LT3990 datasheet(13/20 Pages) LINER

13 / 20 page

LT3990

3990p

the boost capacitor is charged with the energy stored

in the inductor, the circuit will rely on some minimum

load current to get the boost circuit running properly.

This minimum load will depend on input and output volt-

ages, and on the arrangement of the boost circuit. The

minimum load generally goes to zero once the circuit has

started. Figure 5 shows a plot of minimum load to start

and to run as a function of input voltage. In many cases

the discharged output capacitor will present a load to the

switcher, which will allow it to start. The plots show the

worst-case situation where VIN is ramping very slowly.

For lower start-up voltage, the boost diode can be tied to

VIN; however, this restricts the input range to one-half of

the absolute maximum rating of the BOOST pin.

Enable Pin

The LT3990 is in shutdown when the EN pin is low and

active when the pin is high. The rising threshold of the EN

comparator is 1V, with a 30mV hysteresis. This threshold is

accurate when VIN is above 4.2V. If VIN is lower than 4.2V,

tie EN pin to GND to place the part in shutdown.

APPLICATIONS INFORMATION

Figure 4. Two Circuits for Generating the Boost Voltage

Figure 5. The Minimum Input Voltage Depends on

Output Voltage, Load Current and Boost Circuit

Adding a resistor divider from VIN to EN programs the

LT3990 to regulate the output only when VIN is above a

desired voltage (see Figure 6). This threshold voltage,

VIN(EN), can be adjusted by setting the values R3 and R4

such that they satisfy the following equation:

IN EN

()

•

where output regulation should not start until VIN is above

VIN(EN). Note that due to the comparator’s hysteresis,

regulation will not stop until the input falls slightly below

VIN(EN).

LT3990

(4a) For VOUT ≥ 2.2V

BOOST

VIN

VOUT

GND

LT3990

(4b) For VOUT < 2.2V; VIN < 27V

BOOST

VIN

3990 F04

VOUT

GND

LOAD CURRENT (mA)

050

2.5

3.5

5.0

100

200

250

3.0

4.5

4.0

150

300

350

FRONT PAGE APPLICATION

VOUT = 3.3V

TO START

TO RUN

LOAD CURRENT (mA)

050

4.0

5.0

6.5

100

200

250

3990 F05

4.5

6.0

5.5

150

300

350

FRONT PAGE APPLICATION

VOUT = 5V

TO START

TO RUN

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