LTC3589 datasheet(29/50 Pages) LINER

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LTC3589/LTC3589-1/

LTC3589-2

3589fe

Figure 5. Pushbutton Controller State Diagram

OPERATION

Writing a 0 or 1 to the odd bits of voltage change control

respectively. A slew of the DAC is initiated by writing a 1

to an even bit of register VCCR. The DAC output will slew

to either voltage, V1 or V2, as selected by the odd bits of

is detected. At the end of the slewing operation the GO

bits in command register VCCR are cleared.

The slew rate for each regulator is set in the ramp rate

control register VRRCR. Each DAC has independent output

voltage registers, voltage register select, and slew rate and

start controls. The regulators do not have to be enabled

to change the DAC outputs.

The VSTB pin is used to set the DAC controlled output rails

to a low power standby condition. When VSTB is driven

HIGH, all four of the DAC references will immediately slew

to V2. To use VSTB to set the rails to standby voltage,

select V1 for normal rail voltages and V2 for standby rail

voltages. Drive VSTB high to immediately slew all the

DAC outputs to V2. When VSTB is driven LOW, the DAC

outputs will slew to V1.

The default power-up value of all the dynamic target voltage

registers is 11001 corresponding to a DAC output volt-

age of 0.675V. The DTV registers may be reprogrammed

prior to initiating a power-up sequence or at any time for

dynamic slewing.

When a step-down switching regulator output is slewing

down its mode is automatically switched to forced continu-

ous to enable the regulator to sink current. When LDO2 is

slewing down, a 2.5k pull-down is connected to its output.

Table 14 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 01 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 14. 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 the first power application to the LTC3589 VIN pin an

internal power-on reset circuit puts the pushbutton into the

power-down (PDN) state and initiates a one second timer.

The LTC3589 status pin RSTO is pulled low until one second

times out and the always-alive LDO1 is indicating power

good status. After the one second interval the pushbutton

circuit will transition to the power-off (POFF) standby state.

The LTC3589-1/LTC3589-2 powers on directly to the POFF

state bypassing the one second delay. Status pin RSTO will

be released high when LDO1 indicates power good status.

The LTC3589 will not leave the POFF state and enter the

1 SEC

POR

5 SEC

LTC3589

LTC3589-1/LTC3589-2

ON OR

PWR_ON

FAULT OR

PWR_ON OR

HARD RESET

PUP

PON

POFF

PDN

1 SEC

POR

3589 F05

5 SEC

ON OR

PWR_ON

FAULT OR

PWR_ON OR

HARD RESET

PUP

PON

POFF

PDN

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