Datasheet LT1950 (Analog Devices) - 10

LT1950
U U W U APPLICATIO S I FOR ATIO LT1950 Input Supplies, VREF Output and GATE Enable VIN = VIN2 Operation
VIN is the main input supply for the LT1950. VIN2 is the If low VIN operation is not required below approximately input supply for the LT1950 output driver. VIN2 can be 8V on VIN the LT1950 can be configured to run without the provided by shorting the VIN2 pin to the VIN pin or by use of the BOOST pin by shorting the VIN2 pin to the VIN generating VIN2 using the BOOST pin. Waveforms of VIN, pin. Figure 3 shows that both VIN and VIN2 must now VIN2, VREF and GATE switching are shown in Figures 2 and exceed 6.8V to activate the 2.5V VREF output and must 3. Figure 2 represents low VIN operation with VIN2 gener- exceed approximately 8V to enable the output driver ated using the B00ST pin. Figure 3 represents VIN = VIN2 (GATE pin). operation with the BOOST pin open circuit or shorted to ground. 12 VIN2 8
Low V
MIN
IN Operation
4 GATE 3V The LT1950 can be configured to provide a minimum of 0 L1 10V GATE drive for an external N-channel MOSFET from VIN D1 V BOOST IN voltages as low as 3V, if the BOOST pin is used to LT1950 4 generate a second supply at the VIN2 pin (see Figure 2 and VIN VIN2 3 Applications Information “ Generating V C1 IN2 Supply Using 2 VREF BOOST Pin”). The advantage of this configuration is that a 1 lower RDS(ON) is achieved for the external N-channel 0 MOSFET, improving efficiency, versus a controller run- 50µs/DIV 1950 F02 ning at 3V input without boosted gate drive. In addition,
Figure 2. Low VIN Operation
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. 10.2 V This versatility allows optimization of the overall power 8.5 IN = VIN2 6.8 TYPICAL START-UP INPUT supply performance and allows applications which would 5.1 >8.2V 3.4 otherwise not be possible without a more complex topol- ogy. Figure 2 shows that for V 5.0 IN above 2V, the internal VREF VIN switcher at the BOOST pin is enabled. This switch gener- 2.5 BOOST * ates the V LT1950 IN2 supply. As VIN2 ramps up above the 0 V undervoltage lockout threshold of 6.8V the 2.5V reference IN2 C1 10 V GATE REF becomes active and powers up internal control *BOOST PIN CAN BE circuitry. When V 5 IN2 exceeds approximately 8V, the gate LEFT OPEN OR SHORTED TO GROUND driver is enabled. V 0 IN2 is regulated between 10V and 11V, 10µs/DIV 1950 F03 providing a supply to the LT1950 output driver to ensure a minimum of 10V drive at the GATE pin.
Figure 3. VIN = VIN2 Operation
1950fa 10