LTC3811_15 Datasheet, PDF (20/48 Pages) Linear Technology – High Speed Dual, Multiphase Step-Down DC/DC Controller

LTC3811
OPERATION (Refer to the Functional Diagram)
INDIVIDUAL INTV_CC AND DRV_CC
PINS LOCALLY DECOUPLED
(DRV_CC NOT SHOWN)
ONLY PGOOD PIN FOR
MASTER CHANNEL
IS USED (FLOAT SLAVE
CHANNEL PGOOD PINS)
MASTER
V_OUT+
V_OUT–
MASTER
DIFFERENTIAL
AMPLIFIER
USED TO DRIVE
CHANNEL 1
ERROR
AMPLIFIER
SLAVE
CHANNEL
FB PINS ALL
CONNECTED
TO LOCAL
INTV_CC PINS
TO DISABLE
ERROR
AMPLIFIERS
LTC3811
INTV_CC
FB2
DIFF/IN⁺
DIFF/IN^–
PGOOD1
PGOOD2
DIFF/OUT
RUN1
FB1
RUN2
COMP1 SS/TRACK1
COMP2 SS/TRACK2
CLK_OUT SGND
SLAVE
SYNC LTC3811
INTV_CC
FB2
PGOOD1
PGOOD2
FB1
RUN1
COMP1
RUN2
COMP2 SS/TRACK1
SS/TRACK2
CLK_OUT SGND
ON/OFF
CONTROL
ALL
SS/TRACK
PINS
CONNECTED
TOGETHER
ALL RUN
PINS
CONNECTED
TOGETHER
SLAVE
SLAVE
CHANNEL
COMP
PINS ALL
CONNECTED
TO MASTER
CHANNEL
COMP PIN
SYNC LTC3811
INTV_CC
PGOOD1
FB2
PGOOD2
FB1
RUN1
COMP1
RUN2
COMP2 SS/TRACK1
SS/TRACK2
SGND
3811 F04
the amplitude of those for one regulator to be drawn from
the input capacitor. These large amplitude current pulses
increased the total RMS current flowing from the input
capacitor, requiring the use of more expensive input ca-
pacitors and increasing both EMI and losses in the input
capacitor.
With multiphase operation, the two channels of the dual-
switching regulator are operated 180 degrees out of phase.
This effectively interleaves the current pulses drawn by the
switches, greatly reducing the overlap time where they add
together. The result is a significant reduction in total RMS
input current, which in turn allows less expensive input
capacitors to be used, reduces shielding requirements for
EMI and improves real world operating efficiency.
Figure 5 illustrates the benefits of multiphase operation.
Current ripple at the input is reduced by a factor of 1.41
(square root of 2), reducing the size and cost of the input
capacitor. In addition, since power losses are proportional
to I_RMS2, significant efficiency improvements in the input
power path components (batteries, switches, protection
circuitry and PCB traces) can be achieved. Improvements
in both conducted and radiated EMI also directly accrue
as a result of the reduced RMS input current.
SGND BUS ISOLATED FROM
PGND AND INDEPENDENTLY
ROUTED TO NEGATIVE
TERMINAL OF OUTPUT CAPACITOR
Figure 4. LTC3811 Error Amplifier Configuration
for Multiphase Operation
FB pin of a slave phase to INTV_CC. As shown in the Func-
tional Diagram, a comparator detects when the FB pin is
shorted to INTV_CC and three-states this amplifier’s output
and input. The COMP pins for all of the phases can then
be shorted together in order to provide compensation for
the feedback loop, as shown in Figure 4.
Theory and Benefits of Multiphase Operation
Why the need for multiphase operation? Up until the
multiphase family, constant frequency dual switching
regulators operated both channels in phase (i.e., single-
phase operation). This means that both switches turned
on at the same time, causing current pulses of up to twice
20
I_L
5A/DIV
SW1
10V/DIV
SW2
10V/DIV
V_IN = 12V
V_OUT = 1.5V
I_OUT = 16A
1μs/DIV
3811 G41
Figure 5. 2-Phase, Single Output Current Sharing Waveforms
Of course, the improvement afforded by 2-phase opera-
tion is a function of the dual switching regulator’s relative
duty cycles which, in turn, are dependent upon the input
voltage V_IN (Duty Cycle = V_OUT/V_IN).
Figure 6 shows the net ripple current seen by the output
capacitors for the different phase configurations. The
output ripple current is plotted for a fixed output voltage
as the duty factor is varied between 10% and 90% on the
3811f