A8509 View Datasheet(PDF) - Allegro MicroSystems
Part Name
Description
MFG CO.
A8509
Allegro MicroSystems
'A8509' PDF : 28 Pages View PDF
A8509
Wide Input Voltage Range, High Efficiency
4-Channel Fault Tolerant LED Driver
Boost switch overcurrent protection
The boost switch is protected with pulse-by-pulse current limiting
set by the external RSENSE resistor. There also is a secondary
current limit that is sensed on the boost switch.
A8509
RSENSE
SENP SENN
Setting the current sense resistor
The current sense resistor (see figure 18) is set according to the
following formula:
ILIM
=
VSENP
RSENSE
(5)
where VSENP is found in the Electrical Characteristics table, and
RSENSE is the current sense resistor value.
The current limit is calculated by the following formula:
ILIM = IIN(max) +
∆IL
2
(6)
where IIN(max) is the maximum input current, and ΔIL is the
inductor current ripple.
Current sense resistor routing
The current sense resistor must be routed as a differential pair to
minimize measurement accuracy errors. For most current sense
resistors the resistance is measured between the inside edges of
the mounting pads of the RSENSE resistor.
Figure 19 shows correct differential current sensing connections
to the A8509. The individual current sense traces are kept short
and side-by-side to get proper signal voltage levels. The trace for
the positive sense pin (SENP) must be routed to the inside edge
of the mounting pad on the high side of RSENSE. The trace for
the negative sense pin (SENN) must be routed to the inside edge
of the mounting pad on the ground side of RSENSE.
It should be noted that when designing the PCB layout, the trace
for the negative sense pin (SENN) is often automatically merged
with the ground flood fill and with the mounting pad on the
ground side of RSENSE (shown in figure 20). However, the trace
must be kept separate and dedicated, and careful attention must
be given when routing the PCB.
Current
Sense
Figure 18. Simplified schematic of the current sense resistor connections
to the current sense amplifier.
RSENSE
A
A
B
GND
SENN
SENP
A8509LP
Figure 19. Correct layout of current sense resistor traces: (A) connect to
inside edges of pads, (B) parallel and dedicated
RSENSE
B
A
GND
SENN
SENP
A8509LP
Figure 20. Incorrect layout of current sense resistor traces: (A) do not
connect to outside edge of pad, (B) do not merge trace into ground
Allegro MicroSystems, Inc.
13
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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