LTC4000-1
24
40001fa
For more information www.linear.com/LTC4001-1
applicaTions inForMaTion
Note that in this configuration, the recharge threshold is
97.6% of the float voltage level. When the battery voltage
drops below this level, the whole 3-step charging cycle is
reinitiated starting with the bulk charge.
Some systems require trickle charging of an over dis-
charged lead-acid battery. This feature can be included
using the CL pin of the LTC4000-1. In the configuration
shown in Figure 15, when the battery voltage is lower
than 68% of the Absorption level, the pull-up current on
the CL pin is reduced to 10% of the normal pull-up cur-
rent. Therefore, the trickle charge current can be set at
the following level:
I
CLIM
=
MIN 50mV, 0.25µA •R
CL
R
CS
If this feature is not desired, leave the CL pin open to set
the regulation voltage across the charge current sense
resistor (RCS) always at 50mV.
The F LT and CHRG Indicator Pins
The F LT and CHRG pins in the LTC4000-1 provide status
indicators. Table 4 summarizes the mapping of the pin
states to the part status.
Table 4. F LT and CHRG Status Indicator
F LT CHRG STATUS
0 0 NTC Over Ranged – Charging Paused
1 0 Charging Normally
0 1 Charging Terminated and Bad Battery Detected
1 1 V
IBMON
< (V
C/X
– 10mV)
where 1 indicates a high impedance state and 0 indicates
a low impedance pull-down state.
Note that V
IBMON
< (V
CX
– 10mV) corresponds to charge
termination only if the C/X termination is selected. If the
charger timer termination is selected, constant voltage
charging may continue for the remaining charger timer
period even after the indicator pins indicate that V
IBMON
< (V
CX
– 10mV). This is also true when no termination is
selected, constant voltage charging will continue even after
the indicator pins indicate that V
IBMON
< (V
CX
– 10mV).
The BIAS Pin
For ease of use the LTC4000-1 provides a low dropout
voltage regulator output on the BIAS pin. Designed to
provide up to 0.5mA of current at 2.9V, this pin requires
at least 470nF of low ESR bypass capacitance for stability.
Use the BIAS pin as the pull-up source for the NTC resis-
tor networks, since the internal reference for the NTC
circuitry is based on a ratio of the voltage on the BIAS
pin. Furthermore, various 100k pull-up resistors can be
conveniently connected to the BIAS pin.
Setting the Input V
oltage Monitoring Resistor Divider
The falling threshold voltage level for this monitoring
function can be calculated as follows:
R
VM1
=
V
VM _RST
1.193V
– 1
• R
VM2
where R
VM1
and R
VM2
form a resistor divider connected
between the monitored voltage and GND, with the center
tap point connected to the VM pin as shown in Figure 6. The
rising threshold voltage level can be calculated similarly.
Input Voltage Programming
Connecting a resistor divider from V
IN
to the IFB pin en-
ables programming of a minimum input supply voltage.
This feature is typically used to program the peak power
voltage for a high impedance input source. Referring to
Figure 2, the input voltage regulation level is determined
using the following formula:
R
IFB1
=
V
IN_REG
1V
– 1
R
IFB2
Where V
IN_REG
is the minimum regulation input voltage
level, below which the current draw from the input source
is reduced.
Combining the Input Voltage Programming and the
Input Voltage Monitoring Resistor Divider
When connected to the same input voltage node, the input
voltage monitoring and the input voltage regulation resistor
divider can be combined (see Figure 16).
