LTC3589_12 Datasheet, PDF (21/50 Pages) Linear Technology – 8-Output Regulator with Sequencing and I2C

LTC3589/LTC3589-1/
LTC3589-2
OPERATION
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.
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]
0* Wait for Output Below 300mV Before Enable
LTC3589/LTC3589-1 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
occurs whenever a regulator is enabled either initially or
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