LT1950IGN datasheet(10/20 Pages) LINER

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LT1950

sn1950 1950fs

APPLICATIO S I FOR ATIO

LT1950 Input Supplies, VREF Output and GATE Enable

VIN is the main input supply for the LT1950. VIN2 is the

input supply for the LT1950 output driver. VIN2 can be

provided by shorting the VIN2 pin to the VIN pin or by

generating VIN2 using the BOOST pin. Waveforms of VIN,

VIN2, VREF and GATE switching are shown in Figures 2 and

3. Figure 2 represents low VIN operation with VIN2 gener-

ated using the B00ST pin. Figure 3 represents VIN = VIN2

operation with the BOOST pin open circuit or shorted to

ground.

Low VIN Operation

The LT1950 can be configured to provide a minimum of

10V GATE drive for an external N-channel MOSFET from

VIN voltages as low as 3V, if the BOOST pin is used to

generate a second supply at the VIN2 pin (see Figure 2 and

Applications Information “ Generating VIN2 Supply Using

BOOST Pin”). The advantage of this configuration is that a

lower RDS(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 VIN above 2V, the internal

switcher at the BOOST pin is enabled. This switch gener-

ates the VIN2 supply. As VIN2 ramps up above the

undervoltage lockout threshold of 6.8V the 2.5V reference

VREF becomes active and powers up internal control

circuitry. When VIN2 exceeds approximately 8V, the gate

driver is enabled. VIN2 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 VIN Operation

Figure 3. VIN = VIN2 Operation

VIN

µs/DIV

BOOST

VIN2

VIN

LT1950

MIN

VREF

1950 F02

GATE

VIN2

10.2

8.5

6.8

5.1

3.4

5.0

2.5

VIN = VIN2

VREF

GATE

µs/DIV

BOOST

VIN2

VIN

LT1950

TYPICAL START-UP INPUT

>8.2V

*BOOST PIN CAN BE

LEFT OPEN OR

SHORTED TO GROUND

1950 F03

VIN = VIN2 Operation

If low VIN operation is not required below approximately

8V on VIN the LT1950 can be configured to run without the

use of the BOOST pin by shorting the VIN2 pin to the VIN

pin. Figure 3 shows that both VIN and VIN2 must now

exceed 6.8V to activate the 2.5V VREF output and must

exceed approximately 8V to enable the output driver

(GATE pin).

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