LTC3589_1 Datasheet, PDF (20/46 Pages) Linear Technology – 8-Output Regulator with Sequencing and I2C

LTC3589
OPERATION
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 (V_OUTX > V_IN/2) it is possible for the inductor
current to reverse at light loads causing the step-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 V_IN 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 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 ¼
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 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
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
1 Enable Immediately
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
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