Datasheet

15
LT1505
1505fc
APPLICATIONS INFORMATION
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Note that M4 and/or D5 are needed only if the charger
system can be potentially crowbarred.
Figure 10. V
BAT
Crowbar Protection
1505 F09
SHDN
LT1505
D5
1N4148
100k
V
BAT
V
IN
M3
1505 F08
M4
TPO610
R
S4
INFET
LT1505
V
CC
V
IN
Figure 9. V
IN
Crowbar Protection
All battery chargers with fast charge rates require some
means to detect full charge state in the battery to terminate
the high charge current. NiCd batteries are typically charged
at high current until temperature rise or battery voltage
decrease is detected as an indication of near full charge.
The charge current is then reduced to a much lower value
and maintained as a constant trickle charge. An interme-
diate “top off” current may be used for a fixed time period
to reduce 100% charge time.
NiMH batteries are similar in chemistry to NiCd but have
two differences related to charging. First, the inflection
characteristic in battery voltage as full charge is ap-
proached is not nearly as pronounced. This makes it
more difficult to use –∆V as an indicator of full charge,
and an increase in temperature is more often used with a
temperature sensor in the battery pack. Secondly, con-
stant trickle charge may not be recommended.
Instead, a moderate level of current is used on a pulse
basis (≈1% to 5% duty cycle) with the time-averaged
value substituting for a constant low trickle. Please
contact the Linear Technology Applications department
about charge termination circuits.
If overvoltage protection is needed, R3 and R4 in Figure 5
should be calculated according to the procedure described
in the Lithium-Ion Charging section. The V
FB
pin should be
grounded if not used.
Charger Crowbar Protection
If the V
IN
connector of Figure 1 can be instantaneously
shorted (crowbarred) to ground, then a small P-channel FET
M4 should be used to quickly turn off the input
P-channel FET M3 (see Figure 9), otherwise, high reverse
surge current might damage M3. M3 can also be replaced
by a diode if dropout voltage and heat dissipation are not
problems.
Note that the LT1505 will operate even when V
BAT
is
grounded. If V
BAT
of Figure 1 charger gets shorted to
ground very quickly (crowbarred) from a high battery
voltage, slow loop response may allow charge current to
build up and damage the topside N-channel FET M1. A
small diode D5 (see Figure 10) from the SHDN pin to V
BAT
will shut down switching and protect the charger.
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
Layout Considerations
Switch rise and fall times are under 20ns for maximum
efficiency. To prevent radiation, the power MOSFETs, the
SW pin and input bypass capacitor leads should be kept as
short as possible. A Schottky diode (D4 in Figure 1) rated
for at least 1A is necessary to clamp the SW pin and should
be placed close to the low side MOSFET. A ground plane
should be used under the switching circuitry to prevent
interplane coupling and to act as a thermal spreading path.
Note that the inductor is probably the most heat dissipat-
ing device in the charging system. The resistance on a 4A,
15µH inductor, can be 0.03Ω . With DC and AC losses, the
power dissipation can go as high as 0.8W. Expanded
traces should be used for the inductor leads for low
thermal resistance.
The fast switching high current ground path including the
MOSFETs, D4 and input bypass capacitor should be kept
very short. Another smaller input bypass (1µF ceramic)
should be placed very close the chip. The demo board
DC219 should be used for layout reference.