LTC3589 Linear Technology, LTC3589 Datasheet - Page 19

• Manufacturer Part Number: LTC3589
• Description: 8-Output Regulator
• Manufacturer: Linear Technology

OPERATION
turn off the N-channel MOSFET synchronous rectifi er. 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
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 V
due to the continuous operation of the MOSFET switch
and rectifi er. 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
fi xed 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.
OUTX
> V
IN
/2) it is possible for the inductor
IN
current
Set the mode of operation for the step-down switching
regulators by using the I
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 I
register settings used to control the step-down switching
regulators.
Table 4. Step-Down Switching Regulator 1 Command Register
Settings
COMMAND
REGISTER[BIT]
SCR1[1-0]
OVEN[0]
SCR2[0]
VCCR[1]
VCCR[0]
VRRCR[1-0]
B1DTV1[5]
B1DTV1[4-0]
B1DTV2[4-0]
B1DTV2[5]
B1DTV2[6]
B1DTV2[7]
* Denotes Default Power-On Value
11001*
11001*
VALUE
00*
00*
01
10
01
10
11
0*
0*
0*
1
0*
1
0*
1
1
0*
1
0
1
1
1
SETTING
Pulse-Skipping Mode
Burst Mode Operation
Forced Continuous Mode
Disable
Enable
Wait for Output Below 300mV Before Enable
Enable Immediately
Select Register B1DTV1 (V1) Reference
Select Register B1DTV2 (V2) Reference
Initiate Dynamic Voltage Slew
Reference Slew Rate = 0.88mV/μs
Reference Slew Rate = 1.75mV/μs
Reference Slew Rate = 3.5mV/μs
Reference Slew Rate = 7mV/μs
Force PGOOD Low When Slewing
Normal PGOOD Operation When Slewing
DAC Dynamic Target Voltage V1
DAC Dynamic Target Voltage V2
2.25MHz Switching Frequency
1.125MHz Switching Frequency
Switch on Clock Phase 1
Switch on Clock Phase 2
Shutdown Regulator 1 Normally
Keep Regulator 1 Alive
2
C command register SCR1. Each
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LTC3589
2
C command
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3589p