Application Hints
Power Dissipation
Stability Considerations
The constant-voltage mode feedback loop is stable
without an output capacitor provided a battery is
connected to the charger output. With no battery
present, an output capacitor is recommended to
reduce ripple voltage. When using high value, low
ESR ceramic capacitors, 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.
The conditions that cause the SE9016 to reduce
charge current through thermal feedback can be
approximated by considering the power dissipated
in the IC. Nearly all of this power dissipation is
generated
by
the
internal
MOSFET—this
is
calculated to be approximately:
P
D
= (V
CC
– V
BAT
) • I
BAT
The approximate ambient temperature at which the
thermal feedback begins to protect the IC is:
T
A
= 120°C – P
D
θ
JA
T
A
= 120°C – (V
CC
– V
BAT
) • I
BAT
•
θ
JA
Thermal Considerations
Because of the small size
of the thin SOT23
package, it is very important to use a good thermal
PC board layout to maximize the available charge
current. The thermal path for the heat generated by
the IC is from the die to the copper lead frame,
through the package leads, (especially the ground
lead) to the PC board copper. The PC board copper
is the heat sink. The footprint copper pads should
be as wide as possible and expand out to larger
copper areas to spread and dissipate the heat to
the surrounding ambient. Other heat sources on the
board, not related to the charger, must also be
considered when designing a PC board layout
because they will affect overall temperature rise and
the maximum charge current.
V
CC
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 a ceramic capacitor will minimize start-up
voltage transients.
Revision 6/7/2007
Preliminary and all contents are subject to change without prior notice
© Seaward Electronics Inc., 2007. • www.seawardinc.com • Page 7
SEAWARD [ Seaward Electronics Inc. ]
