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A7985A Datasheet(PDF) 10 Page - STMicroelectronics

Part # A7985A
Description  2 A step-down switching regulator for automotive applications
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Manufacturer  STMICROELECTRONICS [STMicroelectronics]
Direct Link  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

A7985A Datasheet(HTML) 10 Page - STMicroelectronics

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Functional description
A7985A
10/44
DocID023128 Rev 5
5
Functional description
The A7985A 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
Soft-start circuitry to limit inrush current during the startup phase
Voltage mode error amplifier
Pulse width modulator and the relative logic circuitry necessary to drive the internal
power switch
High-side driver for embedded P-channel Power MOSFET switch
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 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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