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PDF L5987 Data sheet ( Hoja de datos )
| Número de pieza | L5987 | |
| Descripción | 3A step-down switching regulator | |
| Fabricantes | STMicroelectronics | |
| Logotipo |  |
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L5987
3 A step-down switching regulator
Preliminary Data
Features
■ 3 A DC output current
■ 2.9 V to 18 V input voltage
■ Output voltage adjustable from 0.6 V
■ 250 kHz switching frequency, programmable
up to 1 MHz
■ Internal soft-start and inhibit
■ Low dropout operation: 100 % duty cycle
■ Voltage feed-forward
■ Zero load current operation
■ Over current and thermal protection
■ VFQFPN3x3-8L and HSOP8 package
Applications
■ Consumer:
STB, DVD, DVD recorder, car audio, LCD TV
and monitors
■ Industrial:
PLD, PLA, FPGA, chargers
■ Networking: XDSL, modems, DC-DC modules
■ Computer:
Optical storage, hard disk drive, printers,
audio/graphic cards
■ LED driving
VFQFPN8 3x3 mm HSOP8 exposed pad
Description
The L5987 is a step down switching regulator with
3.5 A (minimum) current limited embedded power
MOSFET, so it is able to deliver up to 3 A current
to the load depending on the application
conditionals Section 5.7 and Section 5.8).
The input voltage can range from 2.9 V to 18 V,
while the output voltage can be set starting from
0.6 V to VIN. Having a minimum input voltage of
2.9 V, the device is suitable also for 3.3 V bus.
Requiring a minimum set of external components,
the device includes an internal 250 kHz switching
frequency oscillator that can be externally
adjusted up to 1 MHz.
The QFN and the HSOP package with exposed
pad allow reducing the RthJA down to 60 °C/W
and 40 °C/W respectively.
Figure 1. Application circuit
August 2008
Rev 1
This is preliminary information on a new product now in development or undergoing evaluation.
Details are subject to change without notice.
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2 Maximum ratings
Maximum ratings
Table 2. Absolute maximum ratings
Symbol
Parameter
Vcc Input voltage
OUT
Output DC voltage
FSW, COMP, SYNCH Analog pin
INH Inhibit pin
FB
PTOT
TJ
Tstg
Feedback voltage
Power dissipation at VFQFPN
TA < 60 °C
HSOP
Junction temperature range
Storage temperature range
Value
20
-0.3 to VCC
-0.3 to 4
-0.3 to VCC
-0.3 to 1.5
1.5.
2
-40 to 150
-55 to 150
Unit
V
W
°C
°C
3 Thermal data
Table 3. Thermal data
Symbol
Parameter
RthJA
Maximum thermal resistance VFQFPN
junction-ambient (1)
HSOP
1. Package mounted on demonstration board.
Value
60
40
Unit
°C/W
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Functional description
5.2 Soft-start
The soft-start is essential to assure correct and safe start up of the step-down converter. It
avoids inrush current surge and makes the output voltage increases monothonically.
The soft-start is performed by a staircase ramp on the non-inverting input (VREF) of the error
amplifier. So the output voltage slew rate is:
Equation 1
SROUT
=
SRVREF
⋅
⎛
⎝
1
+
RR-----12--⎠⎞
where SRVREF is the slew rate of the non-inverting input, while R1and R2 is the resistor
divider to regulate the output voltage (see Figure 6). The soft-start stair case consists of 64
steps of 9.5 mV each one, from 0 V to 0.6 V. The time base of one step is of 32 clock cycles.
So the soft-start time and then the output voltage slew rate depend on the switching
frequency.
Figure 6. Soft-start scheme
Soft-start time results:
Equation 2
SSTIME
=
3----2-----⋅---6---4--
Fsw
For example with a switching frequency of 250 kHz the SSTIME is 8 ms.
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| Páginas | Total 30 Páginas | |
| PDF Descargar | [ Datasheet L5987.PDF ] | |
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