ADP1046-100-EVALZ AD [Analog Devices], ADP1046-100-EVALZ Datasheet - Page 19

• Manufacturer Part Number: ADP1046-100-EVALZ
• Description: Digital Controller for Isolated
• Manufacturer: AD [Analog Devices]

Data Sheet
SYNCHRONOUS RECTIFICATION
SR1 and SR2 are recommended for use as the PWM control
signals when using synchronous rectification. These PWM
signals can be configured much like the other PWM outputs.
An optional soft start can be applied to the synchronous rectifier
PWM outputs. The SR soft start can be programmed using
Register 0x54[1:0].


The advantage of ramping the SR signals is to minimize the
output voltage step that occurs when the SR FETs are turned
on without a soft start. The advantage of turning the SR signals
completely on immediately is that they can help to minimize
the voltage transient caused by a load step.
Using Register 0x54[1], the SR soft start can be programmed to
occur only once (the first time that the SR signals are enabled)
or every time that the SR signals are enabled, for example, when
the system enters or exits light load mode.
When programming the
correct operation of this function by setting the falling edge of
SR1 (t
setting the falling edge of SR2 (t
edge of SR2 (t
Register 0x0F[7] = 1.
SR (SYNCHRONOUS RECTIFIER) DELAY
The
topologies. Every time a PWM signal crosses the isolation barrier
an additional propagation delay is added due to the isolating
components. The
delay (0 ns to 315 ns in steps of 5 ns) using Register 0x79[5:0]. This
delay moves both SR1 and SR2 later in time to compensate for the
added delay due to the isolating components (see Figure 56). In
this way, the edges of all PWM outputs can be aligned, and the
SR delay can be applied separately as a constant dead time.
ADAPTIVE DEAD TIME CONTROL
A set of registers called the adaptive dead time (ADT) registers
(Register 0x68 to Register 0x70) allows the dead time between
PWM edges to be adapted on the fly. The
only when the modulation is below the dead time (primary
current) threshold programmed in Register 0x68. The Analog
Devices software GUI allows the user to easily program the dead
time values, and it is recommended that the GUI be used for
this purpose.
Before ADT is configured, the primary current threshold must
be programmed. Each individual PWM rising and falling edge
(t
offset at no load (zero current).
1
to t
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When SR soft start is disabled (Register 0x54[0] = 0),
the SR signals are turned on to their full PWM duty cycle
values immediately.
When SR soft start is enabled (Register 0x54[0] = 1), the
SR signals ramp up from zero duty cycle to the desired
duty cycle in steps of 40 ns per switching cycle.
10
14
) to a lower value than the rising edge of SR1 (t
) can then be programmed to have a specific dead time
11
is well suited for dc-to-dc converters in isolated
). SR soft start can also be disabled by setting
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allows programming of an adjustable
12
) to a lower value than the rising
to use SR soft start, ensure
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uses the ADT
9
) and by
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This offset can be positive or negative and is relative to the nominal
edge position. When the CS1 current is between zero and the
current threshold, the amount of dead time is linearly adjusted
in steps of 5 ns. The averaging period of the CS1 current and
the speed of the dead time adjustment can also be programmed
in Register 0x70 to accommodate faster or slower adjustment.
For example, if the CS1 threshold is set to 2 A, t
rising edge of 100 ns. If the ADT setting for t
t
the current is 1 A. Similarly, ADT can be applied in the negative
direction.
LIGHT LOAD MODE
The
light load conditions based on the value of CS2. Register 0x3B
and Register 0x7D are used to program the light load mode
thresholds for turn-off and turn-on of SR1, SR2, and other
PWM outputs. Below the light load threshold programmed in
Register 0x3B, the SR outputs are disabled; the user can also
program any of the other PWM outputs to shut down below
this threshold. Light load mode allows the
with interleaved topologies that incorporate automatic phase
shedding at light load.
To prevent the system from oscillating between light load
and normal modes due to the thresholds being programmed
too close to each other, a programmable debounce is provided
in Register 0x7D[5:4]. This debounce prevents the part from
changing state within the programmed interval.
The speed of the SR enable is programmable from 37.5 μs to 300 μs
in four discrete steps using Register 0x7D[3:2]. This ensures that,
in case of a load step, the SR signals (and any other PWM outputs
that are temporarily disabled) can be turned on quickly enough to
prevent damage to the FETs that they are controlling.
The light load mode digital filter is also used during light
load mode.
MODULATION LIMIT
The modulation limit register (Register 0x2E) can be
programmed to apply a maximum duty cycle modulation limit
to any PWM signal, thus limiting the modulation range of any
PWM output. When modulation is enabled, the maximum
modulation limit is applied to all PWM outputs collectively. As
shown in Figure 21, this limit is the maximum time variation
for the modulated edges from the default timing, following the
configured modulation direction. There is no minimum duty
cycle limit setting. Therefore, the user must set the rising edges
and falling edges based on the case with the least modulation.
OUTx
1
moves to 140 ns when the current is 0 A and to 120 ns when
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t
t
RX
FX
can be configured to disable PWM outputs under
Figure 21. Modulation Limit Settings
t
MODULATION_LIMIT
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1
is 40 ns at no load,
1
has a nominal
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to be used