LTC3589EUJ-PBF datasheet(19/44 Pages) LINER

部件名	LTC3589EUJ-PBF

功能描述	8-Output Regulator with Sequencing and I2C

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制造商	LINER [Linear Technology]

网页	http://www.linear.com

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LTC3589

19

3589p

turn off the N-channel MOSFET synchronous rectiﬁer. In

this case the switch node (SW1, SW2, or SW3) goes HIGH

impedance and the switch node will ring. This is discon-

tinuous operation and is normal behavior for a switching

regulator. At very light loads in pulse-skipping mode, the

step down switching regulators will automatically skip

pulses as needed to maintain output regulation. At high

duty cycle (VOUTX > VIN/2) it is possible for the inductor

current to reverse at light loads causing the stepped down

switching regulator to operate continuously. When operat-

ing continuously, regulation and low noise output voltage

are maintained, but input operating current will increase

to a few milliamps.

In the forced continuous mode of operation, the inductor

current is allowed to be less than zero over the full range

of duty cycles. Operating in forced continuous mode is

a lower noise option at light loads than pulse-skipping

operation but with the drawback of higher VIN current

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

operating 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 capaci-

tor 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 ¼ rated output load, the step-down switch-

ing 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.

Register Controls

Table 4, Table 5, and Table 6 show the I2C command

register settings used to control the step-down switching

regulators.

Table 4. Step-Down Switching Regulator 1 Command Register

Settings

COMMAND

REGISTER[BIT]

VALUE

SETTING

SCR1[1-0]

00*

01

10

Pulse-Skipping Mode

Burst Mode Operation

Forced Continuous Mode

OVEN[0]

0*

1

Disable

Enable

SCR2[0]

0*

1

Wait for Output Below 300mV Before Enable

Enable Immediately

VCCR[1]

0*

1

Select Register B1DTV1 (V1) Reference

Select Register B1DTV2 (V2) Reference

VCCR[0]

1

Initiate Dynamic Voltage Slew

VRRCR[1-0]

00*

01

10

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]

0*

1

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]

0

1

2.25MHz Switching Frequency

1.125MHz Switching Frequency

B1DTV2[6]

0*

1

Switch on Clock Phase 1

Switch on Clock Phase 2

B1DTV2[7]

0*

1

Shutdown Regulator 1 Normally

Keep Regulator 1 Alive

* Denotes Default Power-On Value

OPERATION

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