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ADP1046AW Datasheet(PDF) 19 Page - Analog Devices

Part No. ADP1046AW
Description  Digital Controller for Isolated
Download  88 Pages
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com

ADP1046AW Datasheet(HTML) 19 Page - Analog Devices

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Data Sheet
Rev. 0 | Page 19 of 88
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].
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.
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 ADP1046AW to use SR soft start,
ensure correct operation of this function by setting the falling
edge of SR1 (t10) to a lower value than the rising edge of SR1 (t9)
and by setting the falling edge of SR2 (t12) to a lower value than
the rising edge of SR2 (t11). SR soft start can also be disabled by
setting Register 0x0F[7] = 1.
The ADP1046AW is well suited for dc-to-dc converters in
isolated topologies. Every time a PWM signal crosses the isola-
tion barrier an additional propagation delay is added due to the
isolating components. The ADP1046AW allows programming
of an adjustable 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 57). 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.
The ADP1046AW can be configured to disable PWM outputs
under 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 ADP1046AW to be
used 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.
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 22,
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. There-
fore, the user must set the rising edges and falling edges based
on the case with the least modulation.
Figure 22. Modulation Limit Settings
Each LSB in Register 0x2E corresponds to a different time step
size, depending on the switching frequency (see Table 46). The
modulated edges cannot extend beyond one switching cycle.
The GUI provided with the ADP1046AW is recommended for
programming this feature (see Figure 23).
Figure 23. Setting Modulation Limits (Modulation Range Shown by Arrows)

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