose the gain resistor, Rg, for LMP8646
If Rsense = 55 mOhm, then use the equation below to calculate the appropriate gain resistor, R
G
, for
an output current of 1.5A.