IEEE Solid-State Circuits Magazine - Summer 2014 - 61

The secondary impedance can be reflected to the primary side by dividing
by n2. Converting this circuit to a parallel
RLC circuit and assuming operation in
resonance, the structure can be simplified to the primary ESR and an equivalent load resistor as seen in Figure 2 [4].
This model gives rise to two facts:
first, this equivalent load can be modulated on the secondary side. And
since this modulation sequence can
be seen on the primary side, it can be
used for uplink communication and
is better known as load modulation or
load shift keying.
Second, this equivalent load resistor determines the power matching
and thus the efficiency of the inductive link. The overall link efficiency
h 1 h 2 can therefore be calculated and
optimized against the load factor. This
means we select the optimal secondary link capacitance for a certain load
resistance (factor a ) and then select the
appropriate secondary inductance for
resonance. Doing this, load matching is
achieved for the inductive link and the
optimal efficiency becomes
=

k2 Q1 Q2
2

^1 + 1 + k 2 Q 1 Q 2 h

η1
PS

ηT

η2

Class-E

ηL

Tissue

Power
Conditioning
Secondary Coil

Primary Coil

Implanted
Electronics

Figure 1: Telemetric power link efficiency [1] (courtesy of M. Ghovanloo).

link requires automatic tuning or link
compensation for parameter mismatch.
Such link compensation can be employed on both the primary and the
secondary sides, e.g., by tuning the
resonance capacitor. Doing this on the
primary side changes the transmit (TX)
frequency and requires careful consideration of regulations. For example, when
we employ a significant TX power in
the 13.56-MHz ISM band, we have only
14 kHz of bandwidth for TX tuning,
which yields an insufficient 0.1% tuning
range to compensate for link parameter
variations. Employing link tuning on
the secondary side requires a tunable

capacitance in the IMD and typically
leads to large on-chip capacitance.
Continuing along the chain of link
efficiencies shown in Figure 1, the next
important part is the power conditioning inside the IMD. This is because the
telemetric link delivers an ac supply,
which we need to convert to one or
several useful dc supplies for the IMD.
Therefore, we first need efficient rectification and thereafter supply regulation.
The easiest integrated rectifier employs metal-oxide-semiconductor (MOS)
diodes, as shown in Figure 4(a). These
passive MOS rectifiers suffer from a large
voltage drop, which can exceed several

where Q1 and Q 2 are the quality factors of the LC tanks and k is the coupling coefficient.
For illustration, let us assume a
quite high Q = 250 and a not uncommonly seen low coupling k = 0.5%.
The optimal link efficiency is then
as low as 20%; this emphasizes the
need for high k and high Q.
While achieving a good coupling k
is mainly a geometrical issue and thus
a system-engineering task, aiming for
high Q comes with two significant electrical drawbacks. First, a high Q value
translates directly into low bandwidth
of the link, as illustrated in Figure 3(a).
Designing the link with high efficiency
therefore restrains its use for high-datarate telemetry. Second, a high-Q link is
very sensitive to parameter variations
due to parasitic capacitances, bond
wires, and so on. As shown in Figure 3(b),
for a link where Q = 250 and with a 5%
shift in resonant frequency, the ideal
link efficiency (20% in our example) is reduced by 70%, to only 6%. The inductive

C1
+

Power
Amplifier IPRIM

VSEC

Skin

opt/a

External
Battery

Air

C2
L2

Primary
Coil

IPRIM
Requivalent

Body

Secondary
Coil

Figure 2: Weakly coupled, resonant inductive link: equivalent circuit [4].

Gain

h link

ηS
PB

f0
Q=
BW

High Q

0.75
~70%

0.5

Low Q
f0
(a)

0.25

Frequency

13.56 MHz
fTX

∆f

14.2 MHz

fRX

(b)

Figure 3: (a) Link bandwidth versus Q-factor and (b) inductive link sensitivity.

IEEE SOLID-STATE CIRCUITS MAGAZINE

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Table of Contents for the Digital Edition of IEEE Solid-State Circuits Magazine - Summer 2014