LTC3855_15 Datasheet, PDF (13/44 Pages) Linear Technology – Dual, Multiphase Synchronous DC/DC Controller with Differential Remote Sense

LTC3855
Operation
voltage below 0.6V (e.g., SGND). To select pulse-skipping
mode of operation, tie the MODE/PLLIN pin to INTV_CC. To
select Burst Mode operation, float the MODE/PLLIN pin.
When a controller is enabled for Burst Mode operation,
the peak current in the inductor is set to approximately
one-third of the maximum sense voltage even though
the voltage on the I_TH pin indicates a lower value. If the
average inductor current is higher than the load current,
the error amplifier EA will decrease the voltage on the I_TH
pin. When the I_TH voltage drops below 0.5V, the internal
sleep signal goes high (enabling sleep mode) and both
external MOSFETs are turned off.
In sleep mode, the load current is supplied by the output
capacitor. As the output voltage decreases, the EA’s output
begins to rise. When the output voltage drops enough, the
sleep signal goes low, and the controller resumes normal
operation by turning on the top external MOSFET on the
next cycle of the internal oscillator. When a controller is
enabled for Burst Mode operation, the inductor current is
not allowed to reverse. The reverse current comparator
(I_REV) turns off the bottom external MOSFET just before
the inductor current reaches zero, preventing it from
reversing and going negative. Thus, the controller oper-
ates in discontinuous operation. In forced continuous
operation, the inductor current is allowed to reverse at
light loads or under large transient conditions. The peak
inductor current is determined by the voltage on the I_TH
pin. In this mode, the efficiency at light loads is lower than
in Burst Mode operation. However, continuous mode has
the advantages of lower output ripple and less interference
with audio circuitry.
When the MODE/PLLIN pin is connected to INTV_CC, the
LTC3855 operates in PWM pulse-skipping mode at light
loads. At very light loads, the current comparator I_CMP may
remain tripped for several cycles and force the external top
MOSFET to stay off for the same number of cycles (i.e.,
skipping pulses). The inductor current is not allowed to
reverse (discontinuous operation). This mode, like forced
continuous operation, exhibits low output ripple as well as
low audio noise and reduced RF interference as compared
to Burst Mode operation. It provides higher low current
efficiency than forced continuous mode, but not nearly as
high as Burst Mode operation.
Multichip Operations (PHASMD and CLKOUT Pins)
The PHASMD pin determines the relative phases between
the internal controllers as well as the CLKOUT signal as
shown in Table 1. The phases tabulated are relative to
zero phase being defined as the rising edge of the clock
of phase 1.
Table 1.
PHASMD
Phase1
Phase2
CLKOUT
GND
FLOAT
INTVcc
0°
0°
0°
180°
180°
240°
60°
90°
120°
The CLKOUT signal can be used to synchronize additional
power stages in a multiphase power supply solution feeding
a single, high current output or separate outputs. Input
capacitance ESR requirements and efficiency losses are
substantially reduced because the peak current drawn from
the input capacitor is effectively divided by the number
of phases used and power loss is proportional to the
RMS current squared. A two stage, single output voltage
implementation can reduce input path power loss by 75%
and radically reduce the required RMS current rating of
the input capacitor(s).
Single Output Multiphase Operation
The LTC3855 can be used for single output multiphase
converters by making these connections
• Tie all of the I_TH pins together
• Tie all of the V_FB pins together
• Tie all of the TK/SS pins together
• Tie all of the RUN pins together
• Tie all of the ITEMP pins together
• Tie all of the I_LIM pins together, or tie the I_LIM pins to
the same potential
For three or more phases, tie the inputs of the unused dif-
ferential amplifier(s) to ground. Examples of single output
multiphase converters are shown in Figures 20 to 23.
3855f
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