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LTC3589

3589p

is activated by the PWR_ON pin. When the controller enters

the PUP state the open drain WAKE pin releases HIGH.

The WAKE pin is typically used to enable the ﬁrst regulator

in a start-up sequence. The pushbutton state will stay in

PUP for ﬁve seconds before transitioning to the power-on

(PON) state. Before leaving PUP, the PWR_ON pin must be

brought HIGH by the application to indicate that the system

rails are correct. If PWR_ON is not active at the end of ﬁve

seconds the pushbutton controller will continue directly

through PON to the power-down (PDN) state and pull the

WAKE pin down. Three events will cause the pushbutton

to leave the PON state: 1) lowering the PWR_ON pin, 2)

forcing a hard reset by holding the ON pin LOW for ﬁve

seconds, and 3) a fault condition is detected. Fault condi-

tions are low VIN, device over temperature, or extended

undervoltage of one of the regulator outputs. All regulator

enables, the ON input, and PWR_ON signals are inhibited

for one second while in the PDN state. After one second

in PDN the pushbutton controller returns to POFF.

Figure 5. Pushbutton Controller State Diagram

1 SEC

POR

3589 F05

5 SEC

PB400ms OR

PWR_ON

FAULT OR

PWR_ON

PUP

PON

POFF

PDN

Table 13 shows command register and feedback divider

settings to enable slewing step-down switching regulator 1

between 1.2V and 1V in 70μs. The voltage ramp rate

control register bits VRRCR[1:0] are set to 10 which

selects a ramp rate of 1.75mV/μs at the DAC output.

The slew rate at the regulator output is a function of the

feedback resistor divider gain. In this example, the slew

is equal to 1.75 • (1 + 301/499) = 2.8mV/μs. Therefore, a

slew of 200mV will take 70μs. To initiate a change from

1.2V to 1V write 11 to voltage change control register bits

VCCR[1:0]. VCCR[1] selects target register B1DTV2 to

set the regulator reference input to 0.625V. VCCR[0] set

to 1 initiates the dynamic slew to go to the new voltage.

To slew back to 1.2V write 01 to command register bits

VCCR[1:0].

Table 13. Dynamic Slewing Example for Step-Down Switching

Regulator 1

COMMAND

VOUT =1.2V

VOUT =1V

VRRCR[1:0]

Dynamic Slew Rate

VCCR[1]

Select DTV

B1DTV1[4:0]

11111

Resistor Divider Shown

in Figure 3

R1 = 301kΩ

R2 = 499kΩ

B1DTV2[4:0]

10101

PUSHBUTTON OPERATION

State Event Diagram

Figure 5 shows the LTC3589 pushbutton state diagram.

Upon ﬁrst power application to VIN an internal power-on

reset circuit puts the pushbutton into power-down (PDN)

state and initiates a one second timer. Status pin RSTO is

pulled LOW until one second is timed out and the always-

alive LDO1 has indicated power good status. After the one

second interval the pushbutton circuit will transition to the

power-off (POFF) state. The pushbutton will not leave the

POFF state and enter the power-up state (PUP) until ON is

held LOW for at least 400ms (PB400ms) or until PWR_ON

OPERATION

PBSTAT Operation

PBSTAT goes LOW 50ms after the initial pushbutton ap-

plication (ON LOW) and will stay LOW for a minimum of

50ms. PBSTAT will go HIGH coincident with ON going

HIGH unless ON goes HIGH before the 50ms minimum

on-time.

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