LT1950IGN datasheet(12/20 Pages) LINER

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LT1950

sn1950 1950fs

N-channel power MOSFET even though VIN voltage is very

low. High GATE drive voltage reduces MOSFET RDS(ON),

improves efficiency and increases the range of MOSFETs

that can be selected. A small switching regulator at the

BOOST pin, with fixed current limit and fixed off time,

generates the VIN2 supply. With an external inductor

connected between the BOOST pin and VIN (see Figure 5),

the BOOST pin will draw current until approximately

125mA is reached, turn off for 0.5

µsandthenturnbackon.

The cycle is repeated for as long as the switcher is enabled.

By using a diode connected from BOOST to VIN2 and a

capacitor from VIN2 to ground, energy from the external

inductor is transferred to the VIN2 capacitor during the off-

time of the internal switcher. An auxiliary boost converter

is realized providing a supply to the VIN2 pin. The typical

inductor current, VIN2 voltage and BOOST pin voltage

waveforms are shown in Figure 5. When VIN2 reaches 11V,

the internal switcher is disabled. Since VIN2 supplies the

output driver of the LT1950, switching at the GATE pin will

eventually discharge the VIN2 capacitor until VIN2 reaches

a lower level of approximately 10V. At this level the internal

switcher is re-enabled and switches until VIN2 returns to

APPLICATIO S I FOR ATIO

Figure 6. Oscillator Frequency (fOSC) vs ROSC

ROSC (kΩ)

400 450 500

1950 F06

100 150

250

200

300 350

500

450

400

350

300

250

200

150

100

11V. This hysteretic (burst mode) operation for the inter-

nal switcher minimizes power dissipation from VIN.

The VREF output is a 2.5V reference supplying most of the

LT1950 control circuitry. It is available for external use

with maximum current capability of 2.5mA. The pin should

be bypassed to ground using a 0.1

µF capacitor. Internal

undervoltage lockout thresholds for VIN and VIN2 of ap-

proximately 2.6V and 6.8V respectively must be exceeded

before VREF becomes active.

Programming Oscillator Frequency

The oscillator frequency of the LT1950 is programmed

using an external resistor connected between the ROSC pin

and ground. Figure 6 shows typical fOSC vs ROSC resistor

values. The LT1950 is programmable for a free-running

oscillator frequency in the range of 100kHz to 500kHz.

Stray capacitance and potential noise pickup on the ROSC

pin should be minimized by placing the ROSC resistor as

close as possible to the ROSC pin and keeping the area of

the ROSC node as small as possible. The ground side of the

ROSC resistor should be returned directly to the GND

(analog ground) pin.

Figure 5. VIN2 Generation Using the BOOST Pin

BOOST

VIN2

VIN

LT1950

MIN

µs/DIV

0.25

(V)

(A)

1950 F05

BOOST

VIN2

ID1

IL1

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