IEEE Solid-States Circuits Magazine - Fall 2023 - 32

and reliance on tethered optical fiber
and communication systems make
their use in wireless IMDs impractical
or infeasible [17]. LEDs present
distinctive advantages in comparison
to lasers. These include low power
consumption, reliable illumination
stability, and low cost. Recent advancements
in microfabrication have
resulted in a significant reduction in
the size of LEDs while improving their
efficiency [16], [18], [19]. This progress
not only facilitates LED assembly
in the IMD but also enhances IMD miniaturization
and energy efficiency.
The IMD needs a LED driving circuit
to activate the LED periodically
for neuron stimulation. In addition,
>3 V
ac/dc
Vac/dc
Inductive Link
CCS
Inductive Link
SCS
ac/dc
the driver needs to deliver current
pulses that are large enough, e.g., at
the milliampere level, to the LED to
ensure the desired intensity of the
emitted light [20]. This directly correlates
with the effectiveness of neuromodulation.
Traditionally, IMDs have
utilized the constant-current stimulator
(CCS) structure to drive the LED
[Figure 1(a)] [21]. When the switch
is on, a constant-current source will
drive the LED. This approach allows
for the precise control of LED current,
preventing the emission of excess
light that could potentially damage
tissue. However, the large instantaneous
LED current will heavily load
the inductive link, resulting in strong
interference and disruption to the
power supply. In [22], a switched-capacitor-based
stimulator (SCS) is used
to address the loading issue [Figure
1(b)]. More specifically, a storage
capacitor is charged to the supply
voltage by the inductive link during
the charging phase. In the subsequent
stimulation phase, the storage capacitor
is disconnected from the inductive
link and dumps its stored charges to
drive the LED. Meanwhile, a current
limiter sets the upper bound of the
LED current to ensure current safety
control. However, both the CCS and
SCS require a supply voltage larger
than the forward voltage of the LED
(VF). For instance, the blue LED used
>3 V
Vac/dc
Cs
Vac/dc
Vac/dc
Vac/dc
Vac/dc
Nonstimulation Phase
(a)
<1.8 V
Vac/dc
ac/dc
Inductive Link
C1
VB-SCS
Vac/dc
2×Vac/dc
C2
4
2
4
2
10
5
Charging Phase
(c)
Stimulation Phase
2
1
VC2
Stimulation Phase
Charging Phase
(b)
VC2
VC1
VC1
1.8 V
1.8 V
3.6 V
Charging
Charging
ILED
ILED
8 mA
Stimulation Phase
0.2 s5 ms
Output Light
(d)
FIGURE 1: Circuit diagram and operation diagram of three LED drivers for optical stimulation: (a) CCS, (b) SCS, and (c) VB-SCS. (d) Measured
transient waveform of VB-SCS (adapted from [23]). SCS: switched-capacitor-based stimulator; VB-SCS: voltage-boosted switched-capacitorbased
stimulator.
32
FALL 2023
IEEE SOLID-STATE CIRCUITS MAGAZINE
Output Light
LED NoL Curr. (mA) Volt. (V)
Volt. (V)

IEEE Solid-States Circuits Magazine - Fall 2023

Table of Contents for the Digital Edition of IEEE Solid-States Circuits Magazine - Fall 2023

Contents
IEEE Solid-States Circuits Magazine - Fall 2023 - Cover1
IEEE Solid-States Circuits Magazine - Fall 2023 - Cover2
IEEE Solid-States Circuits Magazine - Fall 2023 - Contents
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IEEE Solid-States Circuits Magazine - Fall 2023 - Cover3
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