LTC3789 LINER [Linear Technology], LTC3789 Datasheet - Page 32

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

applicaTions inForMaTion
LTC4000
The Output Voltage Regulation Loop
The feedback signal for the output voltage regulation loop
is the voltage on the OFB pin, which is connected to the
center node of the resistor divider between the output
voltage (connected to C
is compared to an internal reference (1.193V typical) by
the transconductance error amplifier A7. 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 output volt-
age observed by the OFB pin. This loop is shown in detail
in Figure 21.
The simplified loop transmission is as follows:
where Gmo
the output current of the external DC/DC converter, and
R
32
OFB
L
⎡
⎢
⎣
Figure 21. Simplified Linear Model of the Output Voltage
Regulation Loop
R
OV
R
= R
OFB2
OFB
(s) = g
OFB1
LTC4000
⎤
⎥ •
⎦
PULLED HIGH
p
INTERNALLY
R
(s) is the transfer function from V
⎡
⎢
⎣
O7
m7
+ R
R
g
g
m10
L
m7
–
+
⎡
⎢
⎢
⎢
⎢
⎣
OFB2
A10
+
–
⎛
⎜
⎝
• C
A7
= 0.5m
= 0.1m
R
R
L
C
L
.
1.193V
s + 1
R
–
O10
SP
g
) and the FBG pin. This voltage
m10
C
⎤
⎥ • Gmo
⎦
CSP
1
OFB
FBG
ITH
CC
C
s
⎞
⎟ C
⎠
4000 F21
C
R
R
R
s + 1
p
C
OFB1
OFB2
C
(s)
C
+
–
INPUT
⎤
⎥
⎥
⎥
⎥
⎦
•
Gmo
LOAD
C
p
L
(s)
R
L
ITH
to
The Battery Float Voltage Regulation Loop
The battery float voltage regulation loop is very similar to
the output float voltage regulation loop. Instead of observ-
ing the voltage at the OFB pin, the battery float voltage
regulation loop observes the voltage at the BFB pin.
One significant difference is that while the value of R
in the output voltage loop can vary significantly, the
output resistance of the battery float voltage loop is a
small constant value approximately equal to the sum
of the on-resistance of the external PFET (R
the series internal resistance of the battery (R
approximation is valid for any efficient system such that
most of the output power from the battery is delivered to
the system load and not dissipated on the battery inter-
nal resistance or the charging PFET on-resistance. For a
typical system, minimum R
than R
than R
voltage regulation loop.
Figure 22. Simplified Linear Model of the Battery Float
Voltage Regulation Loop
LTC4000
PULLED HIGH
DS(ON)
BAT
INTERNALLY
R
O6
. Figure 22 shows the detail of the battery float
g
g
m10
m6
–
+
A10
+
–
+ R
A7
= 0.5m
= 0.1m
1.136V
BAT
R
O10
and R
FBG
BAT
BFB
ITH
CC
4000 F22
BFB
L
R
R
R
is at least five times larger
is at least 10
C
BFB1
BFB2
C
C
+
–
INPUT
R
R
R
CS
DS(ON)
BAT
Gmo
p
(s)
6
LOAD
times larger
DS(ON)
C
L
BAT
R
). This
L
) and
4000f
L