LTC3552 Datasheet, PDF (19/24 Pages) Linear Technology – Standalone Linear Li-Ion Battery Charger and Dual Synchronous Buck Converter

LTC3552
APPLICATIO S I FOR ATIO
The previous analysis can be repeated to take into account
the power dissipation of the regulator by:
I_BAT
=
120°C – T_A − T_{RISE(REGULATOR)}
(V_IN – V_BAT) • θ_JA
However, the regulator typically dissipates significantly less
heat than the charger (even in worst-case situations), the
calculations here should work well as an approximation.
Moreover, when thermal feedback reduces the charge
current, the voltage at the PROG pin is also reduced
proportionally. It is important to remember that LTC3552
applications do not need to be designed for worst-case
thermal conditions since the IC will automatically reduce
charge current when the junction temperature reaches
approximately 120°C.
In order to deliver maximum charge current under all
conditions, it is critical that the exposed metal pad on
the backside of the LTC3552 package is soldered to rela-
tively large areas of PC board copper with vias to inner
copper layers. 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 40°C/W. As an example, a correctly soldered
LTC3552 can deliver over 800mA to a battery from a 5V
supply at room temperature. Without a good backside
thermal connection, this number will drop considerably.
Battery Charger Stability Considerations
The constant-voltage mode feedback loop is stable with-
out an output capacitor, provided a battery is connected
to the charger output. With no battery present, an output
capacitor on the BAT pin is recommended to reduce ripple
voltage. When using high value, low ESR ceramic capaci-
tors, it is recommended to add a 1Ω resistor in series
with the capacitor. No series resistor is needed if tantalum
capacitors are used. In constant-current mode, the PROG
pin is in the feedback loop, not the battery. The constant-
current mode stability is affected by the impedance at the
PROG pin. With no additional capacitance on the PROG
pin, the charger is stable with program resistor values as
high as 20k; 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 is loaded with a capacitance,
C_PROG, the following equation can be used to calculate
the maximum resistance value for R_PROG:
R_PROG
≤
2π
•
1
10⁵ •
C_PROG
Average, rather than instantaneous charge current may be
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
filter can be used on the PROG pin to measure the average
battery current, as shown in Figure 3. A 10k resistor has
been added between the PROG pin and the filter capacitor
to ensure stability.
LTC3552
PROG
GND
10k
R_PROG
C_FILTER
3552 F03
CHARGE
CURRENT
MONITOR
CIRCUITRY
Figure 3. Isolating Capacitive Load on
PROG Pin and Filtering
V_IN Bypass Capacitor
Many types of capacitors can be used for input bypassing;
however, caution must be exercised when using multilayer
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. Adding a 1.5Ω resistor in series with an X5R
ceramic capacitor will minimize start-up voltage transients.
For more information, see Application Note 88.
3552f
19