Anti-Blooming Design
Anti-blooming shunts excess charge to drain when well exceeds threshold. Trade-off: charge dump efficiency vs full well capacity and linearity.
Linear range (barrier)
40000 e⁻
80 % of the well
In the well
40200 e⁻
barrier + 200 e⁻ leaked, capped at 10000 e⁻
To the drain
19800 e⁻
Spilled into neighbours
0 e⁻
no blooming
Blooming onset
1.040e+6 e⁻
20.8× full well: N_b = T + (W − T)/(1 − η)
Well above the barrier
10000 e⁻
headroom the drain protects
Illustrative hard-threshold drain: below the barrier every electron is kept; above it a fraction (1 − η) still reaches the well until the well is full, and the rest spills. Real lateral and vertical overflow drains turn on gradually, so the response bends over before the barrier.
Model and references
Illustrative. A simplified model that shows trends and orders of magnitude. Don't use its numbers for design. About the tiers
A hard barrier at a fraction of the full well: above it a fixed fraction η of the extra charge goes to the drain, the rest fills the well and then spills (blooming onset N_b = T + (W − T)/(1 − η)). Real overflow drains turn on gradually through sub-threshold barrier current; η here is a lumped parameter, not a device spec.
- Janesick J. R. (2001). Scientific Charge-Coupled Devices. SPIE Press. Blooming, lateral and vertical overflow drains, clocked anti-blooming and their cost in full well and fill factor. doi:10.1117/3.374903