LTM4680 datasheet(60/132 Pages) AD

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LTM4681

Rev. 0

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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 VOUT waveform under time-based sequencing. 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or the IOUT_OC_FAULT_LIMIT is no longer active.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.

SOFT OFF (SEQUENCED OFF)

In addition to a controlled start-up, the LTM4681 also

supports controlled turn-off. The TOFF_DELAY and TOFF_

FALL functions are shown in Figure 26. TOFF_FALL is

processed when the RUNn 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 continu-

ous 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 VOUT will decay

at the natural rate determined by the load 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.

DAC VOLTAGE

ERROR (NOT

TO SCALE)

TIME DELAY OF

200-400ms

DIGITAL SERVO

MODE ENABLED FINAL OUTPUT

VOLTAGE REACHED

TON_MAX_FAULT_LIMIT

TON_RISE

TIME

4681 F25

TON_DELAY

VOUT

Figure 25. Timing Controlled VOUT Rise

TOFF_FALL

TOFF_DELAY

TIME

4681 F26

VOUT

Figure 26. TOFF_DELAY and TOFF_FALL

LTM4680 Datasheet(PDF) 60 Page - Analog Devices

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