LTC3883 Datasheet, PDF (43/112 Pages) Linear Technology – Single Phase Step-Down DC/DC Controller with Digital Power System Management

LTC3883/LTC3883-1
Applications Information
Digital Servo Mode
For maximum accuracy in the regulated output voltage,
enable the digital servo loop by asserting bit 6 of the
MFR_PWM_MODE_LTC3883 command. In digital servo
mode, the LTC3883 will adjust the regulated output voltage
based on the ADC voltage reading. Every 90ms the digital
servo loop will step the LSB of the DAC (nominally 1.375mV
or 0.6875mV depending on the voltage range bit) until the
output is at the correct ADC reading. At power-up this mode
engages after TON_MAX_FAULT_LIMIT unless the limit is
set to 0 (infinite). If the TON_MAX_FAULT_LIMIT is set to
0 (infinite), the servo begins after TON_RISE is complete
and VOUT has exceeded the VOUT_UV_FAULT_LIMIT.
This same point in time is when the output changes from
discontinuous to the programmed mode as indicated
in MFR_PWM_MODE_LTC3883 bits 0 and 1. Refer to
Figure 21 for details on the VOUT waveform under time
based sequencing.
DIGITAL SERVO
MODE ENABLED _{FINAL OUTPUT}
VOLTAGE REACHED
TON_MAX_FAULT_LIMIT
V_OUT
DAC VOLTAGE
ERROR (NOT
TO SCALE)
TIME DELAY OF
MANY SECONDS
TON_DELAY
TON_RISE
TIME
3883 F21
Figure 21. Timing Controlled V_OUT Rise
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.
Soft Off (Sequenced Off)
In addition to a controlled start-up, the LTC3883 also
supports controlled turn-off. The TOFF_DELAY and
TOFF_FALL functions are shown in Figure 22. TOFF_FALL
is processed when the RUN pin goes low or if the part is
commanded off. If the part faults off or GPIO 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 22 so
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
If the 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 the VOUT_UV_FAULT_LIMIT has been exceed or
the IOUT_OC_FAULT_LIMIT is not longer active.
V_OUT
TOFF_DELAY
TOFF_FALL
TIME
3883 F22
Figure 22. TOFF_DELAY and TOFF_FALL
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