Datasheet ALT80800 (Allegro) - 8

Automotive-Grade, Constant-Current 2.0 A ALT80800 PWM Dimmable Synchronous Buck LED Driver Switching Frequency ENABLE AND DIMMING
The ALT80800 operates in fixed on-time mode during switching. The ALT80800 is activated when a logic high signal is applied to The on-time (and hence switching frequency) is programmed the EN (enable) pin and VIN = VVCCIN is above UVLO threshold using an external resistor connected between the TON pin and 4.3 V. The buck converter ramps up the LED current to a target  ground, as given by the following equation: level set by RSENSE when PWM pin = High. t The EN pin is high-voltage tolerant and can be directly connected ON = k × (RON + RINT ) × ( VOUT / VIN ) to a power supply. However, if VEN is higher than the VIN voltage fSW = 1 / [ k × (RON + RINT )] at any time, a series resistor (10 kΩ) is required to limit the current  where k = 0.0111, with f flowing into the EN pin. This resistor is helpful in preventing EN SW in MHz, tON in µs, and RON and RINT (internal resistance, 3 kΩ) in kΩ. from damage in case of reverse-battery connection. This series resistor is not necessary if EN is driven from a logic input. The PWM pin is a logic input pin and is internally pulled up to 2.0 VCC through a resistor. EN pin and PWM pin function as illustrated below: 1.5 EN pin PWM pin VCC LED High Low ON OFF Hz) (M 1.0 High High/Open ON ON f SW Low x Shutdown When the EN pin is forced from high to low, the LED current is 0.5 turned off, but the IC remains in standby mode for up to at least 10 ms. If EN goes high again within this period, the LED current is turned on immediately if PWM pin is high. If EN pin is low for 0.0 0 100 200 300 400 500 more than tOFFDelay, the IC enters shutdown mode to reduce power RON (kΩ) consumption. The next high signal on EN will initialize a full
Figure 6: Switching Frequency vs. t
startup sequence, which includes a startup delay of approximately
ON resistance
150 μs. This startup delay is not present during PWM operation. To minimize the peaks of switching frequency harmonics in EMC Active dimming of the LED is achieved with 2 options: by send- measurement, a dithering feature is implemented. The dithering ing a PWM (pulse-width modulation) signal to the EN pin (while range is internally set at ±5%. The actual switching frequency is PWM = High), or by sending a dimming PWM signal to the swept linearly between 0.95 × fSW and 1.05 × fSW, where fSW is PWM pin (while EN is enabled) as illustrated in the table above. the programmed switching frequency. The rate of modulation for The resulting LED brightness is proportional to the duty cycle of fSW is fixed internally at 12.5 kHz. the applied PWM signal. A practical range for PWM dimming frequency is between 100 Hz (period = 10 ms) and 2 kHz. If the PWM dimming signal at PWM pin is low when the EN pin is high, the LED will be off immediately and IC is alive waiting for next PWM pulse. The internal LDO is still on and can provide bias to the internal and external circuits. In PWM dimming operation and when VIN is above 40 V, a 10 kΩ resistor is needed to be in parallel with a 0.047 µF output  capacitor across the LED string to facilitate BOOT charging dur- ing PWM dimming OFF period. 8 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