This invention is in the field of circuitry for driving light-emitting diodes (LEDs). Embodiments are more specifically directed to LED driver circuitry in transmitters of photoplethysmography (PPG) systems.
Photoplethysmography (PPG) is a modern technology that has proven useful for the measurement of cardiovascular function in humans. According to this technology, fixed wavelength light from a light-emitting diode (LED) is emitted into the skin of a human subject, and is sensed by a photodiode (PD) after transmission through the skin and underlying tissue. The characteristics of the sensed light allows measurement of medical parameters such as oxygenation, pulse rate, respiratory function, and the like.
Conventional PPG sensors include the well-known pulse oximeter, for example of the type that clip-on onto the finger of the patient. Pulse oximeters typically measure the oxygen saturation of circulating blood from a comparison of the absorption of light in the dermis and subcutaneous tissue at two different wavelengths. So-called “wearable” devices such as heart rate sensors also utilize this technology, but need only measure light absorption at a single wavelength.
FIG. 1 generically illustrates the architecture of a conventional PPG system. Transmitter 3 in this system includes LED 2, which has its anode biased by the Vdd power supply voltage and its cathode coupled to ground via LED driver 4. As mentioned above, PPG pulse oximeters will included multiple LEDs 2 (e.g., red and infrared), operated in time-multiplexed fashion. When forward-biased by LED driver 4, LED 2 emits light into the patient, for example the index finger of the patient. Receiver 7 includes photodiode 6, which has its cathode biased at the Vdd power supply voltage and its anode connected to the input of amplifier 8, and which is normally reverse-biased so that photons impinging photodiode 6 will produce a current detectable by amplifier 8. In this manner, photodiode 6 senses the extent to which the light emitted by LED 2 is transmitted through the subject. The output of amplifier 8 is forwarded to the desired processing and analysis circuitry of the PPG system to determine the desired medical measurement, such as the oxygenation of the patient's blood.
The ability of any PPG system to accurately and precisely measure the parameter of interest is based on the signal-to-noise ratio (SNR) of the overall system, considering both its transmitter and receiver. For example, it has been observed that an SNR of at least about 30 dB, for the PPG system as a whole including the transmission channel through patient tissue, is necessary in order to measure pulse rate to an accuracy of 1 beat per minute (bpm). A complicating factor in practice is that the system SNR depends on the perfusion index of the patient, as illustrated in FIG. 2. Perfusion index is the ratio of the AC signal due to pulsatile blood flow to the DC background level of the light signal passing through the patient's peripheral tissue, and depends largely on the health and physical condition of the patient. As such, for the PPG system of FIG. 1, the perfusion index is reflected in the amplitude of the AC pulses of the received light (e.g., as output by amplifier 8) relative to its DC level. As shown in FIG. 2, the system SNR increases at higher perfusion index values.
Conversely, if the SNR of the PPG system can be increased, the system can measure the pulse rate and blood oxygenation in a wider range of patients, particularly those of poorer health and thus lower perfusion indices. FIG. 2 shows two SNR vs. perfusion index plots 9a, 9b. Plot 9a illustrates the relationship of system (transmit—channel—receive) SNR to perfusion index for the case in which the transmitter SNR is 95 dB; as known in the art, to attain the required system SNR, the SNRs for the transmitter and receiver must both be higher than that required system SNR. At that transmit SNR, the PPG system is able to measure pulse rate to an accuracy of one bpm only for patients exhibiting a perfusion index of at least about 0.06. In contrast, plot 9b illustrates that if the transmitter is able to operate at an SNR of 110 dB, pulse rate measurements at an error of 1 bpm can be made for patients with perfusion index values as low as about 0.01. Accordingly, noise in the transmitter of the PPG system is a critical factor in covering a wide range of patients.
So-called “wearable” electronic devices, such as fitness monitoring devices, have recently become popular. In addition to fitness monitoring devices, wearable medical monitoring devices are being contemplated for use in healthcare, for example to monitor the recovery or progress of a patient suffering from a medical condition. As such, the use of PPG to obtain oxygenation, pulse rate, and other measurements by way of a wearable device, particularly such a device that can be worn all day, is desirable. In this context, battery life becomes of critical performance.
In this regard, an important electrical parameter of a transmitter in a battery-powered system, such as a wearable device, is the “headroom” of the LED driver. As well-known by those in the art, it is desirable that battery-powered systems operate at low power supply voltages to reduce power consumption and to reduce the cost of the battery itself. In transmitter 3 of FIG. 1, the voltage drop across LED 2 in its operating state is defined by its material. The headroom, shown as Vhead in FIG. 1, is the voltage required by LED driver 4 beyond the LED voltage drop. Conversely, the minimum Vdd power supply voltage is the sum of the voltage drop across LED 2 and the headroom Vhead of LED driver 4. For battery powered systems, therefore, it is desirable that the headroom Vhead required by LED driver 4 be minimized, especially considering that the output voltage from conventional batteries tends to sag over time.
As mentioned above, it is desirable to minimize power consumption in PPG systems, particularly those in battery-powered wearable devices intended for “all-day” use. Duty cycling of LED driver 4 in transmitter 3 is a common approach to reducing system power consumption. It is therefore desirable for LED driver 4 to exhibit fast switching, and rapid settling times, so that the “on” pulse width can be reduced as much as possible and thus minimizing power consumption.
FIGS. 3a through 3c illustrate examples of conventional LED drivers for PPG systems. The circuit of FIG. 3a is an example of a typical LED driver circuit, such as used in a PPG system as described above. In this circuit, LED 10 has its anode at the Vdd power supply voltage and its cathode connected to the drain of rise time control n-channel MOS transistor 12. Transistor 12 has its source connected to the drain of n-channel driver transistor 14, which is connected to ground via variable resistor 16. Resistor 16 operates to control the current drawn through LED 10 and transistor 12, at a resistance typically set by a digital-to-analog converter (DAC). The gate of transistor 12 receives a control voltage from rise time controller 13, which is an adjustable circuit block that controls the conduction of transistor 12 to attain the desired rise and fall times in the turn-on and turn-off characteristics of LED 10. Amplifier 18 receives a reference voltage VREF at its non-inverting (positive) input and a feedback voltage from the source of transistor 14 at its inverting (negative) input. Output voltage VGATE from amplifier 18 is applied to the gate of driver transistor 14. According to this arrangement, amplifier 18 operates to drive the gate voltage VGATE at driver transistor 14 so that reference voltage VREF at the source node of transistor 14. Reference voltage VREF is modulated to selectively forward bias LED 10.
The LED driver circuit of FIG. 3a provides certain advantages in a PPG system. Specifically, variable resistor 16 tends to reduce the transmitter noise in this circuit, and the ripple exhibited by this circuit is also quite low. However, it has been observed that this arrangement is vulnerable to significant input flicker noise from amplifier 16, degrading transmitter performance. The settling time of this LED driver is also quite slow, due to the bandwidth of amplifier 18. In addition, the LED driver of FIG. 3a is not conducive to implementation in low voltage, battery-powered, applications because of its large headroom voltage, specifically the sum of the drain-to-source overdrive voltages of transistors 12 and 14 plus the voltage drop across resistor 16.
FIG. 3b illustrates a conventional LED driver with very low headroom requirements as useful for a PPG system. In this circuit, power supply 20 applies the Vdd bias to LED 10 through inductor 22. N-channel driver transistor 26 has its source-drain path connected in parallel with LED 10 between inductor 22 and ground. The gate of transistor 26 receives the output of pulse-width modulator (PWM) 24. During the “off” pulses, transistor 26 shunts the inductor current through inductor 22 to ground; during the “on” pulses, the Vdd power supply voltage forward biases LED 10, such that the inductor current is conducted through LED 10 to ground. FIG. 3b illustrates the behavior of output current Iout through LED 10 over a sequence of pulses from PWM 24, illustrating that the output current Iout appears as a sequence of triangle waves. While the headroom required by this LED driver is quite low, it has been observed that transmitter noise is quite high in this arrangement, which reduces the patient coverage as discussed above relative to FIG. 2. In addition, significant ripple is present in the LED driver of FIG. 3b. 
In the circuit of FIG. 3c, LED 10 has its anode at the Vdd power supply voltage and its cathode connected to the drain of n-channel MOS transistor 14; the source of transistor 14 is at ground. A reference voltage VREF is applied to the gate of transistor 14, and is modulated to turn LED 10 on and off, thus controlling the emission of light. This simple driver of FIG. 3c has a low headroom voltage of only the drain-to-source voltage overdrive of transistor 14, and exhibits no ripple. However, because the driver of FIG. 3c is quite noisy, its use in the transmitter of a PPG system will have limited patient coverage, as discussed above relative to FIG. 2.
By way of further background, auto-zeroing techniques for removing offset voltage and drift of operational amplifiers are known in the art, as described in Kugelstadt, “Auto-zero amplifiers ease the design of high-precision circuits”, Analog Applications Journal, 2Q 2005 (Texas Instruments Incorporated), pp. 19-28, incorporated herein by reference.