LT1950 Linear Technology, LT1950 Datasheet - Page 10

• Manufacturer Part Number: LT1950
• Description: Single Switch PWM Controller
• Manufacturer: Linear Technology

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LT1950
APPLICATIO S I FOR ATIO
LT1950 Input Supplies, V
V
input supply for the LT1950 output driver. V
provided by shorting the V
generating V
V
3. Figure 2 represents low V
ated using the B00ST pin. Figure 3 represents V
operation with the BOOST pin open circuit or shorted to
ground.
Low V
The LT1950 can be configured to provide a minimum of
10V GATE drive for an external N-channel MOSFET from
V
generate a second supply at the V
Applications Information “ Generating V
BOOST Pin”). The advantage of this configuration is that a
lower R
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
switcher at the BOOST pin is enabled. This switch gener-
ates the V
undervoltage lockout threshold of 6.8V the 2.5V reference
V
circuitry. When V
driver is enabled. V
providing a supply to the LT1950 output driver to ensure
a minimum of 10V drive at the GATE pin.
10
IN
IN2
IN
REF
is the main input supply for the LT1950. V
voltages as low as 3V, if the BOOST pin is used to
, V
becomes active and powers up internal control
IN
REF
DS(ON)
Operation
and GATE switching are shown in Figures 2 and
IN2
IN2
supply. As V
is achieved for the external N-channel
using the BOOST pin. Waveforms of V
IN2
IN2
U
exceeds approximately 8V, the gate
is regulated between 10V and 11V,
U
REF
IN
IN2
IN2
Output and GATE Enable
operation with V
IN
pin to the V
IN2
above 2V, the internal
ramps up above the
W
pin (see Figure 2 and
IN2
Supply Using
IN
IN2
IN2
pin or by
U
IN2
IN
can be
gener-
= V
is the
IN2
IN
,
V
If low V
8V on V
use of the BOOST pin by shorting the V
pin. Figure 3 shows that both V
exceed 6.8V to activate the 2.5V V
exceed approximately 8V to enable the output driver
(GATE pin).
12
10.2
8
4
0
4
3
2
1
0
8.5
6.8
5.1
3.4
5.0
2.5
IN
10
0
5
0
= V
GATE
V
V
REF
IN
IN2
= V
IN
IN
IN2
the LT1950 can be configured to run without the
operation is not required below approximately
Operation
V
GATE
IN2
10 s/DIV
Figure 3. V
Figure 2. Low V
IN
= V
IN2
IN
Operation
Operation
IN
MIN
REF
3V
and V
TYPICAL START-UP INPUT
SHORTED TO GROUND
LT1950
IN2
output and must
*BOOST PIN CAN BE
LT1950
V
BOOST
LEFT OPEN OR
IN
V
BOOST
IN2
V
IN
pin to the V
IN2
V
IN2
>8.2V
must now
sn1950 1950fs
*
L1
D1
C1
C1
1950 F02
1950 F03
IN