LT1511CSW View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LT1511CSW
Linear Technology
'LT1511CSW' PDF : 16 Pages View PDF
LT1511
APPLICATIONS INFORMATION
off instantly when the input is dead shorted to avoid large
current surges from the battery back through the charger
into the FET. Gate capacitance slows turn-off, so a small
P-channel (Q2) is to discharge the gate capacitance quickly
in the event of an input short. The body diode of Q2 creates
the necessary pumping action to keep the gate of Q1 low
during normal operation. Note that Q1 and Q2 have a VGS
spec limit of 20V. This restricts VIN to a maximum of 20V.
For low dropout operation with VIN > 20V consult factory.
Optional Connection of Input Diode and
Current Sense Resistor
The typical application shown in Figure 1 on the first page
of this data sheet shows a single diode to isolate the VCC
pin from the adapter input. This simple connection may be
unacceptable in situations where the main system power
must be disconnected from both the battery and the
adapter under some conditons. In particular, if the adapter
is disconnected or turned off and it is desired to also
CLP
LT1511
CLN
SW
VCC
PARASITIC
INTERNAL
L1
DIODE
RS1
R7
500Ω
+ C1
1µF
+
CIN
D3
RS4
ADAPTER
IN
TO
SYSTEM
POWER
1511 F12a
Figure 12a. Standard Connection
R7
500Ω
CLP +
LT1511
C1
1µF
CLN
SW
VCC
PARASITIC
INTERNAL
L1
DIODE
+
CIN D3
RS1
RS4
D4
ADAPTER
IN
TO
SYSTEM
POWER
1511 F12b
disconnect the system load from the battery, the system
will remain powered through the parasitic diode from the
SW pin to the VCC pin.
The circuit in Figure 12b allows system power to go to 0V
without drawing battery current by adding an additional
diode, D4. To ensure proper operation, the LT1511 current
sense amplifier inputs (CLP and CLN) were designed to
work above VCC and not to draw current from VCC when the
inputs are pulled to ground by a powered-down adapter.
Layout Considerations
Switch rise and fall times are under 10ns for maximum
efficiency. To prevent radiation, the catch diode, SW pin
and input bypass capacitor leads should be kept as short
as possible. A ground plane should be used under the
switching circuitry to prevent interplane coupling and to
act as a thermal spreading path. All ground pins should be
connected to expanded traces for low thermal resistance.
The fast-switching high current ground path, including the
switch, catch diode and input capacitor, should be kept
very short. Catch diode and input capacitor should be
close to the chip and terminated to the same point. This
path contains nanosecond rise and fall times with several
amps of current. The other paths contain only DC and/or
200kHz tri-wave and are less critical. Figure 13 indicates
the high speed, high current switching path. Figure 14
shows critical path layout. Contact Linear Technology for
an actual LT1511 circuit PCB layout or Gerber file.
SWITCH NODE
L1
HIGH
VIN
CIN
FREQUENCY
CIRCULATING
D1
PATH
VBAT
COUT
BAT
Figure 13. High Speed Switching Path
LT1511 • F13
Figure 12b. Modified Input Diode Connection
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.
15
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