A8518 Datasheet, PDF (15/30 Pages) Allegro MicroSystems – Fully integrated 42 V MOSFET for boost converter

A8518 and A8518-1
Wide Input Voltage Range, High-Efficiency,
Fault-Tolerant LED Driver
V_COMP
V_OUT
V_PWM
I_LED(TOTAL)
V_PWM
I_LED(TOTAL)
Figure 9: Typical PWM Diagram Showing V_OUT, I_LED,
and COMP Pins, as well as the PWM Signal. (PWM dim-
ming frequency is 500 Hz 1% duty cycle.)
Another important feature of the A8518 is the PWM signal to
LED current delay. This delay is typically less than 500 ns, which
allows for greater LED current accuracy at low-PWM dimming
duty cycles.
The error introduced by LED turn-on delay is partially offset by
LED turn-off delay. Therefore, a PWM pulse width of under
1 µs is still feasible, but the percentage error of LED current will
increase with narrower pulse width.
Figure 11: Falling Edge PWM Signal to Total LED Cur-
rent I_LED(TOTAL) Turn-Off Delay. Time base = 100 ns
APWM Pin
APWM
R_ISET
ISET
ISET
Current
Mirror
APWM ISET
Current
Adjust Block
PWM
LED Driver
V_PWM
I_LED(TOTAL)
Figure 10: Rising Edge PWM Signal to Total LED Cur-
rent I_LED(TOTAL) Turn-On Delay. Time base = 100 ns
Figure 12: Simplified Block Diagram of APWM ISET
Block
The APWM pin is used in conjunction with the ISET pin (see
Figure 12). This is a digital signal pin that internally adjusts
the I_ISET current. The typical input signal frequency is between
40 kHz and 1 MHz. The duty cycle of this signal is inversely
proportional to the percentage of current that is delivered to
the LED (see Figure 13). As an example, a system that deliv-
ers I_ILED(TOTAL) = 240 mA would deliver I_ILED(TOTAL) = 180
mA when an APWM signal with a duty cycle of 25% is applied.
When this pin is not used it should be tied to AGND.
Allegro MicroSystems, LLC
15
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com