Datasheet

LTC4095
14
4095fa
APPLICATIONS INFORMATION
In constant-current mode, the PROG pin is in the feed-
back loop rather than the battery voltage. Because of the
additional pole created by any PROG pin capacitance,
capacitance on this pin must be kept to a minimum. With
no additional capacitance on the PROG pin, the battery
charger is stable with program resistor values as high
as 25k. However, additional capacitance on this node
reduces the maximum allowed program resistor. The pole
frequency at the PROG pin should be kept above 100kHz.
Therefore, if the PROG pin has a parasitic capacitance,
C
PROG
, the following equation should be used to calculate
the maximum resistance value for R
PROG
:
R
C
PROG
PROG
≤
1
210
5
π ••
The stability of the constant-current loop also needs to
be considered when average, rather than instantaneous,
battery current is of interest to the user. For example, if a
switching power supply operating in low current mode is
connected in parallel with the battery, the average current
being pulled out of the BAT pin is typically of more interest
than the instantaneous current pulses. In such a case, a
simple RC fi lter an be used on the PROG pin to measure
the average battery current as shown in Figure 7. A 10k
resistor has been added between the PROG pin and the
fi lter capacitor to ensure stability.
Board Layout Considerations
In order to deliver maximum charge current under all
conditions, it is critical that the exposed metal pad on
the backside of the LTC4095 package is soldered to the
PC board ground. Correctly soldered to a 2500mm
2
double-sided 1oz. copper board the LTC4095 has a ther-
mal resistance of approximately 60°C/W. Failure to make
thermal contact between the Exposed Pad on the backside
of the package and the copper board will result in thermal
resistances far greater than 60°C/W. As an example, a
correctly soldered LTC4095 can deliver over 950mA to a
battery from a 5V supply at room temperature. Without
a backside thermal connection, this number could drop
to less than 500mA.
IN Bypass Capacitor
Many types of capacitors can be used for input bypassing;
however, caution must be exercised when using multi-layer
ceramic capacitors. Because of the self-resonant and high
Q characteristics of some types of ceramic capacitors, high
voltage transients can be generated under some start-up
conditions, such as connecting the charger input to a live
power source. For more information, refer to Application
Note 88.
4095 F04
C
FILTER
7
2
CHARGE
CURRENT
MONITOR
CIRCUITRY
R
PROG
LTC4095
PROG
GND
10k
Figure 7. Isolating Capacitive Load on PROG Pin and Filtering