Oscillating circuit

Details Oscillating circuit Oscillating circuit

2001-03-29

2002-08-20

Mis, David (Department: 2817)

Oscillators

Solid state active element oscillator

Transistors

C331S105000, C331S175000, C331S185000

Reexamination Certificate

active

06437651

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an oscillating circuit, and particularly, to an oscillating circuit to be used for a video signal receiving device having an improved VCO (voltage controlled oscillator) free run frequency characteristic, particularly, which serves to detect a video in a PLL (phase-locked loop) type video detecting circuit, for example.
In the PLL type video detecting circuit, a VCO circuit for locking to an input signal to detect a video is used. The VCO circuit regulates a free run frequency into an IF frequency (for example, 45.75 MHz specified for U.S.) to be used by means of an externally provided resonance circuit
51
formed by a variable coil L
1
and a capacitor C
1
as shown in FIG.
4
. The resonance circuit
51
is connected to an oscillator
52
provided in an IC through an IC pin
54
of a lead frame (not shown) and a bonding pad
53
for connecting with an IC terminal. The resonance circuit may be constituted by an internal element (a capacitor C
2
shown in a dotted line of
FIG. 4
) of an integrated circuit and the externally provided variable coil L
1
in place of the externally provided capacitor C
1
.
In a VCO for video detection, a free run frequency is determined by the resonance circuit
51
and is locked to an input frequency through the PLL operation of an APC (automatic phase control) circuit. When an IC power is turned ON, an electric potential of an island provided under the bonding pad
53
is changed so that a floating capacity of the bonding pad is varied, resulting in a drift of the VCO free run frequency. In the case in which an AFT (automatic frequency tuning) voltage is to be generated by utilizing a change in the APC voltage with respect to the frequency, the drift appears as a drift of an AFT voltage. Since the AFT has a very high sensitivity, the drift causes a center voltage to be shifted.
For this reason, when a VCO coil is to be varied and regulated to generate a desirable AFT voltage, a VCO oscillating frequency cannot be regulated accurately or the regulation takes a great deal of time and labor if a drift is caused while the IC power is ON.
The cause will be described below with reference to a sectional view showing a bonding pad structure in FIG.
5
. An IC bonding pad
63
is provided on an island formed in a P-type semiconductor substrate
61
which will be hereinafter referred to as an N-type epitaxial layer
62
, for example. The epitaxial layer
62
provided under the pad is set in a floating state on an electric potential basis.
The bonding pad
63
is formed of a metal layer, a MOS capacitance Cm is formed between the epitaxial layer
62
and a single-layered metal
63
A provided through an oxide film
64
, and a junction capacitance Cj is formed between the (N-type) epitaxial layer
62
and the (P-type) substrate
61
(see
FIGS. 5
to
7
). Values of the parasitic capacities are changed depending on a difference in an electric potential between the epitaxial layer
62
and the substrate
61
.
In the case in which the epitaxial layer
62
provided under the pad
63
is set in the floating state, differences in an electric potential determined by the values of the MOS capacitance Cm and the junction capacitance Cj are caused between the pad
63
and the epitaxial layer
62
and between the epitaxial layer
62
and the substrate
61
when a voltage is applied to the pad
63
. Consequently, a very small leakage current (dark current) is generated from the epitaxial layer
62
to the substrate
61
of a chip. Therefore, a long time is taken until the electric potential of the epitaxial layer
62
is stabilized to be equal to that of the substrate
61
. Therefore, the value of the parasitic capacity between the single-layered metal
63
A and the epitaxial layer
62
is successively changed before the electric potential of the epitaxial layer
62
is stabilized to be equal to that of the substrate
61
.
This change in the capacity influences the capacity of the resonance circuit
51
so that a frequency drift is generated when the power supply is turned ON.
SUMMARY OF THE INVENTION
In consideration of the problems, the invention is intended for fixing an electric potential of an epitaxial layer provided under a bonding pad to which a resonance circuit for a VCO is connected to a predetermined electric potential through a resistor, thereby increasing a speed of a change in the electric potential of the epitaxial layer and quickly stabilizing a value of a parasitic capacity, resulting in an improvement in a drift when a power supply is turned ON.
Moreover, an island provided under the bonding pad is fixed to have a predetermined electric potential through a resistor formed in another island.
Furthermore, the predetermined electric potential to which an electric potential of the island provided under the bonding pad is to be fixed is a Vcc voltage or a GND voltage.
Moreover, the bonding pad is formed in a position shifted from a position where a contact portion to be connected to the resistor is wire-bonded.


REFERENCES:
patent: 6320474 (2001-11-01), Kimaya et al.

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