Power-up circuit for analog circuits

Details Power-up circuit for analog circuits Power-up circuit for analog circuits

2000-05-16

2001-09-04

Zweizig, Jeffrey (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Signal converting, shaping, or generating

Synchronizing

Reexamination Certificate

active

06285223

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a start-up circuit particularly useful in conjunction with a low power analog circuit.
BACKGROUND OF THE INVENTION
Low power systems having relatively small currents flowing therein, typically use sleep states in which circuit portions are powered down when not needed to conserve battery charge. Charging capacitors in the circuit up to operating voltage using these small currents requires long periods of time. To overcome the problem of slow power-up, a node to be charged may be brought to power supply voltage through a sufficiently large transistor for an amount of time dictated by a clock. Upon expiration of the appropriate time period, the charging is ceased and the circuit is allowed to settle back to the operating level. The disadvantage of pulling the node to a supply voltage to power-up a circuit is that it has to settle down afterwards. This may take considerable time if the currents available are relatively small. A further disadvantage is that the circuit requires a clock adding additional circuitry and hence consuming additional power.
Another method known to increase power-up speed includes using a kick-start circuit to pump current into a circuit to be powered up. The kick-start circuit provides current to transistors in the circuit being powered up. When the transistors are charged sufficiently, a transistor that produces a logic signal is turned on. The signal then turns the kick-start circuit off, leaving the attached circuitry in a powered-up state.
The disadvantage of a kick-start circuit is that charge pumped into the circuit to be powered up is not related to the amount of charge required to charge the capacitor in the kick-start circuit. Therefore, the kick-start circuit may overshoot the desirable level of charge, and hence, a period of settling down may be necessary.
FIG. 1
depicts a known start-up circuit
100
used in conjunction with a voltage reference circuit
102
. Start-up circuit
100
is shown by dotted lines. Voltage reference circuit
102
has two possible equilibrium points, one of which corresponds to zero voltage and zero current, and a second, non-zero equilibrium point, which corresponds to a useful reference voltage. Therefore, voltage reference circuit
102
must be designed to choose only the non-zero equilibrium point to establish the reference voltage. Start-up circuit
100
is provided to allow voltage reference circuit
102
to utilize only the desired equilibrium point. If voltage reference circuit
102
is at the undesired equilibrium point, the voltage is zero and therefore, I
1
and I
2
are zero. Consequently, transistor
104
provides current in transistor
106
which then moves voltage reference circuit
102
to the non-zero equilibrium point. Transistor
104
's source voltage increases as the desired equilibrium point is approached. This causes the current through transistor
104
to decrease. When voltage reference circuit
102
reaches the non-zero equilibrium point, the current through transistor
106
will be substantially the same as the current through transistor
108
. Transistor
110
and resistor
112
set the gate bias voltage for transistor
104
. Voltage reference circuit
102
is on within a gate bias voltage window. Therefore, the gate bias voltage must be high enough to turn voltage reference circuit
102
on but must not exceed the upper limit of the voltage window.
FIG. 2
depicts a kick-start circuit. When current flows in the transistors of the main part of the circuit or band gap reference, the kick-start circuit is turned off. This occurs because MP
4
mirrors the current into MN
6
which drives the gate of MN
3
high and pulls down the drain node of MN
3
. Driving this node low turns off the current mirrors in the kick-start circuit, so it stops sourcing and sinking current to the band gap reference circuit. R
3
ensures that current flows in the kick-start circuit when the band gap reference circuit is powered down.
Conventional circuits do not provide the accuracy and speed desirable to power-up low power systems. Accordingly, there is a need for a start-up circuit that provides a targeted current quickly without significantly overshooting or falling short of the targeted value.
SUMMARY OF THE INVENTION
A start-up circuit is disclosed for supplying current to an analog circuit. The start-up circuit provides current to an analog circuit quickly and accurately. The start-up circuit comprises a capacitor connected to a current mirror. Upon a power-up signal input to the start-up circuit the capacitor discharges through the reference transistor of the current mirror. The capacitor discharge causes the current mirror to provide a current to the analog circuit.


REFERENCES:
patent: 6107789 (2000-08-01), Fryer et al.
patent: 6163468 (2000-12-01), Barnes
patent: 6191644 (2001-02-01), Srinath et al.

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