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LTC4000IGN#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P30P31-P33P34-P36P37-P39P40-P40
LTC4000
28
4000fb
For more information www.linear.com/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.
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
105 15
4000 F16
25200–10 –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
105 15
4000 F17
25200–10 –5–15
5ms/DIV
–20
V
IBMON
(mV)
5mV/DIV
15
–10
–5
0
5
10
–15
105 15
4000 F18
25200–10 –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.
LTC4000
29
4000fb
For more information www.linear.com/LTC4000
applicaTions inForMaTion
This same procedure is then repeated for the other four
loops: the input current regulation, the output voltage
regulation, the battery float voltage regulation and finally
the charge current regulation when V
OFB
> V
OUT(INST_ON)
.
Note that the resulting optimum values for each of the loops
may differ slightly. The final values of C
C
and R
C
are then
selected by combining the results and ensuring the most
conservative response for all the loops. This usually entails
picking the largest value of C
C
and the smallest value of
R
C
based on the results obtained for all the loops. In this
particular example, the value of C
C
is finally set to 47nF
and R
C
= 14.7kΩ.
BOARD LAYOUT CONSIDERATIONS
In the majority of applications, the most important param-
eter of the system is the battery float voltage. Therefore,
the user needs to be extra careful when placing and routing
the feedback resistor R
BFB1
and R
BFB2
. In particular, the
battery sense line connected to R
BFB1
and the ground return
line for the LTC4000 must be Kelvined back to where the
battery output and the battery ground are located respec-
tively. Figure 19 shows this
Kelvin sense configuration.
For accurate current sensing, the sense lines from R
IS
and R
CS
(Figure 19) must be Kelvined back all the way
to the sense resistors terminals. The two sense lines of
each resistor must also be routed close together and away
from noise sources to minimize error. Furthermore, cur-
rent filtering capacitors should be placed strategically to
ensure that very little AC current is flowing through these
sense resistors as mentioned in the applications section.
The decoupling capacitors C
IN
and C
BIAS
must be placed
as close to the LTC4000 as possible. This allows as short
a route as possible from C
IN
to the IN and GND pins, as
well as from C
BIAS
to the BIAS and GND pins.
In a typical application, the LTC4000 is paired with an
external DC/DC converter. The operation of this converter
often involves high dV/dt switching voltage as well as high
currents. Isolate these switching voltages and currents
from the LTC4000 section of the board as much as pos-
sible by using good board layout practices. These include
separating noisy power and signal grounds, having a good
low impedance ground plane, shielding whenever neces-
sary, and routing sensitive
signals as short as possible
and away from noisy sections of the board.
Figure 19. Kelvin Sense Lines Configuration for LTC4000
4000 F19
V
IN
CSN
CLN
IN
CSP
BAT
GND
LTC4000
R
C
ITHGND
SWITCHING
CONVERTER
BGATE
ITH
IIDCC
C
C
IGATE
R
BFB1
R
IS
R
CS
R
BFB2
BFB
FBG
SYSTEM LOAD
LTC4000
30
4000fb
For more information www.linear.com/LTC4000
applicaTions inForMaTion
APPENDIX—THE LOOP TRANSFER FUNCTIONS
When a series resistor (R
C
) and capacitor (C
C
) is used
as the compensation network as shown in Figure 11, the
transfer function from the input of A4-A7 to the ITH pin
is simply as follows:
V
ITH
V
FB
(s) = g
m4-7
R
C
–
1
g
m10
C
C
s + 1
R
O4-7
• C
C
s
where g
m4-7
is the transconductance of error amplifier A4-
A7, typically 0.5mA/V; g
m10
is the output amplifier (A10)
transconductance, R
O4-7
is the output impedance of the
error amplifier, typically 50mΩ; and R
O10
is the effective
output impedance of the output amplifier, typically 10mΩ
with the ITH pin open circuit.
Note this simplification is valid when g
m10
• R
O10
• R
O4-7
• C
C
= A
V10
• R
O4-7
• C
C
is much larger than any other
poles or zeroes in the system. Typically A
V10
• R
O4-7
= 5 •
10
10
with the ITH pin open circuit. The exact value of g
m10
and R
O10
depends on the pull-up current and impedance
connected to the ITH pin respectively.
In most applications, compensation of the loops involves
picking the right values of R
C
and C
C
. Aside from picking
the values of R
C
and C
C
, the value of g
m10
may also be
adjusted. The value of g
m10
can be adjusted higher by
increasing the pull-up current into the ITH pin and its
value can be approximated as:
g
m10
=
I
ITH
+ 5µA
50mV
The higher the value of g
m10
, the smaller the lower limit
of the value of R
C
would be. This lower limit is to prevent
the presence of the right half plane zero.
Even though all the loops share this transfer function from
the error amplifier input to the ITH pin, each of the loops
has a slightly different dynamic due to differences in the
feedback signal path.
The Input Current Regulation Loop
The feedback signal for the input current regulation loop
is the sense voltage across the input current sense resis-
tor (R
IS
).
This voltage is amplified by a factor of 20 and compared
to the voltage on the IL pin by the transconductance er-
ror amplifier (A4). This amplifier then drives the output
transconductance amplifier (A10) to appropriately adjust
the voltage on the ITH pin driving the external DC/DC
converter to regulate the input current across the sense
resistor (R
IS
). This loop is shown in detail in Figure 20.
The simplified loop transmission is:
L
IC
(s) = g
m4
R
C
–
1
g
m10
C
C
s + 1
C
C
s
•
20R
IS
R2
CIIMON
s + 1
( )
R1+ R2
( )
C
IIMON
s + 1
• Gmi
p
(s)
where Gmi
p
(s) is the transfer function from V
ITH
to the
input current of the external DC/DC converter.
Figure 20. Simplified Linear Model of the Input Current
Regulation Loop
IN
CC
1V
A8
g
m8
= 0.33m
A4
g
m4
= 0.5m
A10
g
m10
= 0.1m
ITH
LTC4000
IN CLN
R
IS
I
IN
IIMON
IL
C
IIMON
C
IN
+
–
–
+
+
–
–
C
C
4000 F20
R
C
R1
60k
R
O4
R
O10
R
IL
R2
20k
50µA
BIAS
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LTC4000IGN#TRPBF

Mfr. #:
Buy LTC4000IGN#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Battery Management High Voltage, High Current Controller for Battery Charging and Power Management
Lifecycle:
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