Linear Technology LTC4000, Datasheet

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LTC4000
14
4000fb
For more information www.linear.com/LTC4000
operaTion
If the system load requires more power than is available
from the input, the battery ideal diode controller provides
supplemental power from the battery. When the battery
voltage discharges below 97.1% of the float voltage
(V
BFB
< V
RECHRG(FALL)
), the automatic recharge feature
initiates a new charge cycle.
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
CS
) does not
exceed the programmed full charge current.
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.
Figure 3. Charge Current Regulation Loop
Figure 4. Battery Float Voltage Regulation Loop with FBG
Figure 5. Output Voltage Regulation Loop with FBG
CSP
CC
1V
A5
ITH
LTC4000 CSP CSN
R
IS
BAT PMOS
TO SYSTEM
IBMON
CL
C
IBMON
(OPTIONAL)
C
CSP
+ –
–
+
+
–
–
C
C
TO DC/DC
4000 FO3
R
C
60k
50µA AT NORMAL
5µA AT TRICKLE
BIAS
A9
g
m
= 0.33m
R
CL
CC
1.136V
ITH
LTC4000
BFB
BAT
FBG
–
+
C
C
TO DC/DC
4000 FO4
R
C
R
BFB2
R
BFB1
A6
+
–
CC
1.193V
ITH
LTC4000
OFB
CSP
FBG
–
+
C
C
TO DC/DC
4000 FO5
R
C
R
OFB2
R
OFB1
A7
+
–
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
IN
< 3V to ensure that the float voltage resis-
tor divider does not consume battery current when the
battery is the only available power source. For V
IN
≥ 3V,
the typical resistance from the FBG pin to GND is 100Ω.
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
BGATE(ON)
and the PMOS behaves like
a fixed value resistor (R
DS(ON)
).
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