Datasheet ALT80800 (Allegro) - 9

Automotive-Grade, Constant-Current 2.0 A ALT80800 PWM Dimmable Synchronous Buck LED Driver PWM DIMMING RATIO
It is possible to pull ADIM pin below 0.4 V to achieve lower The brightness of the LED string can be changed by adjusting the than 20% analog dimming. However, if the average LED cur- PWM duty cycle at the EN pin as follows: rent determined by ADIM becomes too low and is below half the Dimming ratio = PWM on-time / PWM period inductor current ripple, negative current will flow through the inductor. To prevent such cases from happening, it is suggested For example, by selecting a PWM period of 5 ms (200 Hz PWM fre- that ADIM voltage should meet the condition below: quency) and a PWM on-time of 5 µs, a dimming ratio of 0.1% can be achieved. This is sometimes referred to as “1000:1 dimming.” ADIM > RSENSE / 0.2 × (VIN – VOUT) / L × D × T In an actual application, the minimum dimming ratio is deter- where D is duty cycle, D ≈ VOUT / VIN, T is switching period, mined by various system parameters, including: V T = 1 / f IN , VOUT , SW, L is the inductance. inductance, LED current, switching frequency, PWM frequency, For example, when RSENSE = 0.2 Ω, RON = 178 kΩ, L = 33 µH,  and fault flag usage. The device is easily capable of PWM on- VIN = 12 V, VOUT = 5.2 V, ADIM voltage should be above 0.21 V, time as short as 5 µs; however, if fault flag for open/short LED  i.e. 11% level, to avoid negative inductor current. detection is required, it should be above 130 µs due to the fault  mask timer. ADIM pin can be used in conjunction with PWM dimming to provide wider LED dimming range over 1000:1. In addition, the IC
ANALOG DIMMING
can provide thermal foldback protection by using an external NTC In addition to PWM dimming, the ALT80800 also provides an (negative temperature coefficient) thermistor, as shown below: analog dimming feature. When VADIM is over 2.0 V, the LED cur- rent is at 100% level (as defined by the SENSE resistor). When VCC VADIM is below 2 V, the LED current decreases linearly down to R1 20% at V R ADIM = 0.4 V. This is shown in the following figure: S ADIM V 200 mV CSREG NTC RP ±6 mV (100%) 100 mV
Figure 8: Using an External NTC Thermistor
ADIM pin
to Implement Thermal Foldback
40 mV voltage 0 ADIM is tied to 5 V (or VCC) if never used for analog dimming, 0.4 V 1 V 2 V or always less than 2.5 V when used for analog dimming. For
Figure 7: ADIM Pin Voltage Controls SENSE Reference
long term reliability, or extended period with extreme temperature
Voltage (hence LED current)
condition, it is better to keep ADIM always less than 2.5 V. 9 Allegro MicroSystems 955 Perimeter Road Manchester, NH 03103-3353 U.S.A. www.allegromicro.com Document Outline Features and Benefits Description Applications Package Typical Application Circuit Selection Guide Specifications Absolute Maximum Ratings Thermal Characteristics Pinout Diagrams and Terminal List Tables Functional Block Diagrams Electrical Characteristics Functional Description Application Circuit Diagrams