Datasheet ALT80800 (Allegro) - 7

Automotive-Grade, Constant-Current 2.0 A ALT80800 PWM Dimmable Synchronous Buck LED Driver FUNCTIONAL DESCRIPTION
The ALT80800 is a synchronous buck regulator designed for
Synchronous Regulation
driving a high-current LED string. It uses average current mode control to maintain constant LED current and consistent bright- The ALT80800 integrates an N-channel DMOS as the low-side ness. The LED current level is easily programmable by selection switch to implement synchronous regulation for LED drivers, as of an external sense resistor, with a value determined as follows: shown in Figure 4. iLED = VCSREG / RSENSE VIN where VCSREG = VCSH – VCSL = 0.2 V typical. Boot Charger If necessary, a resistor can be inserted in series with the CSL pin BOOT C to fine-tune the LED current, as shown below: BOOT L SW Rsc
VOUT
iCSH Floating
i_L
i Gate Driver
SW
LED CSH GND CLED + V R Integrated CSREG V CSL Radj SENSE SENSE Switch –
ALT80800
– + i
Figure 4: Synchronous Buck LED Driver
CSL × Radj iCSL The Synchronous configuration can effectively pull down SW to ground by forcing the low-side synchronous switch on even VCSREG = iLED × RSENSE + iCSL × Radj with small inductor current, as shown in Figure 5. Therefore, the Therefore BOOT capacitor can be charged normally every switch cycle to  i ensure the normal operation of buck LED drivers. LED = (VCSREG – iCSL × Radj) / RSENSE
Figure 3: How To Fine-Tune LED Current Using Radj
For example, with a desired LED current of 1.4 A, the required RSENSE = 0.2 V / 1.4 A = 0.143 Ω. But the closest power resistor  available is 0.13 Ω. Therefore, the difference is Radj × iCSL = 0.2 V – 1.4 A × 0.13 Ω = 0.018 V where iCSL = 75 µA typical Radj = 0.018 V / 75 µA = 240 Ω
Figure 5: Normal SW waveform with SR configuration
Note that the effects of Radj are only applicable when the output
when VIN ≈ VOUT: VIN = 5.4 V, VOUT = 5.14 V
voltage is at least 5 V. If the LED string voltage is below 5 V,
(2 white LEDs)
Radj = 0 should be used. The LED current is further modulated by the ADIM (Analog Dimming) pin voltage. This feature can be used for LED bright- ness calibration, or for thermal foldback protection. See Analog Dimming section for details. 7 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