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LT3587

3587fc

APPLICATIONS INFORMATION

Setting The Output Voltages and The Boost3 Output

Current

The LT3587 has a trimmed internal feedback resistor. A 1M

feedback resistor from each output pin to its corresponding

feedback pin sets the outputs to 15V for Boost1, –8V for the

inverter and 15V for Boost3. Note that only one resistor is

needed to set the output voltage for each channel. Set the

output voltages according to the following formulas:

RFB1 = ((VVOUT1/1.22V) – 1) • 88.5k

RFB2 = |VNEG|/8μA

RVFB3 = ((VVOUT3/0.8V) – 1) • 56.3k

Asdescribedinprevioussections,Boost3canbeconﬁgured

as a boost current regulator. When conﬁgured as such, set

the output current according to the following formula:

RIFB3 = 200 • (0.8V/IVOUT3)

In order to maintain accuracy, use high precision resistors

when setting any of the channels output voltage and/or

the Boost3 output current (1% is recommended).

Soft-Start

The LT3587 has two soft-start control pins: EN/SS1 and

EN/SS3. The EN/SS1 pin controls the soft-start for both

the Boost1 and the inverter, while the EN/SS3 pin controls

the soft-start for the Boost3. Each of these soft-start pins

is pulled up internally with a 1μA current source.

Connecting a capacitor from the EN/SS1 pin to ground

programs a soft-start ramp for the Boost1 and the inverter

channels. Use an open-drain transistor to pull this pin low

to shut down both the Boost1 and the inverter. Turning off

this transistor allows the 1μA pull-up current to charge the

soft-start capacitor. When the voltage at the EN/SS1 pins

goes above 200mV, the regulation loops for Boost1 and

the inverter are enabled. The VC1 node voltage follows the

EN/SS1 voltage as it continues to ramp up to ensure slow

start-up on the Boost1 channel. The VC2 node follows the

ramp voltage minus 0.7V. This ensures that the inverter

starts up after the Boost1, but still has a slow ramping

output to avoid large start-up currents. The Boost1 and

the inverter regulation loops are free running with full

inductor current when the voltage at the EN/SS1 pin is

above 2.5V.

Connecting a capacitor from the EN/SS3 pin to ground

sets up a soft-start ramp for the Boost3 channel. As the

1μA current charges up the capacitor, the Boost3 regula-

tion loop is enabled when the EN/SS3 pin voltage goes

above 200mV. The VC3 node voltage follows the EN/SS3

voltage as it ramps up ensuring slow start-up on the

Boost3 channel. When the voltage at the EN/SS3 pin is

above 2V, the Boost3 regulation loop is free running with

full inductor current.

Start Sequencing

The LT3587 also has internal sequencing circuitry that

inhibits the inverter channel from operating until the feed-

back voltage of the Boost1 voltage (at the FB1 pin) reaches

about 1.1V (87% of the ﬁnal voltage). This ensures that

the Boost1 output voltage is near regulation before any

negative voltage is generated at the inverter output.

Figure 6 contrasts the start-up sequencing without any

soft-start capacitor, and with a 10nF soft-start capacitor.

Figure 6. VEN/SS1, VOUT1, VNEG, IVIN with No Soft-Start

Capacitor, and with a 10nF Soft-Start Capacitor

3587 F06a

IVIN

500mA/DIV

VVOUT1

10V/DIV

VNEG

10V/DIV

VEN/SS1

2V/DIV

400μs/DIV

0mA

3587 F06b

IVIN

500mA/DIV

VVOUT1

10V/DIV

VNEG

10V/DIV

VEN/SS1

2V/DIV

4ms/DIV

0mA

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