ADuM6400/ADuM6401/ADuM6402/ADuM6403/ADuM6404 Data Sheet
Rev. A | Page 24 of 28
For each output channel with CL greater than 15 pF, the
additional capacitive supply current is given by
IAOD = 0.5 × 10−3 × ((CL − 15) × VISO) × (2f − fr); f > 0.5 fr (3)
where:
CL is the output load capacitance (pF).
VISO is the output supply voltage (V).
f is the input logic signal frequency (MHz); it is half the input
data rate expressed in units of Mbps.
fr is the input channel refresh rate (Mbps).
CURRENT LIMIT AND THERMAL OVERLOAD
PROTECTION
The ADuM640x 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 VISO to ground occurs.
At first, the ADuM640x reaches its maximum current, which is
proportional to the voltage applied at VDD1. Power dissipates on
the primary side of the converter (see Figure 16). 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 ADuM6400/ADuM6401/ADuM6402/ADuM6403/
ADuM6404 power input, data input channels on the primary
side, and data input channels on the secondary side are all
protected from premature operation by undervoltage lockout
(UVLO) circuitry. Below the minimum operating voltage, the
power converter holds its oscillator inactive, and all input channel
drivers and refresh circuits are idle. Outputs remain in a high
impedance state to prevent transmission of undefined states
during power-up and power-down operations.
During application of power to VDD1, the primary side circuitry
is held idle until the UVLO preset voltage is reached. At that
time, the data channels initialize to their default low output
state until they receive data pulses from the secondary side.
When the primary side is above the UVLO threshold, the data
input channels sample their inputs and begin sending encoded
pulses to the inactive secondary output channels. The outputs
on the primary side remain in their default low state because no
data comes from the secondary side inputs until secondary side
power is established. The primary side oscillator also begins to
operate, transferring power to the secondary power circuits.
The secondary VISO voltage is below its UVLO limit at this point;
the regulation control signal from the secondary side is not being
generated. The primary side power oscillator is allowed to free
run under these conditions, 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 VDD1. When the
regulation point is reached, the regulation control circuit pro-
duces the regulation control signal that modulates the oscillator
on the primary side. The VDD1 current is then reduced and is
proportional to the load current. The inrush current is less than
the short-circuit current shown in Figure 16. The duration of
the inrush current depends on the VISO loading conditions and
on the current and voltage available at the VDD1 pin.
As the secondary side converter begins to accept power from
the primary, the VISO voltage starts to rise. When the secondary
side UVLO is reached, the secondary side outputs are initialized
to their default low state until data is received from the corre-
sponding primary side input. It can take up to 1 μs after the
secondary side is initialized for the state of the output to
correlate to the primary side input.
Secondary side inputs sample their state and transmit it to the
primary side. Outputs are valid about 1 μs after the secondary
side becomes active.
Because the rate of charge of the secondary side power supply is
dependent on loading conditions, the input voltage, and the output
voltage level selected, take care that the design allows the con-
verter sufficient time to stabilize before valid data is required.
When power is removed from VDD1, the primary side converter
and coupler shut down when the UVLO level is reached. The
secondary side stops receiving power and starts to discharge.
The outputs on the secondary side hold the last state that they
received from the primary side. Either the UVLO level is reached
and the outputs are placed in their high impedance state, or the
outputs detect a lack of activity from the primary side inputs
and the outputs are set to their default low value before the
secondary power reaches UVLO.