LTC4449 datasheet(52/108 Pages) LINER

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LTC3882-1

Rev A

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APPLICATIONS INFORMATION

Figure 29. Time-Based VOUT Turn-On

Figure 30. Time-Based VOUT Turn-Off

As shown in Figure 29, it is important to remember that the

hysteresis given in the Electrical Characteristics (EC) table

appliesaboveVOUT_UV_FAULT_LIMIT,whichspecifiesthe

UV limit when the output is falling out of regulation. For

turn-on, the output must rise above this programmed limit

plus the hysteresis to avoid exceeding TON_MAX_FAULT_

LIMIT. PGOOD is indicated at that point. For this reason,

VOUT_UV_FAULT_LIMITshouldbemorethan27mVbelow

the programmed output voltage in low range and more

than 54mV below VOUTinhighrange.Thereisafixeddelay

and other timing uncertainty associated with all changes

in output voltage controlled by the LTC3882-1. A nominal

fixed timing delay of 270µs exists to process any change

in output voltage, including soft start/stop, margining and

generalchangesinVOUT_COMMANDvalue.Thestartofall

time-based output operations occur with an uncertainty of

±50µs and have a nominal step resolution of 100µs. This

meanstheminimumTON_DELAYorTOFF_DELAYthatthe

LTC3882-1 can produce will range from 220µs to 320µs,

not including basic oscillator tolerances. For software-

based output changes (e.g., margining), this algorithmic

delay begins when the STOP bit is received on the serial

bus. An example of this minimum turn on/off delay and

step-wise output control can be seen in Figure 31, where

TON_DELAY = 0s and TON_RISE = 1ms.

To effectively implement sequencing and synchronized

ramping between rails controlled by LTC digital power

products, two signals should be shared between all

controlling ICs: SHARE_CLK and RUN (CONTROL pin

on LTC297x products). This facilitates synchronized rail

sequencing on or off based on shared input supply state

(VIN_ONthreshold),externalhardwarecontrol(RUNpin),

or PMBus commands (possibly using global addressing).

Figure 32 shows an example of output supply sequencing

using TON_DELAY.

Usingthisscheme,conventionalcoincidentandratiometric

tracking can also be emulated by setting equivalent turn-

on/off delays and appropriate rise and fall times as shown

in Figure 33 and Figure 34.

Inaddition,theseschemescaneasilybemixedandmatched

to create any necessary ramping controls, some of which

might prove difficult to implement with conventional

analog-only controllers. These programmable features

DAC VOLTAGE

ERROR (NOT

TO SCALE)

TIME DELAY OF

<1S, TYPICAL

DIGITAL SERVO

MODE ENABLED FINAL OUTPUT

VOLTAGE REACHED

TON_MAX_FAULT_LIMIT

TON_RISE

TIME

38821 F29

TON_DELAY

VOUT

PGOOD

VOUT_UV_FAULT_LIMIT

RUN

HYSTERESIS

(NOT TO SCALE)

TOFF_FALL

TOFF_DELAY

TIME

38821 F30

VOUT

RUN

greatly simplify actual system development because rails

can be re-sequenced without a hardware change as final

product requirements evolve. The LTpowerPlay GUI and

LTC3882-1 onboard EEPROM can be used for this task,

avoidingtheneedforfirmwaredevelopmenttomodifyturn

on/off relationships between rails. Entire power systems

can then easily be scaled up or down, facilitating reuse of

proven hardware macro designs.

Voltage-Based Output Sequencing

The LTC3882-1 is capable of voltage-based output se-

quencing. For concatenated events between members of

the LTC388x family, it is possible to control one RUN pin

from a PGOOD pin of a different controller as shown in

LTC4449 Datasheet(PDF) 52 Page - Linear Technology

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