LTM4664 datasheet(67/138 Pages) AD

67 / 138 page

LTM4664A

Rev. 0

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VOUT_UV_FAULT_LIMIT. This same point in time is

when the output changes from discontinuous to the pro-

grammed mode as indicated in MFR_PWM_MODE bit 0.

Refer to Figure 29 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 exceeded

or the IOUT_OC_FAULT_LIMIT is no longer active.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 LTM4664A PSM

also supports controlled turn-off. The TOFF_DELAY and

TOFF_FALL functions are shown in Figure 30. TOFF_FALL

is processed when the RUN pin goes low or if the part is

commanded off. If the part faults off or FAULT_Cn 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 30

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 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

4664A F29

TON_DELAY

VOUT

Figure 29. Timing Controlled VOUT Rise

TOFF_FALL

TOFF_DELAY

TIME

4664A F30

VOUT

Figure 30. TOFF_DELAY and TOFF_FALL

DUAL 25A/30A PSM APPLICATIONS INFORMATION

LTM4664 Datasheet(PDF) 67 Page - Analog Devices

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