LTC1760 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1760' PDF : 44 Pages View PDF
LTC1760
APPLICATIONS INFORMATION
As is often the case, the wall adapter will usually have at
least a +10% current limit margin and many times one
can simply set the adapter current limit value to the actual
adapter rating (see Figure 9 & Table 1).
Table 1. Common RCL Resistor Values
Adapter
Rating A
RCL Value*
RCL Power
(Ω) 1% Dissipation (W)
RCL Power
Rating (W)
1.5
0.06
0.135
0.25
1.8
0.05
0.162
0.25
2
0.045
0.18
0.25
2.3
0.039
0.206
0.25
2.5
0.036
0.225
0.5
2.7
0.033
0.241
0.5
3
0.030
0.21
0.5
*Values shown above are rounded to nearest standard value.
Table 1 RCL values take into account LTC1760 C-grade 5% tolerance for VCL1.
Extending System to More than 2 Batteries
The LTC1760 can be extended to manage systems with
more than 3 sources of power. Contact Linear Technology
Applications Engineering for more information.
Charge Termination Issues
Batteries with constant-current charging and voltage-
based charger termination might experience problems
with reductions of charger current caused by adapter
limiting. It is recommended that input limiting feature be
defeated in such cases. Consult the battery manufacturer
for information on how your battery terminates charging.
Setting Charger Output Current Limit
The LTC1760 current DAC and the PWM analog circuitry
must coordinate the setting of the charger current. Failure
to do so will result in incorrect charge currents.
Table 2. Recommended Resistor Values
IMAX (A)
1
RSENSE (Ω) 1%
0.100
RSENSE (W)
0.25
RILIMIT (Ω) 1%
0
2
0.05
0.25
10k
3
0.025
0.5
33k
4
0.025
0.5
Open or short to VCC2
Warning
DO NOT CHANGE THE VALUE OF RILIMIT DURING OPERA-
TION. The value must remain fixed and track the RSENSE
value at all times. Changing the current setting can result
in currents that greatly exceed the requested value and
potentially damage the battery or overload the wall adapter
if no input current limiting is provided.
Setting Charger Output Voltage Limit
The value of an external resistor connected from the VLIMIT
pin to GND determines one of five voltage limits that are
applied to the charger output value. See Table 3. These
limits provide a measure of safety with a hardware restric-
tion on charging voltage, which cannot be overridden by
software. This voltage sets the limit that will be applied
to the battery as reported by battery. Since the battery
internal voltage monitor point is the actual cell voltage,
you may see higher voltages, up to 512mV higher, at the
external charger terminals due to the voltage servo loop
action. See Operations “Section 3.7” for more information
on the voltage servo system.
Table 3. Recommended Resistor Values for RVLIMIT
VMAX
RVLIMIT (Ω) 1%
Up to 8.4V
0 (Short to ground)
Up to 12.6V
10k
Up to 16.8V
33k
Up to 21.0V
100k
Up to 32.7V (No Limit)
Open or short to VCC2
Inductor Selection
Higher operating frequencies allow the use of smaller
inductor and capacitor values. A higher frequency gener-
ally results in lower efficiency because of MOSFET gate
charge losses. In addition, the effect of inductor value
on ripple current and low current operation must also be
considered. The inductor ripple current ΔIL decreases with
higher frequency and increases with higher VIN.
ΔIL
=
(
1
f)(L)
VOUT
⎛
⎝⎜
1−
VOUT
VIN
⎞
⎠⎟
Accepting larger values of ΔIL allows the use of low
inductances, but results in higher output voltage ripple
and greater core losses. A reasonable starting point for
setting ripple current is ΔIL = 0.4(IMAX). In no case should
ΔIL exceed 0.6(IMAX) due to limits imposed by IREV and
1760fa
37
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