2000 January

11

Philips Semiconductors

Product specification

Frame Transfer CCD Image Sensor

FT 18

Application information

Current handling

One of the purposes of VPS is to drain the holes that are generated

during exposure of the sensor to light. Free electrons are either

transported to the VRD connection and, if excessive (from over-

exposure), free electrons are drained to VNS. No current should

flow into any VPS connection of the sensor. During high overexposure

a total current 10 to 15mA through all VPS connections together

may be expected. The PNP emitter follower in the circuit diagram

(figure 6) serves these current requirements.

VNS drains superfluous electrons as a result of overexposure. In

other words, it only sinks current. During high overexposure a total

current of 10 to 15mA through all VNS connections together may be

expected. The NPN emitter follower in the circuit diagram meets

these current requirements.

Decoupling of DC voltages

All DC voltages should be decoupled with a 100nF decoupling

capacitor. This capacitor must be mounted as close as possible to

the sensor pin. Further noise reduction (by bandwidth limiting) is

achieved by the resistors in the connections between the sensor

and its voltage supplies. The electrons that build up the charge

packets that will reach the floating diffusions only add up to a small

current, which will flow through VRD.Therefore a large series resistor

in the VRD connection may be used.

Outputs

To limit the on-chip power dissipation, the output buffers are designed

with open source outputs. Outputs to be used should therefore be

loaded with a current source or more simply with a resistance to

GND. In order to prevent the output (which typically has an output

impedance of about 400

Ω) from bandwidth limitation as a result of

capacitive loading, load the output with an emitter follower built from

a high-frequency transistor. Mount the base of this transistor as close

as possible to the sensor and keep the connection between the

emitter and the next stage short. The CCD output buffer can easily

be destroyed by ESD. By using this emitter follower, this danger is

suppressed; do NOT reintroduce this danger by measuring directly

on the output pin of the sensor with an oscilloscope probe. Instead,

measure on the output of the emitter follower. Slew rate limitation is

prevented by avoiding a too-small quiescent current in the emitter

follower; about 10mA should do the job. The collector of the emitter

follower should be decoupled properly to suppress the Miller effect

from the base-collector capacitance.

A CCD output load resistor of 3.3k

Ω typically results in a bandwidth

of 110MHz. The bandwidth can be enlarged to about 130MHz by

using a resistor of 2.2k

Ω instead, which, however, also enlarges the

on-chip power dissipation.

Device protection

The output buffers of the FT 18 are likely to be damaged if VPS

rises above SFD or RD at any time. This danger is most realistic

during power-on or power-off of the camera. The RD voltage should

always be lower than the SFD voltage.

Never exceed the maximum output current. This may damage the

device permanently. The maximum output current should be limited

to 6mA.

Be especially aware that the output buffers of these image sensors

are very sensitive to ESD damage.

Because of the fact that our CCDs are built on an n-type substrate,

we are dealing with some parasitic npn transistors.To avoid activation

of these transistors during switch-on and switch-off of the camera,

we recommend the application diagram of figure 6.