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参数资料
| 型号： | LT3587EUD#PBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING REGULATOR, 1200 kHz SWITCHING FREQ-MAX, PQCC20 |
| 封装： | 3 X 3 MM, LEAD FREE, PLASTIC, QFN-20 |
| 文件页数： | 5/24页 |
| 文件大小： | 395K |
| 代理商： | LT3587EUD#PBF |
LT3587
13
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,Boost3canbecongured
as a boost current regulator. When congured 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 nal 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
0V
0mA
3587 F06b
IVIN
500mA/DIV
VVOUT1
10V/DIV
VNEG
10V/DIV
VEN/SS1
2V/DIV
4ms/DIV
0mA
0V
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