Datasheet ALT80800 (Allegro) - 10

Automotive-Grade, Constant-Current 2.0 A ALT80800 PWM Dimmable Synchronous Buck LED Driver OUTPUT VOLTAGE AND DUTY CYCLE
During SW on-time: The figure below provides simplified equations for approximat- ing output voltage. The output voltage of a buck converter is iRIPPLE = (VIN – VOUT) / L × tON = (VIN – VOUT) / L × t × D approximately given as: where D = tON / t. VOUT ≈ VIN × D , D = tON / (tON + tOFF) During SW off-time: where D is the duty cycle. iRIPPLE = VOUT / L × tOFF = VOUT / L × t × (1 – D)
V
Simplified equation for output voltage:
IN
V MOS OUT = VIN × D HS CIN More precisely: L VOUT = (VIN – iAVG × RDSON(HS)) × D – (1 – D) × RDSON(LS) × iAVG
SW
– (DCR + RSENSE) × iAVG where DCR is the internal resistance of the inductor, RSENSE is i the current sensing resistance, RDSON(HS) is the on-resistance L RSENSE of high-side switch, R
V
DSON(LS) is the on-resistance of low-side
OUT
MOS switch, i LS AVG is the average current through inductor and equal to LED current.
GND
VSW VIN 0 t –VD iL i –VD RIPPLE t tON tOFF Period, t Note: VD = RDSON(LS) × IL
Figure 9: Simplified Waveforms for a Buck Converter
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