Datasheet

Functional description L7985
8/43 Doc ID 022446 Rev 3
5 Functional description
The L7985 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. 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 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