Patent Application
20080269724
The present invention relates to implantable medical devices. In particular, the present invention relates to a charge control for controlling charging of a capacitor from a battery and subsequently delivering stored energy from the capacitor to a pump motor.
Implantable drug delivery devices are used to provide patients with long-term dosage or infusion of a drug or other therapeutic agent. Implantable drug delivery devices may be categorized as either passive or active devices.
Passive drug delivery devices typically rely upon a pressurized drug reservoir to deliver the drug. The reservoir may be filled using a syringe. The drug is then delivered to the patient using force provided by the pressurized reservoir.
Active drug delivery devices include a pump or metering system to deliver the drug into the patient's system. The pump is electrically powered to deliver the drug from a reservoir through a catheter to a selected location within the patient's body. The pump typically includes a battery as its power source for both the pump and for the electronic circuitry used to control flow rate of the pump and to communicate through telemetry to an external device to allow programming of the pump.
Battery life is an important consideration for all implantable medical devices. With an implantable drug delivery device, efficiency of the driver circuitry that powers the pump motor is an important consideration. In one type of driver configuration, the pump motor is driven from electrical energy stored by a storage capacitor. The capacitor serves as a low-impedance, short-term energy reservoir to deliver sufficient power to the pump motor during assertion. During pump operation, the motor will be asserted periodically for a short period of time to provide a pulse flow of the drug, and followed by a longer period until the next assertion.
The efficiency of the driver circuitry can have an important effect on the lifetime of the battery, overall volume of the device (including battery size, capacitor size, and size of the circuitry required), and on the overall cost of the device. Considerations in the overall efficiency of the driver include the efficiency of charging the storage capacitor, and the efficiency of delivering energy stored in the storage capacitor to the pump motor.
An implantable drug delivery device includes a pump motor, a battery, and a driver powered by the battery for operating the motor. The driver includes a storage capacitor for storing electrical energy from the battery, a charge control for charging the storage capacitor, and a motor control for delivering the electrical energy from the storage capacitor to the pump motor. The charge control delivers charging current from the battery to the capacitor based upon a charging rate value, a minimum battery voltage value, sensed charging current, and sensed battery voltage.
Battery 12 acts as a power source that provides all of the electrical energy for operation of implantable drug delivery device 10. In particular, battery 12 provides the electrical energy to power device electronics 14, as well as the power used by motor driver 18 to generate electrical pulses delivered to motor 16 to pump a drug or other therapeutic agent to a desired location within the patient's body. Battery 12 can make use of any battery technology consistent with the lifetime, physical size, and performance requirements for an implantable battery. The battery technologies can include, for example, CSVO cathode technology that delivers medium capacity and high pulse current during operation. Another alternative is hybrid cathode technology that features high energy density but also has high source resistance.
Device electronics 14 typically include a microprocessor or other programmable digital electronics, together with associated memory and timing circuitry for controlling and coordinating the operation of device 10. Device electronics 14 may also include an antenna and transceiver for RF telemetry, to allow communication with an external device, so that drug delivery device 10 can be programmed to deliver a drug at a selected rate.
Motor 16 is, in one embodiment, a solenoid type pump. When the motor is asserted, a solenoid coil is energized, which produces an electromagnetic field causing a solenoid plunger or actuator to move. Motor 16 may also include a spring bias, which returns the actuator to its original position when the solenoid coil is no longer energized. Motor 16 typically is asserted or energized for a relatively short time period, with a relatively long period between successive assertions. The delivery rate of the pump will depend on the period of time between successive assertions of the motor that produce a pump stroke. Assertion time of motor 16 may be on the order of milliseconds (e.g. 5 milliseconds) and the period between motor assertion will vary with delivery rate and may be on the order of several seconds (e.g. 3 seconds).
Motor driver 18 isolates motor 16 from battery 12 through charge controller 20 and motor control 26. Motor 16 is driven by energy stored in storage capacitor Cl, rather than directly from battery 12. As a result, a low impedance load presented by motor 16 is not directly connected to battery 12, and therefore does not cause a decrease or droop in battery voltage each time a motor assertion occurs. The stability of the battery voltage is important to proper functioning of device electronics 16, as well as the electrical devices of driver 18.
Power delivered by motor control 26 to motor 16 is provided from storage capacitor C1. Charge controller 20, in conjunction with monitor 22 and firmware interface 24, controls the charging of storage capacitor C1 to enhance charging efficiency. Charge controller 20 delivers a programmable substantially constant charging current to storage capacitor C1 during each charging operation. This provides improved efficiency, because storage capacitor C1, when it begins charging, is capable of accepting a large amount of current, while providing a very slow increase in voltage. A high charging current during initial charging results in additional energy loss in the internal resistance of battery 12. By maintaining charging current at a substantially constant level throughout the charging operation, less energy loss occurs in battery 12, and the charging efficiency is improved.
Monitor 22 receives inputs representing sensed charge current from charge controller 20, sensed battery voltage BV, and sensed capacitor voltage CV. Monitor 22 provides charge controller 20 with a Charge Control signal that controls operation of the switches within charge controller 20. The Charge Control signal is a function of sensed battery voltage BV, charge current, a programmable charge rate value and a programmable minimum battery voltage value (provided to monitor 22 by firmware interface 24). Monitor 22 controls the Charge Control signal so that the charge current will be maintained at or near the charge rate value. If battery voltage BV begins to droop, for example as a result of operation of device electronics 14, monitor 22 will modify the Charge Control signal to reduce or even stop charging until the current draw from device electronics 14 is reduced and battery voltage BV increases above the minimum battery voltage value. In one embodiment, as the battery voltage BV increases, monitor 22 will vary the Charge Control signal to gradually increase the charge current until it is restored to the programmable charge rate value provided by firmware interface 24.
The coordination of the power demands of motor driver 18 with the demands of other loads operated by device electronics 14 prevents battery voltage droop that may adversely effect operation of device electronics 14. It also enhances efficiency of charging by curtailing or reducing the charging operation when battery voltage is low.
Monitor 22 also controls the discharging of storage capacitor Cl by motor control 28. Monitor 22 receives a minimum charge voltage value for storage capacitor Cl, a maximum charge value for storage capacitor Cl, and a charge time (which is the time period between motor assertions, and determines pump delivery rate). All three values are programmable through device electronics 14 and firmware interface 24. In other words, all of the programmable values provided to monitor 22 can be changed, as desired, by downloading new values via telemetry to device electronics 14, which then provides those values to firmware interface 24.
Monitor 22 uses the sensed battery voltage BV and capacitor voltage CV to determine when capacitor 26 is charged sufficiently so that motor control 26 can assert motor 16 by delivering electrical energy from storage capacitor C1 to motor 16.
Monitor 22 determines when capacitor voltage CV has reached the minimum charge value, which is provided by firmware interface 24. Monitor 22 continues to monitor voltage CV to determine whether a maximum charge voltage is reached. The maximum charge voltage is a programmable percentage of the sensed battery voltage.
If capacitor voltage CV reaches the maximum charge voltage before the charge time has expired, monitor 22 provides a Charge Complete signal to motor control 26. In response to the Charge Complete signal, motor control 26 causes current from storage capacitor C1 to be delivered to motor 16 for a time period ton sufficient to produce a full stroke of the solenoid pump.
If the charge time expires before a maximum charge voltage has been achieved by storage capacitor C1, but the minimum charge voltage was reached, then monitor 22 still produces the Charge Complete signal. In other words, even though a maximum charge not achieved on storage capacitor C1, motor 16 will again be asserted as long as there is at least the minimum charge on storage capacitor C1.
If the charge time interval expires without the capacitor voltage CV reaching the minimum charge value, then monitor 22 provides a Failed Charge signal to both device electronics 14 and firmware interface 24. The Failed Charge signal may represent only a temporary condition, or may signal a longer term problem affecting operation of implantable drug delivery device 10. Device electronics 14 can provide a signal via telemetry to an external device to indicate that a failed charge condition has occurred.
The Failed Charge signal can also be used to modify the programmed values (or select alternative values) that are provided by firmware interface 24 to monitor 22. A change in values may result in the next operating cycle successfully charging storage capacitor C1 to at least the minimum charge voltage. For example, in response to a Failed Charge signal, the charge rate may be modified to increase the charge current delivered by charge controller 20 to storage capacitor C1.
Firmware interface 24 allows the programmed values or set points used by monitor 22 to be changed to offer different modes of operation. For example, during initial setup of drug delivery device 10, prior to the implantation, device 10 may be filled with a fill fluid such as water that must be removed so that device 10 can be filled with the drug. By providing a command to device electronics 14 by telemetry, a fast operating mode can be initiated to accelerate the pumping of the fill fluid in preparation for being filled with a drug. This can be done by changing the charge time, which changes the rate at which motor 16 is asserted. Other set points, such as the charge rate, also may be changed in order to accelerate charging of storage capacitor C1 to accommodate a higher pump rate.
Charge controller 20 includes electronic switches M1 and M2, inductor L1 and sense resistor RS. Switches M1 and M2 of charge controller 20 are operated by the Charge Control signal delivered by monitor 22. Switches M1 and M2 are operated simultaneously so that one switch is on while the other is off.
When switch M1 is on, current iBAT from battery 12 flows through M1, inductor L1, and sense resistor RS to storage capacitor C1. Switch M2 is turned off, as is switch M3 of motor control 26. As a result, all of the battery current iBAT flows through switch M1 and inductor L1, and then through sense resistor RS to capacitor C1. Thus, iBAT equals iL1 equals iC1.
When the current flowing through sense resistor RS reaches the charge rate set point, as indicated by the difference between voltage V1 and voltage V2, monitor 22 changes the Charge Control signal so that M1 is turned off and M2 is turned on. The current flowing in resistor L1 at the time that M1 and M2 change state represents stored energy that otherwise could be lost. By providing a current path through transistor M2, a charging circuit is maintained which allows the energy stored in inductor L1 to be transferred to storage capacitor C1. When the current through sense resistor RS diminishes, monitor 22 again reverses switches M1 and M2 so that current again can flow through M1, L1 and RS due to storage capacitor C1. The active transfer circuit formed by switch M1, switch M2, and inductor L1, in conjunction with the current sensing provided by resistor RS, provides high efficiency charging of storage capacitor C1 from battery 12. The charging current is maintained substantially constant at a level set by the charge rate value provided by firmware interface 24 to monitor 22. This increases the efficiency of charging by not permitting extremely high currents, and thus high losses in battery 12, when charging of storage capacitor C1 first begins following a motor assertion.
In the embodiment shown in
Once storage capacitor C1 has been charged and monitor 22 produces a Charge Complete signal, switch M3 of motor control 26 is turned on. This establishes a current path from storage capacitor C1 through terminal 34, motor components RM and LM, and switch M3 to terminal 36. During the discharge of storage capacitor C1 to motor 16, switch M1 of charge controller 20 is turned off, so that battery 12 is isolated from motor 16. The charging cycle begins again after motor assertion is complete and switch M3 is again turned off.
Monitor section 22A includes differential amplifiers 40 and 42, comparator 44, programmable references 46 and 48, and backoff algorithm 50. Voltages V1 and V2 represent voltages measured on opposite sides of current sense resistance RS in
Amplifier 42 compares battery voltage BV with a programmable reference value produced by programmable reference 46 in response to the minimum battery value from firmware interface 24. The output of amplifier 42 is provided to backoff algorithm 50, which provides an input to programmable reference 48 that is used in conjunction with the charge rate set point to provide a reference level to the inverting input of comparator 44. The reference level can range from zero up to maximum level representing the maximum current defined by the charge rate set point. When battery voltage droops to below the minimum battery level, backoff algorithm 50 will cause the reference level to comparator 44 to be decreased. This decrease may be all the way to zero, or to some predefined percentage of the charge rate set point. As battery voltage then increases above the minimum battery voltage, backoff algorithm 50 provides an input that causes programmable reference 48 to vary the reference level until it reaches a maximum defined by the charge rate set point.
The output of comparator 44 is the Charge Control signal controls the state of switches M1 and M2 in
Monitor section 22B monitors capacitor voltage CV and battery voltage BV to determine when charging of storage capacitor C1 has been successful and is complete. Monitor section 22B includes comparators 52 and 54, programmable references 56 and 58, and programmable timer 60. Comparator 52, in conjunction with programmable reference 56, determines when a minimum charge of storage capacitor C1 has been completed. Comparator 52 compares capacitor voltage CV with a minimum charge level produced by programmable reference 56 in response to the minimum charge set point from firmware interface 24. When capacitor voltage CV exceeds the minimum charge level, a Minimum Charge Complete signal is supplied by comparator 52 to programmable timer 60.
Comparator 54 and programmable reference 58 determine when a maximum charge has been achieved. Programmable reference 58 produces a maximum charge level based upon the sensed battery voltage BV and a maximum charge percentage set point received from firmware interface 24. Comparator 54 compares the sensed capacitor voltage CV with the maximum charge level, which is a percentage of the sensed battery voltage BV. When capacitor voltage CV exceeds the maximum charge level, a Maximum Charge Complete signal is supplied to programmable timer 60.
Programmable timer 60 defines a charge time or time interval that represents the time between successive assertions of motor 16. This charge time, therefore, defines the pump delivery rate of implantable drug delivery device 10.
Each time a Charge Complete or Charge Failed signal is produced by programmable timer 60, it resets and begins a new charge time period. The length of the charge time period is based upon a charge time set point received from firmware interface 24. If programmable timer 60 receives a Maximum Charge Complete signal before the time charge interval expires, it generates a Charge Complete signal. It will also produce a Charge Complete signal if the Minimum Charge Complete signal has been received by the time that the charge time interval has expired. In either case, the Charge Complete signal allows motor control 26 to assert motor 16. If the charge time interval times out without the minimum charge complete signal having been generated, programmable timer 60 produces a Charge Failed signal.
The motor driver of the present invention provides a more efficient, programmable charging of a storage capacitor, which is then used to deliver pulses to operate a pump motor. The motor driver provides isolation between the battery and the motor, and coordinates the charging of the capacitor with other loads presented to the battery by the electronics of the implantable drug delivery device.
Although specific circuits have been illustrated, other implementations of the invention may use different components, circuits and technologies. For example,
Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.
