datasheetbank_Logo
Integrated circuits, Transistor, Semiconductors Search and Datasheet PDF Download Site

LT1239(V2) View Datasheet(PDF) - Linear Technology

Part Name
Description
View to exact match
LT1239
(Rev.:V2)
Linear
Linear Technology Linear
LT1239 Datasheet PDF : 12 Pages
1 2 3 4 5 6 7 8 9 10 Next Last
LT1239
APPLICATIONS INFORMATION
system power and the backup batteries. The comparator
output will be driven high if the output of the 5V system
supply is greater than the 4.85V output of regulator 2.
Regulator 2 will act as a diode to prevent current flow from
the 5V system supply back into the backup battery. Cur-
rent flow into the output of regulator 2, with the output
pulled up to 5V, will be limited to approximately 6µA and
will flow to ground. If the main 5V system supply drops
below the 4.85V output of regulator 2 the comparator will
switch off and regulator 2 will provide power to the
memory. The comparator combined with regulator 2 and
the batteries provide an uninterruptable power source to
the memory and power monitoring circuitry.
Choosing Current Limiting Resistors
Due to UL safety considerations, circuits used to charge
lithium-ion batteries must have external resistors (passive
components) to limit the available charge current in the
event of a failure in the charging circuit. The LT1239 allows
these resistors to be placed in series with the output
transistor of the regulator 1 as shown in Figure 5. The
current limiting resistor (R4) will be in series with the main
charge current path but will be inside the feedback loop of
regulator 1. Because the resistors are inside the feedback
loop they will not affect output voltage regulation in
normal operating conditions. The resistors should be
selected so that they limit the charge current below the
maximum level specified by the battery manufacturer. For
a typical 3.4V, 50mA rechargeable backup cell (Panasonic
VL2330) the maximum charge current is specified at
300mA. Most users will choose to limit the current well
below the maximum charge current. It is important to note
that these resistors can also limit the charge current
during normal operation. Since the charge current for a
typical lithium-ion button cell is normally less than 20mA,
limited by the internal impedance of the cells during a
constant voltage charge, the current limiting resistors do
not significantly affect the charge times for the backup
cells. The worst case would occur if the regulator failed as
a short and the main battery is at its maximum charge
voltage. The current limiting resistor (R4) must be chosen
to limit the current to less than the manufacturers maxi-
mum charging current with the difference between the
main battery voltage and the backup battery voltage dropped
across it.
10
For example with a main battery voltage of 24V max, a
backup battery voltage of 6.8V and a maximum charge
current of 300mA, R4 must be greater than (24V – 6.8V) /
300mA, R4 > 57.
R4 can also be used to limit the power dissipated by
regulator 1 as shown in the following section. C1 is needed
for stability in circuits with protection resistors (R4).
The power dissipation in R4 during fault conditons can be
significant. it will be equal to:
(VINL – VBATTERY)2
R4
Power resistors with ratings greater than 0.25W or fusable
resistors may be required.
Thermal Considerations
The power dissipation of this device is made up of several
components.They are the power dissipation of each regu-
lator, the comparator and the error amplifier. The largest
component will be due to the power in regulator 1, when
the charge current for the batteries is the highest and the
input voltage to regulator 1 is at the maximum. In most
systems this condition only occurs for a short period after
the backup battery has been completely discharged. Both
regulators have thermal limiting circuitry which limits the
power in the regulator when the junction temperature
reaches about 100°C. The thermal limit temperature is set
low because the device is designed to work with batteries
specified to run at ambient temperatures below 60°C. The
power in regulator 1 can be limited with external resistors
placed in the feedback loop as shown in Figure 5. In
lithium-ion systems these resistors are required for safety
reasons.
The power in regulator 1 will be equal to:
[(VMAINBATTERY – VBACKUPBATTERY) × ICHG] – (ICHG × R4)
Note that for circuits with a current limiting resistor (R4)
the worst-case power point occurs when ICHG is equal to
the maximum charging current/2.
Example: [(24V – 6.8V) × (71mA/2)] – [(71mA/2) × 240]
= 300mW
This is the only significant component of power dissipation
in the device and this condition will only occur when the
 

Share Link: 

datasheetbank.com [ Privacy Policy ] [ Request Datasheet ] [ Contact Us ]