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L7986 Datasheet(PDF) 8 Page - STMicroelectronics

Part # L7986
Description  3 A step-down switching regulator
PDF  43 Pages
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Manufacturer  STMICROELECTRONICS [STMicroelectronics]
Direct Link  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

L7986 Datasheet(HTML) 8 Page - STMicroelectronics

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Functional description
L7986
8/43
DocID022450 Rev 6
5
Functional description
The L7986 device is based on a “voltage mode”, constant frequency control. The output
voltage VOUT is sensed by the feedback pin (FB) compared to an internal reference (0.6 V)
providing an error signal that, compared to a fixed frequency sawtooth, controls the on- and
off-time of the power switch.
The main internal blocks are shown in the block diagram in Figure 3. They are:
A fully integrated oscillator that provides sawtooth to modulate the duty cycle and the
synchronization signal. Its switching frequency can be adjusted by an external resistor.
The voltage and frequency feed-forward are implemented.
The soft-start circuitry to limit inrush current during the startup phase.
The voltage mode error amplifier.
The pulse width modulator and the relative logic circuitry necessary to drive the internal
power switch.
The high-side driver for embedded P-channel power MOSFET switch.
The peak current limit sensing block, to handle overload and short-circuit conditions.
A voltage regulator and internal reference. To supply the internal circuitry and provide
a fixed internal reference.
A voltage monitor circuitry (UVLO) that checks the input and internal voltages.
A thermal shutdown block, to prevent thermal runaway.
Figure 3. Block diagram
PEAK
CURRENT
LIMIT
OSCILLATOR
S
R
Q
THERMAL
SHUTDOWN
SOFT-
START
EN
TRIMMING
UVLO
0.6V
REGULATOR
&
BANDGAP
1.254V
3.3V
SYNCH
&
PHASE SHIFT
EN
FB
COMP
FSW
GND
SYNCH
OUT
VCC
DRIVER
E/A
PWM
PEAK
CURRENT
LIMIT
OSCILLATOR
S
R
Q
THERMAL
SHUTDOWN
SOFT-
START
EN
TRIMMING
UVLO
UVLO
0.6V
REGULATOR
&
BANDGAP
REGULATOR
&
BANDGAP
1.254V
3.3V
SYNCH
&
PHASE SHIFT
EN
FB
COMP
FSW
GND
SYNCH
OUT
VCC
DRIVER
E/A
PWM



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