LTC1479IG View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1479IG' PDF : 24 Pages View PDF
LTC1479
APPLICATIONS INFORMATION
be high enough to withstand the maximum battery or
charger output voltage. In most cases, this will allow the
use of 20V MOSFET switches in the charger path, while
30V switches are used in the main power path.
Inrush Current Sense Resistor, RSENSE
A small valued sense resistor (current shunt) is used by
the three main switch pair drivers to measure and limit the
inrush current flowing through the conducting switch
pair.
It should be noted that the inrush limiting circuit is not
intended to provide short-circuit protection ; but rather, is
designed to limit the large peak currents which flow into or
out of the large power supply capacitors and the battery
packs during power supply switch-over transitions. The
inrush current limit should be set at approximately 2× or
3× the maximum required DC/DC input current.
For example, if the maximum current required by the
DC/DC converter is 2A, an inrush current limit of 6A is set
by selecting a 0.033Ω sense resistor, RSENSE, using the
following formula:
RSENSE = (200mV)/IINRUSH
Note that the voltage drop across the resistor in this
example is only 66mV under normal operating conditions.
Therefore, the power dissipated in the resistor is extremely
small (132mW), and a small 1/4W surface mount resistor
can be used in this application. A number of small valued,
surface mount resistors are available that have been
specifically designed for high efficiency current sensing
applications.
DC Input Monitor Resistor Divider
The DCDIV input continuously monitors the DC power
supply voltage via a two resistor divider network, RDC1 and
RDC2, as shown in Figure 5. The threshold voltage of the
DC good comparator is 1.215V when the power supply
input voltage is rising. Approximately – 35mV of hyster-
esis is provided to ensure clean switching of the compara-
tor when the DC supply voltage is falling.
To minimize errors due to the input bias current of the DC
good comparator, set RDC1 = 12.1k so that approximately
100µA flows through the resistor divider when the desired
DC
SUPPLY
RDC2
1%
RDC1
12.1k
1%
LTC1479
DCIN
1.215V +
DCDIV
–
TO SW A/B
DCINGOOD
1479 F05
Figure 5. DC Monitor Resistor Divider
BAT1
BAT2
VBAT
BATSEL
SWITCH
CONTROL
LOGIC
RB2
1.215V
+
1%
BDIV
–
RB1
121k
1%
LTC1479
LOBAT
1479 F06
Figure 6. Battery Monitor Resistor Divider
threshold is reached. RDC2 is then selected according to
the following formula:
) RDC2 = 12.1k
VGOOD
1.215V
–1
Battery Monitor Resistor Divider
A switch controlled by the BATSEL input connects one of
the two batteries to the VBAT pin and therefore to the top
of the battery resistor divider as shown in Figure 6. The
threshold voltage of the low-battery comparator is 1.215V
when the battery voltage is falling. Approximately +35mV
of hysteresis is provided to ensure clean switching of the
comparator when the battery voltage rises again.
To minimize errors due to the input bias current of the low
battery comparator, assume RB1 = 121k so that approxi-
mately 10µA flows through the resistor divider when the
threshold is reached. RB2 is selected according to the
following formula:
14
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