LTC4000_15 Datasheet, PDF (28/40 Pages) Linear Technology – High Voltage High Current Controller for Battery Charging and Power Management

LTC4000
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.
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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.
10
5
0
–5
–10
–15
–20 –15 –10 –5 0 5 10 15 20 25
5ms/DIV
4000 F16
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
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.
15
10
5
0
–5
–10
–15
–20 –15 –10 –5 0 5 10 15 20 25
5ms/DIV
4000 F17
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
15
10
5
0
–5
–10
–15
–20 –15 –10 –5 0 5 10 15 20 25
5ms/DIV
4000 F18
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
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.
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