LTC3589 datasheet(21/50 Pages) LINER

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LTC3589/LTC3589-1/

LTC3589-2

3589fe

due to the continuous operation of the MOSFET switch

and rectiﬁer. Since the inductor current is allowed to be

negative in forced continuous operation the step-down

switching regulator has the ability to sink output current.

The LTC3589 automatically forces the step-down switching

regulator into forced continuous mode when dynamically

slewing the DAC voltage reference down.

When the LTC3589 step-down switching regulators are in

Burst Mode operation, they automatically switch between

ﬁxed frequency pulse-skipping operation and hysteretic

Burst Mode control as a function of the load current. At

light loads the step-down switching regulators control the

inductor current directly and use a hysteretic control loop

to minimize both noise and switching losses. While in Burst

Mode operation, the output capacitor is charged to a voltage

slightly higher than the regulation point. The step-down

switching regulator then goes into a low power sleep mode

during which the output capacitor provides the load current.

In sleep mode, most of the switching regulator’s circuitry

is powered off to conserve battery power. When the output

voltage drops below the regulation point the regulator’s

circuitry is powered on and another burst cycle begins. As

the load current increases, the time between burst cycles

decreases. Above a load current about one-quarter rated

output load, the step-down switching regulators will switch

to low noise constant-frequency PWM operation.

Set the mode of operation for the step-down switching

regulators by using the I2C command register SCR1. Each

of the three regulators has independent mode control.

A step-down switching regulator may enter a dropout condi-

tion when its input voltage drops to near its programmed

output voltage. For example, a discharging battery voltage

of 3.4V dropping to the regulators programmed output

voltage of 3.3V. When this happens the duty cycle of the

P-channel MOSFET switch is increased until it turns on

continuously with 100% duty cycle. In dropout, the regu-

lators output voltage equals the regulators input voltage

minus the voltage drops across the internal P-channel

MOSFET and the inductor DC resistance.

OPERATION

Table 5, Table 6, and Table 7 show the I2C command register

settingsusedtocontrolthestep-downswitchingregulators.

Table 5. Step-Down Switching Regulator 1 Command Register

Settings

COMMAND

VALUE SETTING

SCR1[1-0]

00*

Pulse-Skipping Mode

Burst Mode Operation

Forced Continuous Mode

OVEN[0]

Disable

Enable

SCR2[0]

LTC3589/LTC3589-1

Wait for Output Below 300mV Before Enable

Enable Immediately

SCR2[0]

LTC3589-2

Enable Immediately

Wait for Output Below 300mV Before Enable

VCCR[1]

Select Register B1DTV1 (V1) Reference

Select Register B1DTV2 (V2) Reference

VCCR[0]

Initiate Dynamic Voltage Slew

VRRCR[1-0]

11*

Reference Slew Rate = 0.88mV/μs

Reference Slew Rate = 1.75mV/μs

Reference Slew Rate = 3.5mV/μs

Reference Slew Rate = 7mV/μs

B1DTV1[5]

Force PGOOD Low When Slewing

Normal PGOOD Operation When Slewing

B1DTV1[4-0]

11001* DAC Dynamic Target Voltage V1

B1DTV2[4-0]

11001* DAC Dynamic Target Voltage V2

B1DTV2[5]

2.25MHz Switching Frequency

1.125MHz Switching Frequency

B1DTV2[6]

Switch on Clock Phase 1

Switch on Clock Phase 2

B1DTV2[7]

Shutdown Regulator 1 Normally

Keep Regulator 1 Alive

* Denotes Default Power-On Value

Soft-Start

Soft-start is accomplished by gradually increasing the

input reference voltage on each step-down switching

regulator from 0V to the dynamic reference DAC output

level at a rate of 0.8V/ms. This allows each output to

rise slowly, helping minimize inrush current required to

charge up the regulator output capacitor. A soft-start cycle

occurs whenever a regulator is enabled either initially or

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