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A7986ATR Datasheet(PDF) 8 Page - STMicroelectronics

Part # A7986ATR
Description  3 A step-down switching regulator for automotive applications
PDF  42 Pages
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Manufacturer  STMICROELECTRONICS [STMicroelectronics]
Direct Link  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

A7986ATR Datasheet(HTML) 8 Page - STMicroelectronics

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Functional description
A7986A
8/42
DocID022801 Rev 5
5
Functional description
The A7986A 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. It supplies internal circuitry and provides a
fixed internal reference
•
A voltage monitor circuitry (UVLO) that checks the input and internal voltages
•
A thermal shutdown block, to prevent thermal run-away.
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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