Datasheet

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
V
OUT
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 UVLOUVLO
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