LTC3889 datasheet(63/118 Pages) AD

63 / 118 page

LTC3888/LTC3888-1/

LTC3888-2

Rev. B

For more information www.analog.com

Figure 37. LTC3888-1 Time-Based Ratiometric Supply Ramping

388812 F37

100ms/DIV

VOUT2

VOUT3

VOUT4

VOUT1

0.5V/DIV

Conventional coincident and ratiometric tracking can also

be emulated by setting equivalent turn-on/off delays and

appropriate rates as shown in Figure 36 and Figure 37.

In addition, these schemes can easily be mixed and

matched to create any necessary ramping controls, some

of which might prove difficult to implement with con-

ventional analog-only controllers. These programmable

features greatly simplify system development because

rails can be resequenced without a hardware change as

final product requirements evolve. The LTpowerPlay GUI

and LTC3888 onboard EEPROM can be used for this task,

avoiding the need for firmware development to modify

turn on/off relationships between rails. Entire power sys-

tems can easily be scaled up or down, facilitating reuse

of proven hardware macro designs.

VOLTAGE-BASED OUTPUT SEQUENCING

The LTC3888 is capable of voltage-based output sequenc-

ing. For concatenated events between members of the ADI

PSM family, it is possible to control one RUN pin from

a GPIO, FAULT or PGOOD pin of a different controller. A

hardware configuration of the type shown in Figure 38

disables the next downstream controller anytime the

upstream output is below the specific UV threshold. If

GPIO or FAULT is used, the controlling output must be

configured to only propagate VOUT_UV_UF, and that

IC must have its MFR_GPIO/FAULT_RESPONSE set to

ignore (0x00). Use of the unfiltered VOUT UV fault limit is

recommended because there is less delay between cross-

ing the UV threshold and the GPIO/FAULT pin releasing.

When GPIO/FAULT UV propagation is utilized, an output

APPLICATIONS INFORMATION

deglitching filter can ensure the control does not toggle

repeatedly at lower values of output transition due to

noise on VOUT. If unwanted transitions still occur with

only the internal filter found on most ADI PSM deviced

(typically 70μs), place a capacitor to ground on the out-

put pin. The RC time-constant of the filter should be low

enough to assure no appreciable delay is incurred. A value

of 300μs to 500μs will provide some additional filtering

without significant delay of the trigger event.

Figure 38. Cascade Sequencing Configuration Example

Figure 39. Cascade Sequencing Waveforms

LTC3882

GPIO0

GPIO1

RUN 1

RUN 0

START

LTC3888-1

388812 F38

RUN 0

PGOOD0

(PULL-UP RESISTORS

TO 3.3V NOT SHOWN)

PGOOD1

TO NEXT CHANNEL

IN THE SEQUENCE

RUN 1

388812 F39

100ms/DIV

VOUT2

VOUT3

VOUT4

VOUT1

0.5V/DIV

When the system is turned off, rails will shut down in the

same order as they turn on, as shown in Figure 39. If a

different sequence is required, the circuit must be rewired

or delays must be added by programming TON_DELAY or

TOFF_DELAY. A fundamental limitation of this application

When the system is turned off, rails will shut down in the

same order as they turn on, as shown in Figure 39. If a

different sequence is required, the circuit must be rewired

or delays must be added by programming TON_DELAY

or TOFF_DELAY. A fundamental limitation of this applica-

tion is the inability of upstream rails to detect a start-up

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