LTC3882-1
52
Rev A
For more information
APPLICATIONS INFORMATION
Figure 29. Time-Based V
OUT
Turn-On
Figure 30. Time-Based V
OUT
Turn-Off
As shown in Figure 29, it is important to remember that the
hysteresis given in the Electrical Characteristics (EC) table
applies above VOUT_UV_FAULT_LIMIT, which specifies the
UV limit when the output is falling out of regulation. For
turn-on, the output must rise above this programmed limit
plus the hysteresis to avoid exceeding TON_MAX_FAULT_
LIMIT. PGOOD is indicated at that point. For this reason,
VOUT_UV_FAULT_LIMIT should be more than 27mV below
the programmed output voltage in low range and more
than 54mV below V
OUT
in high range. There is a fixed delay
and other timing uncertainty associated with all changes
in output voltage controlled by the LTC3882-1. A nominal
fixed timing delay of 270µs exists to process any change
in output voltage, including soft start/stop, margining and
general changes in VOUT_COMMAND value. The start of all
time-based output operations occur with an uncertainty of
±50µs and have a nominal step resolution of 100µs. This
means the minimum TON_DELAY or TOFF_DELAY that the
LTC3882-1 can produce will range from 220µs to 320µs,
not including basic oscillator tolerances. For software-
based output changes (e.g., margining), this algorithmic
delay begins when the STOP bit is received on the serial
bus. An example of this minimum turn on/off delay and
step-wise output control can be seen in Figure 31, where
TON_DELAY = 0s and TON_RISE = 1ms.
To effectively implement sequencing and synchronized
ramping between rails controlled by LTC digital power
products, two signals should be shared between all
controlling ICs: SHARE_CLK and RUN (CONTROL pin
on LTC297x products). This facilitates synchronized rail
sequencing on or off based on shared input supply state
(VIN_ON threshold), external hardware control (RUN pin),
or PMBus commands (possibly using global addressing).
Figure 32 shows an example of output supply sequencing
using TON_DELAY.
Using this scheme, conventional coincident and ratiometric
tracking can also be emulated by setting equivalent turn-
on/off delays and appropriate rise and fall times as shown
in Figure 33 and Figure 34.
In addition, these schemes can easily be mixed and matched
to create any necessary ramping controls, some of which
might prove difficult to implement with conventional
analog-only controllers. These programmable features
DAC VOLTAGE
ERROR (NOT
TO SCALE)
TIME DELAY OF
<1S, TYPICAL
DIGITAL SERVO
MODE ENABLED FINAL OUTPUT
VOLTAGE REACHED
TON_MAX_FAULT_LIMIT
TON_RISE
TIME
38821 F29
TON_DELAY
V
OUT
PGOOD
VOUT_UV_FAULT_LIMIT
RUN
HYSTERESIS
(NOT TO SCALE)
TOFF_FALL
TOFF_DELAY
TIME
38821 F30
V
OUT
RUN
greatly simplify actual system development because rails
can be re-sequenced without a hardware change as final
product requirements evolve. The LTpowerPlay GUI and
LTC3882-1 onboard EEPROM can be used for this task,
avoiding the need for firmware development to modify turn
on/off relationships between rails. Entire power systems
can then easily be scaled up or down, facilitating reuse of
proven hardware macro designs.
Voltage-Based Output Sequencing
The LTC3882-1 is capable of voltage-based output se-
quencing. For concatenated events between members of
the LTC388x family, it is possible to control one RUN pin
from a PGOOD pin of a different controller as shown in
