Datasheet
LTC4089-3
22
40893f
For more information www.linear.com/4089-3
High frequency currents, such as the high voltage input
current of the LTC4089, tend to find their way along
the ground plane on a mirror path directly beneath the
incident path on the top of the board. If there are slits or
cuts in the ground plane due to other traces on that layer,
the current will be forced to go around the slits. If high
frequency currents are not allowed to flow back through
their natural least-area path, excessive voltage will build
up and radiated emissions will occur. See Figure 9.
Figure 8. Suggested Board Layout
Figure 9. Ground Currents Follow Their Incident Path
at High Speed. Slices in the Ground Plane Cause High
Voltage and Increased Emissions.
MINIMIZE D1, L1,
C3, U1, SW PIN LOOP
MINIMIZE TRACE LENGTH
U1 THERMAL PAD
SOLDERED TO PCB.
VIAS CONNECTED TO ALL
GND PLANES WITHOUT
THERMAL RELIEF.
C1 AND D1
GND PADS
SIDE-BY-SIDE
AND SEPERATED
WITH C3 GND PAD
APPLICATIONS INFORMATION
40893 F09
V
IN
and V
HVIN
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 from connecting the charger
input to a hot power source. For more information, refer
to Application Note 88.
Battery Charger Stability Considerations
The constant-voltage mode feedback loop is stable without
any compensation when a battery is connected with low
impedance leads. Excessive lead length, however, may add
enough series inductance to require a bypass capacitor
of at least 1µF from BAT to GND. Furthermore, a 4.7µF
capacitor with a 0.2
to 1 series resistor to GND is
recommended at the BAT pin to keep ripple voltage low
when the battery is disconnected.