LTC4000
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applicaTions inForMaTion
With C
C
= 1µF, R
C
= 10k at V
IN
= 20V, V
BAT
= 7V, V
CSP
regulated at 9.8V and a 0.2A output load condition at
CSP, the transient response for a 100mA charge current
step observed at IBMON is shown in Figure 16.
The transient response now indicates an overall under
damped system. As noted in the empirical loop compensa-
tion section, the value of R
C
is now increased iteratively
until R
C
= 20k. The transient response of the same loop
with C
C
= 22nF and R
C
= 20k is shown in Figure 18.
Figure 16. Transient Response of Charge Current Regulation Loop
Observed at IBMON When V
OFB
is Regulated to V
OUT(INST_ON)
with
C
C
= 1µF, R
C
= 10k for a 100mA Charge Current Step
5ms/DIV
–20
V
IBMON
(mV)
5mV
/DIV
15
–10
–5
0
5
10
–15
10
5
15
4000 F16
25
20
0
–10 –5
–15
The transient response shows a small overshoot with slow
settling indicating a fast minor loop within a well damped
overall loop. Therefore, the value of C
C
is reduced iteratively
until C
C
= 22nF. The transient response of the same loop
with C
C
= 22nF and R
C
= 10k is shown in Figure 17.
Figure 17. Transient Response of Charge Current Regulation Loop
Observed at IBMON When V
OFB
is Regulated to V
OUT(INST_ON)
with
C
C
= 22nF, R
C
= 10k for a 100mA Charge Current Step
Figure 18. Transient Response of Charge Current Regulation Loop
Observed at IBMON When V
OFB
is Regulated to V
OUT(INST_ON)
with
C
C
= 22nF, R
C
= 20k for a 100mA Charge Current Step
5ms/DIV
–20
V
IBMON
(mV)
5mV
/DIV
15
–10
–5
0
5
10
–15
10
5
15
4000 F17
25
20
0
–10 –5
–15
5ms/DIV
–20
V
IBMON
(mV)
5mV
/DIV
15
–10
–5
0
5
10
–15
10
5
15
4000 F18
25
20
0
–10 –5
–15
Note that the transient response is close to optimum
with some overshoot and fast settling. If after iteratively
increasing the value of R
C
, the transient response again
indicates an over damped system, the step of reducing
C
C
can be repeated. These steps of reducing C
C
followed
by increasing R
C
can be repeated continuously until one
arrives at a stable loop with the smallest value of C
C
and
the largest value of R
C
. In this particular example, these
values are found to be C
C
= 22nF and R
C
= 20kΩ.
After arriving at these final values of R
C
and C
C
, the stability
margin is checked by varying the values of both R
C
and
C
C
by 2:1 in all four possible combinations. After which
the setup condition is varied, including varying the input
voltage level and the output load level and the transient
response is checked at these different setup conditions.
Once the desired responses on all different conditions are
obtained, the values of R
C
and C
C
are noted.
