a5975d STMicroelectronics, a5975d Datasheet - Page 12

• Manufacturer Part Number: a5975d
• Description: Up To 3 A Step-down Switching Regulator For Automotive Applications
• Manufacturer: STMicroelectronics

Functional description
5.1
5.2
5.3
12/50
Power supply and voltage reference
The internal regulator circuit (shown in
voltage pre-regulator, the bandgap voltage reference and the bias block that provides
current to all the blocks. The starter supplies the start-up currents to the entire device when
the input voltage goes high and the device is enabled (inhibit pin connected to ground). The
pre-regulator block supplies the bandgap cell with a pre-regulated voltage, V
very low supply voltage noise sensitivity.
Voltage monitor
An internal block continuously senses the V
their thresholds, the regulator begins operating. There is also a hysteresis on the V
(UVLO).
Figure 4.
Oscillator and synchronization
Figure 5
The clock generator provides the switching frequency of the device, which is internally fixed
at 250 kHz. The frequency shifter block acts to reduce the switching frequency in case of
strong overcurrent or short-circuit. The clock signal is then used in the internal logic circuitry
and is the input of the ramp generator and synchronizer blocks.
The ramp generator circuit provides the sawtooth signal, used for PWM control and the
internal voltage feed-forward, while the synchronizer circuit generates the synchronization
signal. The device also has a synchronization pin which can work as both master and slave.
Beating frequency noise is an issue when more than one voltage rail is on the same board.
A simple way to avoid this issue is to operate all the regulators at the same switching
frequency.
The synchronization feature, of a set of the A5975D, is simply obtained by connecting
together their SYNCH pins. The device with highest switching frequency is the master,
shows the block diagram of the oscillator circuit.
Internal circuit
Doc ID 018760 Rev 1
Figure
CC
, V
4) consists of a start-up circuit, an internal
REF
and V
BG
. If the voltages go higher than
REG
, that has a
CC
A5975D