ADuM5000 Data Sheet
Rev. B | Page 12 of 16
With a fast V
DD1
slew rate (200 μs or less), the peak current
draws up to 100 mA/V of V
DD1
. The input voltage goes high
faster than the output can turn on; therefore, the peak current
is proportional to the maximum input voltage.
With a slow V
DD1
slew rate (in the millisecond range), the input
voltage does not change quickly when V
DD1
reaches UVLO. The
current surge is about 300 mA because V
DD1
is nearly constant at
the 2.7 V UVLO point. The behavior during start-up is similar
to when the device load is a short circuit; these values are con-
sistent with the short-circuit current shown in Figure 7.
When starting the device for V
ISO
= 5 V operation, do not limit
the current available to the V
DD1
power pin to less than 300 mA.
The ADuM5000 may not be able to drive the output to the
regulation point if a current-limiting device clamps the V
DD1
voltage during startup. As a result, the ADuM5000 can draw
large amounts of current at low voltage for extended periods.
The output voltage of the ADuM5000 device exhibits V
ISO
over-
shoot during startup. If this could potentially damage components
attached to V
ISO
, then a voltage-limiting device, such as a Zener
diode, can be used to clamp the voltage. Typical behavior is
shown in Figure 12 and Figure 13.
EMI CONSIDERATIONS
It is necessary for the dc-to-dc converter section of the ADuM5000
to operate at 180 MHz to allow efficient power transfer through
the small transformers. This creates high frequency currents that
can propagate in circuit board ground and power planes, causing
edge emissions and dipole radiation between the input and output
ground planes. Grounded enclosures are recommended for
applications that use these devices. If grounded enclosures are
not possible, follow good RF design practices in the layout of
the PCB. See the AN-0971 Application Note for board layout
recommendations.
THERMAL ANALYSIS
The ADuM5000 consists of four internal silicon die, attached to
a split lead frame with two die attach paddles. For the purposes
of thermal analysis, it is treated as a thermal unit with the highest
junction temperature reflected in the θ
JA
from Table 5. The value
of θ
JA
is based on measurements taken with the part mounted
on a JEDEC standard 4-layer board with fine width traces and
still air. Under normal operating conditions, the ADuM5000
operates at full load across the full temperature range without
derating the output current. However, following the recommen-
dations in the PCB Layout section decreases the thermal resistance
to the PCB, allowing increased thermal margin at high ambient
temperatures.
CURRENT LIMIT AND THERMAL OVERLOAD
PROTECTION
The ADuM5000 is protected against damage due to excessive
power dissipation by thermal overload protection circuits. Thermal
overload protection limits the junction temperature to a maximum
of 150°C (typical). Under extreme conditions (that is, high ambient
temperature and power dissipation), when the junction temper-
ature starts to rise above 150°C, the PWM is turned off, which
turns off the output current. When the junction temperature
falls below 130°C (typical), the PWM turns on again, restoring
the output current to its nominal value.
Consider the case where a hard short from V
ISO
to ground occurs.
At first, the ADuM5000 reaches its maximum current, which is
proportional to the voltage applied at V
DD1
. Power dissipates on
the primary side of the converter (see Figure 7). If self-heating of
the junction becomes great enough to cause its temperature to rise
above 150°C, thermal shutdown activates, turning off the PWM
and turning off the output current. As the junction temperature
cools and falls below 130°C, the PWM turns on and power
dissipates again on the primary side of the converter, causing
the junction temperature to rise to 150°C again. This thermal
oscillation between 130°C and 150°C causes the part to cycle
on and off as long as the short remains at the output.
Thermal limit protections are intended to protect the device
against accidental overload conditions. For reliable operation,
externally limit device power dissipation to prevent junction
temperatures from exceeding 130°C.
POWER CONSIDERATIONS
The ADuM5000 converter primary side is protected from pre-
mature operation by undervoltage lockout (UVLO) circuitry.
Below the minimum operating voltage, the power converter
holds its oscillator inactive.
When the primary side oscillator begins to operate, it transfers
power to the secondary power circuits. The secondary V
ISO
voltage
starts below its UVLO limit making it inactive and unable to
generate a regulation control signal. The primary side power
oscillator is allowed to free run under this condition, supplying
the maximum amount of power to the secondary side.
As the secondary side voltage rises to its regulation setpoint,
a large inrush current transient is present at V
DD1
. When the
regulation point is reached, the regulation control circuit produces
the regulation control signal that modulates the oscillator on the
primary side. The V
DD1
current is then reduced and is proportional
to the load current. The inrush current is less than the short-
circuit current shown in Figure 7. The duration of the inrush
depends on the V
ISO
loading conditions and on the current and
voltage available at the V
DD1
pin.