A8508 View Datasheet(PDF) - Allegro MicroSystems
Part Name
Description
MFG CO.
A8508
Allegro MicroSystems
'A8508' PDF : 28 Pages View PDF
A8508
Wide Input Voltage Range, High Efficiency
8-Channel Fault Tolerant LED Driver
Pulse-by-pulse current limit
Figure 21 illustrates the normal waveform for the current sense
signal. The pulse-by-pulse current limit is designed to limit the
current through the external MOSFET to prevent failure. When
the VSENSEP threshold is reached, the IC stops switching to allow
the inductor current to fall. Operation of pulse-by-pulse current
limiting is shown in figure 22.
Secondary boost switch limit
In case there is an inductor short during operation ,the A8508 has
a secondary switch current limit. When this threshold is reached,
the IC immediately shuts down. The level of this current limit is
set above the pulse-by-pulse current limit to protect the switch
from destructive currents when the boost inductor is shorted.
Output overvoltage and undervoltage
protection
IL
C1 VSENSE
Normal spikes
due to switching
The OVP pin on the A8508 controls both the overvoltage (OVP)
and undervoltage (UVP) protection features. The pin circuit is
shown in figure 23. The OVP protection protects the boost con-
verter from excessive voltage levels when the feedback control
loop is broken, usually caused by an open connection from output
voltage to the LEDs. The UVP function provides output voltage-
to-ground short protection when an external disconnect switch is
used. For more detailed information on disconnect switch appli-
cation, see the Undervoltage Protection (UVP) section.
C2
VGATE
C3
For proper operation of this pin, due to the relatively low voltage
level, special care has to be taken during PCB layout. Figure 24 is
an example of a proper PCB layout.
t
Figure 21. Current sense signal (VSENSE) during normal operation,
showing large spikes that are filtered out by a blanking period to avoid
false overcurrent tripping; RSENSE = 10 mΩ; shows inductor current IL (ch1,
1 A/div.), VSENSE (ch2, 20 mV/div.), and gate voltage of the main boost
switch VGATE (ch3), t = 500 ns/div.
A8508
OVP
1.25 V
VOUT
ROVP
OVP
UVP
100 mV
50 μA
IL
Figure 23. Simplified schematic of the Overvoltage Protection section.
VOUT
C2
C1
VCOMP
C3
C4
VGATE
t
Figure 22. Typical pulse-by-pulse current limit; shows IL (ch1, 2 A/div.),
VOUT (ch2, 10 V/div.), VCOMP (ch3, 2 V/div.), and VGATE (ch4),
t = 500 ns/div.
A8508
OVP
A
ROVP
B
VOUT
Figure 24. OVP resistor connections; (A) connection should be short,
(B) connection to VOUT can be long, and ROVP should be as close to the
OVP pin as possible.
Allegro MicroSystems, Inc.
14
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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