LTC3789 LINER [Linear Technology], LTC3789 Datasheet - Page 34

• Manufacturer Part Number: LTC3789
• Description: High Voltage High Current Controller for Battery Charging and Power Management
• Manufacturer: LINER [Linear Technology]

applicaTions inForMaTion
LTC4000
The Battery Charge Current Regulation Loop when
V
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
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.
34
OFB
BIAS
R
O10
Figure 24. Simplified Linear Model of the Charge
Current Regulation Loop with the External Charging
PFET Linearly Regulated
50µA/
5µA
is Regulated to V
R
1V
O5
ITH
CL
R
g
R
m10
CL
C
g
A10
m5
= 0.1m
C
A5
= 0.5m
C
R1
60k
CC
g
m8
OUT(INST_ON)
IBMON
R2
20k
C
= 0.33m
A8
IBMON
+
–
LTC4000
CSP
CSN
BAT
4000 F24
OUT(INST_ON)
C
+
–
g
INPUT
R
R
g
Gmo
CS
BAT
mEXT
1
p
(s)
C
L
).
R
L
The simplified loop transmission is:
where Gmo
output current of the external DC/DC converter, g
the small signal transconductance of the output PFET,
R
the effective resistance value resulting from the parallel
combination of R
flDC

L
⎡
⎢
⎢
⎢
⎢
⎢
⎣
20R
R
⎛
⎜
⎝
CC2
fIDC
R
= R
CS
R
CS
(s) = g
R
L
+ R
CS
+ R
fIDC
•
g
⎡
⎢
⎢
⎣
p
L
mEXT
(s) is the transfer function from V
+ 1/g
(
BAT
C
R1+ R2
(
m5
•
g
R2 • C
⎡
⎢
⎢
⎣
(
⎞
⎟
⎠
L
⎡
⎢
⎢
⎢
⎢
⎣
R
mEXT
s + 1
⎛
⎜
⎝
g
and R
L
R
mEXT
C
C
IBMON
)
 R
g
C
–
IBMON
+ R
fIDC
flDC
g
s + 1
1
m10
C
s + 1
1
BAT
C
.
)
s
C
s + 1
⎤
⎥
⎥
⎥
⎥
⎥
⎦
⎞
⎟ C
⎠
L
• Gmo
and R
)
s + 1
C
⎤
⎥
⎥
⎦
s + 1
•
⎤
⎥
⎥
⎦
L
p
•
//R
(s)
⎤
⎥
⎥
⎥
⎥
⎦
•
flDC
represents
ITH
mEXT
to the
4000f
is