ltc1760 Linear Technology Corporation, ltc1760 Datasheet - Page 34

• Manufacturer Part Number: ltc1760
• Description: Dual Smart Battery System Manager
• Manufacturer: Linear Technology Corporation

LTC1760
APPLICATIO S I FOR ATIO
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 Table 1).
Table 1. Common R
*Values shown above are rounded to nearest standard value.
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
Warning
DO NOT CHANGE THE VALUE OF R
TION. The value must remain fixed and track the R
value at all times. Changing the current setting can result
34
Rating A
I
Adapter
MAX
1.5
1.8
2.3
2.5
2.7
1
2
3
4
2
3
(A)
R
SENSE
CL
RCL Value*
(Ω) 1%
0.100
0.025
0.025
U
Resistor Values
0.045
0.039
0.036
0.033
0.030
0.05
0.06
0.05
(Ω) 1%
U
Dissipation (W)
R
RCL Power
SENSE
0.25
0.25
0.135
0.162
0.206
0.225
0.241
0.5
0.5
0.18
0.27
W
ILIM
(W)
DURING OPERA-
R
ILIM
RCL Power
Rating (W)
U
Open
10k
33k
0.25
0.25
0.25
0.25
(Ω) 1%
0.5
0.5
0.5
0
SENSE
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 V
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 3.6 for more information
on the voltage servo system.
Table 3. Recommended Resistor Values for R
V
Up to 8.4V
Up to 12.6V
Up to 16.8V
Up to 21.0V
Up to 32.7V (No Limit)
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 ∆I
with higher frequency and increases with higher V
Accepting larger values of ∆I
inductances, but results in higher output voltage ripple
and greater core losses. A reasonable starting point for
setting ripple current is ∆I
∆I
MAX
L
∆I
exceed 0.6(I
L
=
( )( )
f L
1
V
MAX
OUT
) due to limits imposed by IREV and
⎛
⎜
⎝
1
−
L
V
V
= 0.4(I
OUT
R
0Ω (Short to ground)
10k
33k
100k
Open (or short to V
IN
VLIM
L
⎞
⎟
⎠
± 1%
allows the use of low
MAX
). In no case should
VLIM
CC2
L
)
decreases
IN
LIMIT
.
1760f