LT1505 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1505' PDF : 16 Pages View PDF
LT1505
APPLICATIONS INFORMATION
if the SHDN pin is not used, otherwise internal pull-down
current will keep the SHDN pin low and switching off when
power turns on.
Each TGATE and BGATE pin has a 50k pull-down resistor
to keep the external power FETs off when shut down or
power is off.
Note that maximum operating VCC is 24V. For short
transients the LT1505 can be operated as high as 27V. For
VCC higher than 24V it is preferred to use the VC pin to shut
down. If the SHDN pin has to be used to shut down at VCC
higher than 24V, the Figure 5 pull-up circuit must be used
to slow down the VGBIAS ramp-up rate when the SHDN pin
is released. Otherwise, high surge current charging the
bypass capacitor might damage the LT1505. For VCC less
than 24V, only a 100k resistor and no capacitor is needed
at SHDN pin to VIN for pull-up.
Synchronization
The LT1505 can be synchronized to a frequency range
from 240kHz to 280kHz. With a 200ns one-shot timer on
chip, the LT1505 provides flexibility on the synchronizing
pulse width. Sync pulse threshold is about 1.2V (Figure 6).
Nickel-Cadmium and Nickel-Metal-Hydride Charging
The circuit in the 4A Lithium Battery Charger (Figure 1) can
be modified to charge NiCd or NiMH batteries. For
example, 2-level charging is needed; 2A when Q1 is on,
and 200mA when Q1 is off (Figure 7).
For 2A full current, the current sense resistor (RS1) should
be increased to 0.05Ω so that enough signal (10mV) will
be across RS1 at 0.2A trickle charge to keep charging
current accurate.
For a 2-level charger, R1 and R2 are found from:
R1= (2.465)(4000)
ILOW
R2 = (2.465)(4000)
IHI − ILOW
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.
24V ≤ VCC < 27V
VIN
LT1505
3M
SHDN
OPEN DRAIN
3.3µF
1505 F05
Figure 5
5V TO 20V
5k
SYNC
VN2222
PULSE WIDTH > 200ns
Figure 6
LT1505
1505 F06
LT1505
PROG
CPROG
1µF
R1
49.3k
R2
5.49k
Q1
1505 F07
Figure 7. 2-Level Charging
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,
constant trickle charge may not be recommended.
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