A8510
Wide Input Voltage Range, High Efficiency
Fault Tolerant LED Driver
Setting the current sense resistor
The typical threshold for the current sense circuit is 180 mV,
when RADJ is 0 Ω. This voltage can be trimmed by the RADJ
resistor. The typical trip point should be set at about 3 A, which
coincides with the cycle-by-cycle current limit minimum thresh-
old. A sample calculation is done below:
Given: 2.85 A of input current, and the calculated maximum
value of the sense resistor, RSC = 0.063 Ω.
The RSC chosen is 0.056 Ω, a standard value.
VDD
The VDD pin provides regulated bias supply for internal circuits.
Connect the capacitor CVDD with a value of 0.1 μF or greater to
this pin.
Shutdown
If the EN/PWM pin is pulled low for more than tPWML , the
device enters shutdown mode and clears all internal fault regis-
ters. As an example, at a 2-MHz clock frequency, the maximum
PWM low period, while avoiding shutdown, is 16 ms. In shut
down, the IC disables all current sources and waits until the
EN/PWM pin goes high to re-enable the IC and proceed with
power-up.
Also:
RADJ = (VSENSETRIP – VADJ ) / IADJ
The typical trip point voltage is calculated as:
Fault protection during operation
(5) The A8510 constantly monitors the state of the system to deter-
mine if any fault conditions occur during normal operation. The
response to a triggered fault condition is summarized in the Fault
Mode table, on the next page.
VADJ = 2.85 A × 0.056 Ω = 0.160 V
RADJ = (0.180 – 0.160 V) / (20.3 μA) = 1.0 kΩ
Input UVLO
When VIN and VSENSE rise above the UVLO enable hysteresis
(VUVLOrise + VUVLOhys ), the A8510 is enabled. A8510 is disabled
when VIN falls below the VUVLOfall threshold for more than 50 μs.
This lag is to avoid shutting down because of momentary glitches
in the input power supply.
The possible fault conditions that the device can detect are: Open
LED pin, LED pin shorted to GND, shorted inductor, VOUT short
to GND, SW pin shorted to GND, ISET pin shorted to GND, and
input disconnect switch source shorted to GND.
Note the following:
• Some of the protection features might not be active during
startup, to prevent false triggering of fault conditions.
• Some of these faults will not be protected if the input
disconnect switch is not being used. An example of this is
VOUT short to ground.
Allegro MicroSystems, Inc.
21
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com