LTC1980
12
1980f
APPLICATIO S I FOR ATIO
WUUU
An external resistor divider (Figure 3) can be used to
program other float voltages. Resistor values are found
using the following equation:
R10 = R11 • (V
FLOAT
– V
REF
)/V
REF
where V
REF
= 1.225V. The suggested value for R11 is
100k. Use 1% or better resistors.
Setting DC/DC Converter Output Voltage
From Figure 5, select the following resistors based on
output voltage V
REG
:
R8 = R14 • (V
REG
– V
REF
)/V
REF
where V
REF
= 1.225V, suggested value for R14 is 100k, 1%.
LDO Operation
The LTC1980 provides an uninterrupted power supply for
the system load. When a wall adapter is connected and
operating, power is taken from the wall adapter to charge
the batteries and supply power to the system. In applica-
tions where an unregulated wall adapter is used but a
regulated voltage is needed by the system, an external P-
channel MOSFET pass transistor may be added to the
LTC1980 to create a low dropout linear regulator.
From Figure 5, select the following resistors based on the
output voltage V
LDO
:
R5 = R6 • (V
LDO
– V
REF
)/V
REF
where V
REF
= 1.225V, suggested value for R6 is 100k, 1%.
This is the voltage that will be seen when operating from
a higher voltage wall adapter. When operating from the
batteries (as a regulator), the load will see either this
voltage or the voltage set by the PWM regulator, which-
ever is less, minus any drops in the pass transistor.
Placing a large-valued capacitor from the drain of this
MOSFET to ground creates output compensation.
Wall Adapter Comparator Threshold
From Figure 5, select the following resistors based on the
wall adapter comparator threshold V
WATH
:
R15 = R7(V
WATH
– V
IH1
)/V
IH1
where V
IH1
= 1.226V, suggested value for R7 is 100k. Use
1% resistors.
MODE Pin Operation
The following truth table describes MODE pin operation.
Burst Mode operation is disabled during battery charging
to reduce broadband noise inherent in Burst Mode opera-
tion. (Refer to the LT1307 data sheet for details).
POWER FLOW MODE PIN OPERATING MODE
Battery Charger 0 Disabled
Battery Charger Open Enabled Continuous
Battery Charger 1 Enabled Continuous
DC/DC converter 0 Disabled
DC/DC converter Open Enabled Burst Mode Operation
DC/DC converter 1 Enabled Continuous
Logic 1 = V
BIAS1
(Pin 13) Logic 0 = GND
The MODE pin should be decoupled with 200pF to ground
when left open.
Snubber Design
The values given in the applications schematics have been
found to work quite well for most applications. Care
should be taken in selecting other values for your applica-
tion since efficiency may be impacted by a poor choice. For
a detailed look at snubber design, Application Note 19 is
very helpful.
Frequency Compensation
Load step testing can be used to empirically determine
compensation. Application Note 25 provides information
on the technique. To adjust the compensation for the DC/
DC converter, adjust C12 and R13 (in Figure 5). Battery
charger current loop compensation is set by C11 and
battery charger float voltage compensation is set by C8.
Component Selection Basics
The application circuits work well for most 1- and 2-cell
Li-Ion, 0.5A to 1A output current designs. The next section
highlights the component selection process. More infor-
mation is available in Application Note 19.
