LTC4040_15 Datasheet, PDF (19/26 Pages) Linear Technology – 2.5A Battery Backup Power Manager

LTC4040
Applications Information
Programming the Input Current Limit
and CLPROG Monitor
The input current limit is programmed by connecting a
series resistor between the V_IN and CLN pins. To limit the
total system current to I_SYSLIM, the value of the required
resistor can be calculated using the following equation:
R_S
=
25mV
I_SYSLIM
For example, to set the current limit to 2A, the se-
ries resistor should be 12.5mΩ. As discussed in the
Operations section, the part does not limit the system
current but reduces the charge current to zero in case the
system load exceeds this limit.
The voltage on the CLPROG pin always represents the total
system current I_SYS through the external series resistance,
R_S. 800mV on CLPROG represents the full-scale current
set by R_S. The system current can be calculated from the
CLPROG pin voltage by using the following equation:
I_SYS
=
V_CLPROG
32 •R_S
For example, if the CLPROG pin voltage is 600mV and
R_S is 12.5mΩ, then the total system current is 1.5A. As
shown in the block diagram, the CLPROG pin is not buff-
ered internally. So it is important to isolate this pin before
connecting to an ADC or any other monitoring device.
Failure to do so would degrade the accuracy of this circuit.
Programming the Boost Output Voltage
The boost converter output voltage in backup mode can
be programmed for any voltage from 3.5V to 5V by using
a resistor divider from the V_SYS pin to GND via the BSTFB
pin such that:
V_SYS
=
V_BSTFB
•


1+
R_FB1
R_FB2


=
0.8V
•


1+
R_FB1
R_FB2


V_BSTFB is 0.8V. See the Block Diagram. Typical values for
R_FB1 and R_FB2 are in the range of 40k to 2M. Too small a
resistor will result in a large quiescent current whereas
too large a resistor coupled with any parasitic BSTFB pin
capacitance will create an additional pole and may cause
loop instability.
Programming the Reset Comparator
Threshold
The threshold for the reset comparator can be programmed
by using a resistor divider from the V_SYS pin to GND via
the RSTFB pin such that:
V_{SYS(RST )}
=
V_RSTFB
•


1+
R_FB1
R_FB2


=
0.74V
•


1+
R_FB1
R_FB2


V_RSTFB is 0.74V. See the Block Diagram. Typical values
for R_FB1 and R_FB2 are in the range of 40k to 2M. In most
applications, the BSTFB and RSTFB pins can be shorted
together and only one resistor divider between V_SYS and
GND is needed to set the V_SYS voltage during backup
mode and the reset threshold 7.5% below the V_SYS pro-
grammed voltage.
Choosing the External Resistor for the
overvoltage Protection (OVP) Module
In an overvoltage condition, the OVSNS pin will be clamped
at 6V. The external 6.2k resistor must be sized appropriately
to dissipate the resultant power. For example, a 1/8W 6.2k
resistor can have at most P_MAX • 6.2kΩ = 28V applied
across its terminals. With the 6V at OVSNS, the maximum
overvoltage magnitude that this resistor can withstand is
34V. A 1/4W 6.2k resistor raises the value to 45V. The
OVSNS pin’s absolute maximum current rating of 10mA
imposes an upper limit of 68V protection.
For more information www.linear.com/LTC4040
4040fa
19