Datasheet

LT1950
10
1950fa
APPLICATIO S I FOR ATIO
WUU
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LT1950 Input Supplies, V
REF
Output and GATE Enable
V
IN
is the main input supply for the LT1950. V
IN2
is the
input supply for the LT1950 output driver. V
IN2
can be
provided by shorting the V
IN2
pin to the V
IN
pin or by
generating V
IN2
using the BOOST pin. Waveforms of V
IN
,
V
IN2
, V
REF
and GATE switching are shown in Figures 2 and
3. Figure 2 represents low V
IN
operation with V
IN2
gener-
ated using the B00ST pin. Figure 3 represents V
IN
= V
IN2
operation with the BOOST pin open circuit or shorted to
ground.
Low V
IN
Operation
The LT1950 can be configured to provide a minimum of
10V GATE drive for an external N-channel MOSFET from
V
IN
voltages as low as 3V, if the BOOST pin is used to
generate a second supply at the V
IN2
pin (see Figure 2 and
Applications Information “ Generating V
IN2
Supply Using
BOOST Pin”). The advantage of this configuration is that a
lower R
DS(ON)
is achieved for the external N-channel
MOSFET, improving efficiency, versus a controller run-
ning at 3V input without boosted gate drive. In addition,
typical controllers running at low input voltages have the
limitation of only being able to use logic level MOSFETs.
The LT1950 allows a greater range of usable MOSFETs.
This versatility allows optimization of the overall power
supply performance and allows applications which would
otherwise not be possible without a more complex topol-
ogy. Figure 2 shows that for V
IN
above 2V, the internal
switcher at the BOOST pin is enabled. This switch gener-
ates the V
IN2
supply. As V
IN2
ramps up above the
undervoltage lockout threshold of 6.8V the 2.5V reference
V
REF
becomes active and powers up internal control
circuitry. When V
IN2
exceeds approximately 8V, the gate
driver is enabled. V
IN2
is regulated between 10V and 11V,
providing a supply to the LT1950 output driver to ensure
a minimum of 10V drive at the GATE pin.
Figure 2. Low V
IN
Operation
Figure 3. V
IN
= V
IN2
Operation
12
8
4
0
V
IN
50µs/DIV
BOOST
V
IN2
V
IN
LT1950
L1
D1
C1
MIN
3V
4
3
2
1
0
V
REF
1950 F02
GATE
V
IN2
BOOST
V
IN2
10.2
8.5
6.8
5.1
3.4
5.0
2.5
0
10
5
0
V
IN
= V
IN2
V
REF
GATE
10µs/DIV
BOOST
V
IN2
V
IN
LT1950
TYPICAL START-UP INPUT
>8.2V
C1
*
*BOOST PIN CAN BE
LEFT OPEN OR
SHORTED TO GROUND
1950 F03
BOOST
V
IN2
V
IN
= V
IN2
Operation
If low V
IN
operation is not required below approximately
8V on V
IN
the LT1950 can be configured to run without the
use of the BOOST pin by shorting the V
IN2
pin to the V
IN
pin. Figure 3 shows that both V
IN
and V
IN2
must now
exceed 6.8V to activate the 2.5V V
REF
output and must
exceed approximately 8V to enable the output driver
(GATE pin).