LTC3811_15 Datasheet, PDF (19/48 Pages) Linear Technology – High Speed Dual, Multiphase Step-Down DC/DC Controller

LTC3811
OPERATION (Refer to the Functional Diagram)
The typical capture range of the LTC3811’s PLL is ap-
proximately 125kHz to 1.1MHz, with a guarantee over
all manufacturing variations to be between 175kHz and
900kHz. The amplitude of the sync pulse to the LTC3811
should be greater than 1.8V and the minimum pulse width
should be greater than 200ns.
Using the CLKOUT and PHASEMODE Pins in
Multiphase Applications
The LTC3811 features two pins (CLKOUT and PHASEMODE)
that allow multiple LTC3811 ICs to be daisy-chained to-
gether in multiphase applications. The clock output signal
on the CLKOUT pin can be used to synchronize additional
power stages in a multiphase power supply solution feeding
a single high current output or even separate outputs. The
PHASEMODE pin is used to adjust the phase relationship
between channel 1 and channel 2, as well as the phase
relationship between channel 1 and CLKOUT, as summa-
rized in Table 3. The phases are calculated relative to the
zero degrees, defined as the rising edge of the top gate
driver output of channel 1, TG1.
The PHASEMODE input comparators are referenced to an
internal divider from INTV_CC that has 33% and 67% INTV_CC
thresholds. For 6-phase operation, connect PHASEMODE
to an external divider from INTV_CC with equal value resis-
tors (e.g., 100k), so that PHASEMODE is always 50% of
INTV_CC.
Table 3
# PHASES
2
3
4
6
12
IC # PHASEMODE CLKOUT CONNECTS TO
1
0V
N/A
1
INTV_CC
MODE/SYNC of IC # 2
2
0V
N/A
1
0V
MODE/SYNC of IC # 2
2
0V
N/A
1
50% INTV_CC MODE/SYNC of IC # 2
2
50% INTV_CC MODE/SYNC of IC # 2
3
50% INTV_CC
N/A
1
50% INTV_CC MODE/SYNC of IC # 2
2
50% INTV_CC MODE/SYNC of IC # 3
3
0V
MODE/SYNC of IC # 4
4
50% INTV_CC MODE/SYNC of IC # 5
5
50% INTV_CC MODE/SYNC of IC # 6
6
50% INTV_CC
N/A
Remote Sensing Using the Differential Amplifier
The LTC3811 has a differential amplifier for true remote
sensing of the output voltage. The sensing connections
should be returned from the load back to the differential
amplifier’s inputs through a common, tightly coupled pair
of PCB traces. The differential amplifier rejects common
mode signals capacitively or inductively radiated into the
feedback PCB traces, as well as ground loop disturbances.
The differential amplifier output signal is typically divided
down and compared with the internal precision 0.6V volt-
age reference by the error amplifier.
The differential amplifier utilizes four precision internal
resistors to enable instrumentation-type measurement
of the output voltage. The amplifier has a gain of 1.000,
contains a CMOS rail-to-rail output stage, and is optimized
for low input offset and high bandwidth.
The output voltage is set by an external resistive divider
according to the following formula:
V_OUT
=
0.6
•
⎡⎣⎢₁₊
R2⎤
R1_⎦⎥
where R2 and R1 are the upper and lower divider resistors,
respectively. The differential amplifier was optimized for
divider currents in the range of 100μA to 600μA, meaning
that R1 in the equation above should be 1k to 6k.
Using the LTC3811 Operational Error Amplifiers in
Multiphase Applications
The LTC3811 error amplifiers are true operational ampli-
fiers, meaning that they have high DC gain and low output
impedance. In previous generations of multiphase control-
lers, such as the LTC1628 family, the error amplifiers were
transconductance amplifiers, meaning that they could be
connected in parallel for multiphase applications.
Multiphase applications using the LTC3811 will use one
operational error amplifier as the master and will disable
all of the slave phase error amplifiers. Typically, the chan-
nel 1 amplifier for phase = 0º will be used as the master
and phases 2 through n (up to 12 phases) will serve as
slaves. To disable the slave error amplifiers but still use
their current comparators and power stages, connect the
3811f
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