LTC3589-2_15 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 discontinuous 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 operating 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
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3589fg