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LTM4700
APPLICATIONS INFORMATION
This same point in time is when the output changes from
discontinuous to the programmed mode as indicated in
MFR_PWM_MODE bit 0. Refer to Figure 25 for details on
the V
waveform under time-based sequencing. If the
OUT
TON_MAX_FAULT_LIMIT is set to a value greater than 0
and the TON_MAX_FAULT_RESPONSE is set to ignore
0x00, the servo begins:
1. After the TON_RISE sequence is complete
2. After the TON_MAX_FAULT_LIMIT time is reached;
and
3. After th VOUT_UV_FAULT_LIMIT has been exceed or
the IOUT_OC_FAULT_LIMIT is no longer active.
If the TON_MAX_FAULT_LIMIT is set to a value greater
than 0 and the TON_MAX_FAULT_RESPONSE is not set
to ignore 0x00, the servo begins:
1. After the TON_RISE sequence is complete
2. After the TON_MAX_FAULT_LIMIT time has expired
and both VOUT_UV_FAULT and IOUT_OC_FAULT are
not present.
The maximum rise time is limited to 1.3 seconds.
In a PolyPhase configuration it is recommended only one
of the control loops have the digital servo mode enabled.
This will assure the various loops do not work against each
other due to slight differences in the reference circuits.
TON_MAX_FAULT_LIMIT
V
OUT
TON_RISE
TON_DELAY
Figure 25. Timing Controlled V
54
DIGITAL SERVO
MODE ENABLED FINAL OUTPUT
VOLTAGE REACHED
TIME DELAY OF
DAC VOLTAGE
200-400ms
ERROR (NOT
TO SCALE)
TIME
4700 F25
Rise
OUT
For more information
SOFT OFF (SEQUENCED OFF)
In addition to a controlled start-up, the LTM4700 also sup-
ports controlled turn-off. The TOFF_DELAY and TOFF_FALL
functions are shown in Figure 26. TOFF_FALL is processed
when the RUN pin goes low or if the part is commanded
off. If the part faults off or FAULTn is pulled low externally
and the part is programmed to respond to this, the output
will three-state rather than exhibiting a controlled ramp.
The output will decay as a function of the load. The output
voltage will operate as shown in Figure 26 as long as the
part is in forced continuous mode and the TOFF_FALL
time is sufficiently slow that the power stage can achieve
the desired slope. The TOFF_FALL time can only be met if
the power stage and controller can sink sufficient current
to assure the output is at zero volts by the end of the fall
time interval. If the TOFF_FALL time is set shorter than
the time required to discharge the load capacitance, the
output will not reach the desired zero volt state. At the end
of TOFF_FALL, the controller will cease to sink current and
V
will decay at the natural rate determined by the load
OUT
impedance. If the controller is in discontinuous mode, the
controller will not pull negative current and the output
will be pulled low by the load, not the power stage. The
maximum fall time is limited to 1.3 seconds. The shorter
TOFF_FALL time is set, the larger the discrete steps in the
TOFF_FALL ramp will appear. The number of steps in the
ramp is equal to TOFF_FALL/0.1ms.
V
OUT
TOFF_DELAY
Figure 26. TOFF_DELAY and TOFF_FALL
www.analog.com
TIME
4700 F26
TOFF_FALL
Rev. B
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