LTC4000
33
4000fb
For more information
The Battery Charge Current Regulation Loop when
V
OFB
is Regulated to V
OUT(INST_ON)
When the battery voltage is below the instant-on level,
the external charging PFET is driven linearly to regulate
the voltage at the output (connected to the CSP pin). The
output voltage is regulated such that the voltage at the OFB
pin is equal to the instant-on threshold (V
OUT(INST_ON)
).
If this external PFET regulation is fast compared to the
unity crossover frequency of the battery charge current
regulation loop, then the voltage at the output can be con-
sidered a small signal ground. However, in the LTC4000
the external PFET regulation is purposely made slow to
allow for a broader selection of possible PFETs to be used.
Therefore, the linear model of the PFET has to be included
in the analysis of the charge current regulation loop. The
detail of this loop is shown in Figure 24.
Figure 24. Simplified Linear Model of the Charge
Current Regulation Loop with the External Charging
PFET Linearly Regulated
The simplified loop transmission is:
L
CC2
(s)
=
g
m5
R
C
–
1
g
m10
C
C
s
+
1
C
C
s
•
20R
CS
•
R2 • C
IBMON
s
+
1
(
)
R1
+
R2
(
)
C
IBMON
s
+
1
•
R
L
R
fIDC
+
R
L
•
1
R
L
P R
fIDC
(
)
C
L
s
+
1
•
C
g
g
mEXT
s
+
1
R
CS
+
R
BAT
R
fIDC
C
g
g
mEXT
s
+
1
• Gmo
p
(s)
where Gmo
p
(s) is the transfer function from V
ITH
to the
output current of the external DC/DC converter, g
mEXT
is
the small signal transconductance of the output PFET,
R
flDC
= R
CS
+ 1/g
mEXT
+ R
BAT
and R
L
//R
flDC
represents
the effective resistance value resulting from the parallel
combination of R
L
and R
flDC
.
applicaTions inForMaTion
CC
A8
g
m8
= 0.33m
A10
g
m10
= 0.1m
ITH
INPUT
CSP
CSN
LTC4000
BAT
C
C
4000 F24
R
C
Gmo
p
(s)
R2
20k
R1
60k
R
O10
R
O5
–
+
C
L
R
L
C
IBMON
IBMON
CL
+
–
R
BAT
1
g
mEXT
R
CS
1V
A5
g
m5
= 0.5m
R
CL
+
–
+
–
–
50µA/
5µA
BIAS
C
g
