Data Sheet ADuM5240/ADuM5241/ADuM5242
Rev. B | Page 11 of 16
APPLICATIONS INFORMATION
DC-TO-DC CONVERTER
The dc-to-dc converter section of the ADuM524x works on
principles that are common to most modern power supply
designs. V
DD
power is supplied to an oscillating circuit that
switches current into a chip scale air core transformer. Power is
transferred to the secondary side where it is rectified to a high
dc voltage. The power is then linearly regulated down to about
5.2 V and supplied to the secondary side data section and to the
V
ISO
pin for external use. This design allows for a physically
small power section compatible with the 8-lead SOIC packaging
of this device. Active feedback was not implemented in this
version of isoPower for reasons of size and cost.
Because the oscillator runs at a constant high frequency inde-
pendent of the load, excess power is internally dissipated in the
output voltage regulation process. Limited space for transformer
coils and components also adds to internal power dissipation.
This results in low power conversion efficiency, especially at low
load currents.
The load characteristic curve in Figure 8 shows that the V
DD
current is typically 80 mA with no V
ISO
load and 110 mA at full
V
ISO
load at the V
DD
supply pin.
Alternate supply architectures are possible using this technology.
Addition of a digital feedback path allows regulation of power
on the primary side. Feedback would allow significantly higher
power, efficiency, and synchronization of multiple supplies at the
expense of size and cost. Future implementations of isoPower
includes feedback to achieve these performance improvements.
The ADuM524x can be operated with the internal dc-to-dc
enabled or disabled. With the internal dc-to-dc converter
enabled, the isolated supply of Pin 8 provides the output power
as well as power to the secondary-side circuitry of the part.
The internal dc-to-dc converter state of the ADuM524x is
controlled by the input V
DD
voltage, as defined in Table 6. In
normal operating mode, V
DD
is set between 4.5 V and 5.5 V and
the internal dc-to-dc converter is enabled. When/if it is desired
to disable the dc-to-dc converter, V
DD
is lowered to a value
between 2.7 V and 4.0 V. In this mode, V
ISO
power is supplied
externally by the user and the signal channels of the ADuM524x
continue to operate normally.
There is hysteresis into the V
DD
input voltage detect circuit.
Once the dc-to-dc converter is active, the input voltage must be
decreased below the turn-on threshold to disable the converter.
This feature ensures that the converter does not go into
oscillation due to noisy input power.
PROPAGATION DELAY-RELATED PARAMETERS
Propagation delay is a parameter that describes the time it takes
a logic signal to propagate through a component. The propagation
delay to a logic low output may differ from the propagation
delay to a logic high.
INPUT (
Ix
)
OUTPUT (V
Ox
)
t
PLH
t
PHL
50%
50%
06014-012
Figure 12. Propagation Delay Parameters
Pulse width distortion is the maximum difference between
these two propagation delay values and is an indication of how
accurately the timing of the input signal is preserved.
Channel-to-channel matching refers to the maximum amount
the propagation delay differs between channels within a single
ADuM524x component.
Propagation delay skew refers to the maximum amount the
propagation delay differs between multiple ADuM524x
components operating under the same conditions.
DC CORRECTNESS AND MAGNETIC FIELD
IMMUNITY
Positive and negative logic transitions at the isolator input cause
narrow (~1 ns) pulses to be sent to the decoder via the transformer.
The decoder is bistable and is, therefore, either set or reset by
the pulses, indicating input logic transitions. In the absence of
logic transitions at the input for more than 1 μs, a periodic set
of refresh pulses indicative of the correct input state are sent to
ensure dc correctness at the output. If the decoder receives no
internal pulses of more than about 5 μs, the input side is assumed
to be unpowered or nonfunctional, in which case the isolator
output is forced to a default state by the watchdog timer circuit
(see Table 12 through Table 14).
The limitation on the magnetic field immunity of the ADuM524x
is set by the condition in which induced voltage in the receiving
coil of the transformer is sufficiently large to either falsely set or
reset the decoder. The following analysis defines the conditions
under which this may occur. The 3 V operating condition of the
ADuM524x is examined because it represents the most susceptible
mode of operation.
The pulses at the transformer output have an amplitude greater
than 1.0 V. The decoder has a sensing threshold at about 0.5 V, thus
establishing a 0.5 V margin in which induced voltages can be
tolerated. The voltage induced across the receiving coil is given by
V = (−dβ/dt)Σπr
n
2
; n = 1, 2, … , N
where:
β is magnetic flux density (gauss).
N is the number of turns in the receiving coil.
r
n
is the radius of the n
th
turn in the receiving coil (cm).
