Datasheet

L5970AD
4/11
3 Functional Description
The main internal blocks are shown in Fig. 4, where is reported the device block diagram. They are:
● A voltage regulator that supplies the internal circuitry. From this regulator, a 3.3V reference
voltage is externally available.
● A voltage monitor circuit that checks the input and internal voltages.
● A fully integrated sawtooth oscillator whose frequency is500KHz
● Two embedded current limitations circuitries which control the current that flows through the
power switch. The Pulse by Pulse Current Limit forces the power switch OFF cycle by cycle
if the current reaches an internal threshold, while the Frequency Shifter reduces the switch-
ing frequency in order to strongly reduce the duty cycle.
● A transconductance error amplifier.
● A pulse width modulator (PWM) comparator and the relative logic circuitry necessary to drive
the internal power.
● An high side driver for the internal P-MOS switch.
● An inhibit block for stand-by operation.
● A circuit to realize the thermal protection function.
Figure 4. Block Diagram
3.1 POWER SUPPLY & VOLTAGE REFERENCE
The internal regulator circuit (shown in Figure 2) consists of a start-up circuit, an internal voltage Prereg-
ulator, the Bandgap voltage reference and the Bias block that provides current to all the blocks.
The Starter gives the start-up currents to the whole device when the input voltage goes high and the de-
vice is enabled (inhibit pin connected to ground).
The Preregulator block supplies the Bandgap cell with a preregulated voltage V
REG
that has a very low
supply voltage noise sensitivity.
3.2 VOLTAGES MONITOR
An internal block senses continuously the V
cc
, V
ref
and V
bg
. If the voltages go higher than their thresholds, the
regulator starts to work. There is also an hysteresis on the V
CC
(UVLO).
INHIBIT
VOLTAGES
MONITOR
PEAK TO PEAK
CURRENT LIMIT
THERMAL
SHUTDOWN
E/A
PWM
1.235V
+
-
-
+
OSCILLATOR
D
Ck
Q
FREQUENCY
SHIFTER
TRIMMING
SUPPLY
1.235V 3.5V
DRIVER
V
REF
BUFFER
LPDMOS
POWER
FB
SYNC
COMP
INH
V
REF
GND OUT
VCC
D00IN1125