LTM4686BIVPBF datasheet(53/130 Pages) AD

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LTM4686B

Rev. 0

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APPLICATIONS INFORMATION

If the TON_MAX_FAULT_LIMITn is set to a value greater

than 0 and the TON_MAX_FAULT_RESPONSEn is set to

ignore (0x00), the servo begins:

1. After the TON_RISEn sequence is complete

2. After the TON_MAX_FAULT_LIMITn time is reached;

and

3. After the VOUT_UV_FAULT_LIMITn has been exceed

or the IOUT_OC_FAULT_LIMITn is no longer active.

If the TON_MAX_FAULT_LIMITn is set to a value greater

than 0 and the TON_MAX_FAULT_RESPONSEn is not set

to ignore (0X00), the servo begins:

1. After the TON_RISEn sequence is complete;

2. After the TON_MAX_FAULT_LIMITn time has expired

and both VOUT_UV_FAULTn and IOUT_OC_FAULTn

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 LTM4686B also

supports controlled turn-off. The TOFF_DELAYn and TOFF_

FALLn functions are shown in Figure 3. TOFF_FALLn is

processed when the RUNn pin goes low or if the module is

commanded off. If the module faults off or GPIOn is pulled

low externally and the module is programmed to respond

to this (MFR_GPIO_RESPONSEn = 0xC0), the output three-

states (becomes high impedance) rather than exhibiting a

controlled ramp. The output then decays as a function of

the load.

The output voltage operates as shown in Figure 3 so

long as the part is in forced continuous mode and the

TOFF_FALLn time is sufficiently slow that the power stage

can achieve the desired slope. The TOFF_FALLn 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_FALLn

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_FALLn, the controller

ceases to sink current and VOUTn decays at the natural rate

determined by the load impedance. If the controller is in

discontinuous mode, the controller does not pull negative

current and the output becomes pulled low by the load,

not the power stage. The maximum fail time is limited to

1.3 seconds. The number of steps in the ramp is equal to

TOFF_FALL/0.1ms.Therefore, the shorter the TOFF_FALLn

setting, the more jagged the TOFF_FALLn ramp appears.

TOFF_FALLn

TOFF_DELAYn

TIME

4686B F03

VOUTn

RUNn

Figure 3. TOFF_DELAYn and TOFF_FALLn

UNDERVOLTAGE LOCKOUT

The LTM4686B is initialized by an internal threshold-

based UVLO where SVIN must be approximately 4V and

INTVCC, VDD33, VDD25 must be within approximately

20% of the regulated values. In addition, VDD33 must

be within approximately 7% of the targeted value before

the LTM4686B releases its RUNn pins. After the part has

initialized, an additional comparator monitors SVIN. The

VIN_ON threshold must be exceeded before the power

sequencing can begin. When SVIN drops below the VIN_

OFF threshold, the LTM4686B ceases PWM action and

SVIN must increase above the VIN_ON threshold before

the controller will restart. The normal start-up sequence

will be allowed after the VIN_ON threshold is crossed.

It is possible to program the contents of the NVM in the

application if the VDD33 supply is externally driven. This

activates the digital portion of the LTM4686B without

engaging the high voltage sections. PMBus communica-

tions are valid in this supply configuration. If SVIN has not

been applied to the LTM4686B, MFR_COMMON[3] will

LTM4686BIVPBF 数据表(PDF) 53 Page - Analog Devices

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