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PDF L7981 Data sheet ( Hoja de datos )

Número de pieza L7981
Descripción 3A step-down switching regulator
Fabricantes STMicroelectronics 
Logotipo STMicroelectronics Logotipo



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L7981
3 A step-down switching regulator
Features
3 A DC output current
4.5 V to 28 V input voltage
Output voltage adjustable from 0.6 V
250 kHz switching frequency, programmable
up to 1 MHz
Internal soft-start and enable
Low dropout operation: 100% duty cycle
Voltage feed-forward
Zero load current operation
Overcurrent 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
Suitable for LED driving
Figure 1. Application circuit
VFQFPN8 3x3
HSOP8 exposed pad
Description
The L7981 is a step down switching regulator with
3.7 A (minimum) current limited embedded power
MOSFET, so it si able to deliver up to 3 A current
to the load depending on the application
conditions.
The input voltage can range from 4.5 V to 28 V,
while the output voltage can be set starting from
0.6 V to VIN.
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 packages with exposed
pad allow reducing the RthJA down to 60°C/W and
40°C/W respectively.
July 2010
Doc ID 15182 Rev 3
1/44
www.st.com
44
http://www.Datasheet4U.com

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L7981 pdf
L7981
2 Maxim
um ratings
Maximum ratings
Table 2. Absolute maximum ratings
Symbol
Parameter
Vcc
OUT
FSW, COMP, SYNCH
EN
FB
PTOT
TJ
Tstg
Input voltage
Output DC voltage
Analog pin
Enable pin
Feedback voltage
Power dissipation VFQFPN
at TA < 60°C
HSOP
Junction temperature range
Storage temperature range
Value
30
-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
junction-ambient (1)
1. Package mounted on demonstration board.
VFQFPN
HSOP
Value
60
40
Unit
°C/W
Doc ID 15182 Rev 3
5/44

5 Page





L7981 arduino
L7981
Functional description
5.2 Soft-star t
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
SR OUT
=
SR
VR
EF
⎝⎛⎠⎞1
+
R---1----
R2
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 7). 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 7. Soft-start scheme
Soft-start time results:
Equation 2
SSTIME
=
3--2----6---4-------
Fsw
For example with a switching frequency of 250kHz the SSTIME is 8ms.
Doc ID 15182 Rev 3
11/44

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