LTC3589-2_15 Datasheet, PDF (21/50 Pages) Linear Technology – 8-Output Regulator with Sequencing and I2C

LTC3589/LTC3589-1/
LTC3589-2
OPERATION
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
condition 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 regulators output voltage equals the regulators
input voltage minus the voltage drops across the internal
P-channel MOSFET and the inductor DC resistance.
Table 5, Table 6, and Table 7 show the I²C command
register settings used to control the step-down switching
regulators.
Table 5. Step-Down Switching Regulator 1 Command Register
Settings
COMMAND
VALUE SETTING
REGISTER[BIT]
SCR1[1-0]
00* Pulse-Skipping Mode
01 Burst Mode Operation
10 Forced Continuous Mode
OVEN[0]
0* Disable
1 Enable
SCR2[0]
LTC3589/
LTC3589-1
0* Wait for Output Below 300mV Before Enable
1 Enable Immediately
SCR2[0]
LTC3589-2
0* Enable Immediately
1 Wait for Output Below 300mV Before Enable
VCCR[1]
0* Select Register B1DTV1 (V1) Reference
1 Select Register B1DTV2 (V2) Reference
VCCR[0]
1 Initiate Dynamic Voltage Slew
VRRCR[1-0]
00 Reference Slew Rate = 0.88mV/µs
01 Reference Slew Rate = 1.75mV/µs
10 Reference Slew Rate = 3.5mV/µs
11* Reference Slew Rate = 7mV/µs
B1DTV1[5]
0* Force PGOOD Low When Slewing
1 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* 2.25MHz Switching Frequency
1 1.125MHz Switching Frequency
B1DTV2[6]
0* Switch on Clock Phase 1
1 Switch on Clock Phase 2
B1DTV2[7]
0* Shutdown Regulator 1 Normally
1 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
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3589fg
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