LTC3789 LINER [Linear Technology], LTC3789 Datasheet - Page 15

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

operaTion
Charge Current Regulation
The first loop involved in a normal charging cycle is the
charge current regulation loop (Figure 3). As with the input
current regulation loop, this loop also drives the ITH and
CC pins. This loop ensures that the charge current sensed
through the charge current sense resistor (R
exceed the programmed full charge current.
(OPTIONAL)
Battery Voltage Regulation
Once the float voltage is reached, the battery voltage regu-
lation loop takes over from the charge current regulation
loop (Figure 4).
The float voltage level is programmed using the feedback
resistor divider between the BAT pin and the FBG pin with
the center node connected to the BFB pin. Note that the
ground return of the resistor divider is connected to the
FBG pin. The FBG pin disconnects the resistor divider
load when V
tor divider does not consume battery current when the
battery is the only available power source. For V
the typical resistance from the FBG pin to GND is 100Ω.
Output Voltage Regulation
When charging terminates and the system load is com-
pletely supplied from the input, the PMOS connected to
BGATE is turned off. In this scenario, the output voltage
regulation loop takes over from the battery float voltage
regulation loop (Figure 5). The output voltage regulation
loop regulates the voltage at the CSP pin such that the
output feedback voltage at the OFB pin is 1.193V.
C
CSP
IBMON
R
CL
Figure 3. Charge Current Regulation Loop
IBMON
CL
IN
C
BIAS
< 3V to ensure that the float voltage resis-
CSP
50µA AT NORMAL
5µA AT TRICKLE
CSP
R
IS
A9
g
CSN
60k
m
= 0.33m
1V
TO SYSTEM
BAT PMOS
+
–
–
A5
–
+
LTC4000
ITH
CC
CS
4000 FO3
) does not
R
C
IN
C
C
TO DC/DC
≥ 3V,
Battery Instant-On and Ideal Diode
The LTC4000 controls the external PMOS connected to the
BGATE pin with a controller similar to the input ideal diode
controller driving the IGATE pin. When not charging, the
PMOS behaves as an ideal diode between the BAT (anode)
and the CSN (cathode) pins. The controller (A2) regulates
the external PMOS to achieve low loss conduction by driv-
ing the gate of the PMOS device such that the voltage drop
from the BAT pin to the CSN pin is 8mV. When the ability
to deliver a particular current with an 8mV drop across
the PMOS source and drain is exceeded, the voltage at
the gate clamps at V
a fixed value resistor (R
The ideal diode behavior allows the battery to provide cur-
rent to the load when the input supply is in current limit
or the DC/DC converter is slow to react to an immediate
load increase at the output. In addition to the ideal diode
behavior, BGATE also allows current to flow from the CSN
pin to the BAT pin during charging.
There are two regions of operation when current is flowing
from the CSN pin to the BAT pin. The first is when charg-
ing into a battery whose voltage is below the instant-on
threshold (V
tion, the controller regulates the voltage at the CSP pin
Figure 4. Battery Float Voltage Regulation Loop with FBG
Figure 5. Output Voltage Regulation Loop with FBG
R
R
R
R
BFB1
BFB2
OFB1
OFB2
OFB
BAT
BFB
FBG
CSP
OFB
FBG
< V
1.136V
1.193V
OUT(INST_ON)
BGATE(ON)
LTC4000
LTC4000
DS(ON)
+
–
+
–
A6
A7
and the PMOS behaves like
).
–
+
–
+
). In this region of opera-
ITH
ITH
CC
CC
4000 FO4
4000 FO5
LTC4000
R
R
C
C
C
C
C
C
TO DC/DC
TO DC/DC
15
4000f