LTC3589_12 Datasheet, PDF (20/50 Pages) Linear Technology – 8-Output Regulator with Sequencing and I2C

LTC3589/LTC3589-1/
LTC3589-2
OPERATION
STEP-DOWN SWITCHING REGULATORS
Output Voltage Programming
Each of the step-down converters uses a dynamically
slewing DAC output for its reference. The full-scale output
voltage is set by using a resistor divider connected from
the step-down switching regulator output to the feedback
pins (B1_FB, B2_FB, and B3_FB), as shown in Figure 3.
Set the output voltage of step-down switching regulators
using the following formula:
V_OUT


= _1+

R1^
R2_
•
(0.3625 + BxDTVx
•
0.0125)(V)
BxDTVx is the decimal value of the five bit binary number
in the I²C BxDTV1 or BxDTV2 command registers. BxDTV1
and BxDTV2 default to 11001 to output a reference voltage
of 0.675V. Typical values for R1 are in the range of 40k
to 1M. The capacitor C_FB cancels the pole created by the
feedback resistors and the input capacitance on the FB pin
and also helps to improve load step transient response.
A value of 10pF is recommended for most applications.
Experimentation with capacitor sizes between 10pF and
33pF may yield improved transient response.
EN
MODE
PWM
CONTROL
PV_IN
SW
L1
5
DAC
FB
0.3625V
TO 0.75V
C_FB R1
C_OUT
R2
3589 F03
Figure 3. Step-Down Switching Regulator
Application Circuit
Operating Modes
The step-down switching regulators include three possible
operating modes to meet the noise and power needs of a
variety of applications.
In pulse-skipping mode, at the start of every cycle, a latch
is set that turns on the main P-channel MOSFET switch.
During the cycle, a current comparator compares the
peak inductor current to the output of an error amplifier.
The output of the current comparator resets the latch.
At this time the P-channel MOSFET switch turns off and
the N-channel MOSFET synchronous rectifier turns on.
The N-channel MOSFET synchronous rectifier will turn
off when the end of the clock cycle is reached or if the
inductor current drops through zero. Using this method
of operation, the error amplifier adjusts the peak inductor
current to deliver the required output power. All necessary
loop compensation is internal to the step-down switching
regulator requiring only a single ceramic output capacitor
for stability. At light loads in pulse-skipping mode, the
inductor current may reach zero on each pulse that will
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
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