Data Sheet ADuM6000
Rev. D | Page 13 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, so the peak current is proportional to
the maximum input voltage.
With a slow V
DD1
slew rate (in the millisecond range), the input
voltage is not changing quickly when V
DD1
reaches the UVLO
minimum voltage. The current surge is approximately 300 mA
because V
DD1
is nearly constant at the 2.7 V UVLO voltage. The
behavior during startup is similar to when the device load is a
short circuit; these values are consistent 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 ADuM6000 device 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 ADuM6000 device can
draw large amounts of current at low voltage for extended
periods of time.
The output voltage of the ADuM6000 device exhibits V
ISO
overshoot during startup. If this overshoot could potentially
damage components attached to V
ISO
, 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
The dc-to-dc converter section of the ADuM6000 must 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 primary and
secondary 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 ADuM6000 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
value 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
ADuM6000 operates at full load across the full temperature
range without derating the output current. However, following
the recommendations 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 ADuM6000 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 temperature starts to rise above 150°C, the PWM is
turned off, turning off the output current. When the junction
temperature drops 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 ADuM6000 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 is activated, turning off
the PWM and turning off the output current. As the junction
temperature cools and drops 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 ADuM6000 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 regula-
tion 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 propor-
tional to the load current. The inrush current is less than the
short-circuit current shown in Figure 7. The duration of the
inrush current depends on the V
ISO
loading conditions and on
the current and voltage available at the V
DD1
pin.
