LTC3860_15 Datasheet, PDF (22/36 Pages) Linear Technology – Dual, Multiphase Step-Down Voltage Mode DC/DC Controller with Current Sharing

LTC3860
APPLICATIONS INFORMATION
the TRACK/SS2 pin voltage instead of 0.6V. The start-up
of V_OUT2 may ratiometrically track that of V_OUT1, according
to a ratio set by a resistor divider (Figure 11c):
V_{OUT1 =} R2A _• R_TRACKA + R_TRACKB
V_OUT2 R_TRACKA
R2B + R2A
For coincident tracking (V_OUT1 = V_OUT2 during start-up),
R2A = R_TRACKA
R2B = R_TRACKB
The ramp time for V_OUT2 to rise from 0V to its final
value is:
t_SS2
= t_SS1 •
0.6
V_{OUT 1 F}
• ^{RTRACKA + RTRACKB}
R_TRACKA
For coincident tracking,
t_SS2
=
t_SS1
•
V_OUT2F
V_OUT1F
where V_OUT1F and V_OUT2F are the final, regulated values
of V_OUT1 and V_OUT2. V_OUT1 should always be greater than
V_OUT2 when using the TRACK/SS2 pin for tracking. If no
tracking function is desired, then the TRACK/SS2 pin may
be tied to a capacitor to ground, which sets the ramp time
to final regulated output voltage. It is only possible to track
another supply that is slower than the internal soft-start
ramp. At the completion of tracking, the TRACK/SS pin
must be >620mV, so as not to affect regulation accuracy
and to ensure the part is in CCM mode.
Feedback Loop Compensation
The LTC3860 is a voltage mode controller with a second
dedicated current sharing loop to provide excellent phase-
to-phase current sharing in multiphase applications. The
current sharing loop is internally compensated.
While Type 2 compensation for the voltage control loop
may be adequate in some applications (such as with the
use of high ESR bulk capacitors), Type 3 compensation,
along with ceramic capacitors, is recommended for opti-
mum transient response. Referring to Figure 12, the error
amplifiers sense the output voltage at V_OUT.
The positive input of the error amplifier is connected to
an internal 600mV reference, while the negative input is
connected to the FB pin. The output is connected to COMP,
which is in turn connected to the line feedforward circuit
and from there to the PWM generator. To speed up the
overshoot recovery time, the maximum potential at the
COMP pin is internally clamped.
Unlike many regulators that use a transconductance (g_m)
amplifier, the LTC3860 is designed to use an inverting
summing amplifier topology with the FB pin configured
as a virtual ground. This allows the feedback gain to be
tightly controlled by external components, which is not
possible with a simple g_m amplifier. In addition, the voltage
feedback amplifier allows flexibility in choosing pole and
zero locations. In particular, it allows the use of Type 3
compensation, which provides a phase boost at the LC
pole frequency and significantly improves the control loop
phase margin.
V_OUT
C3
C2
C1
R2
R1 R3 _–
FB
R_B
+
V_REF
–1
GAIN
0
COMP
PHASE
+1
–1
BOOST
FREQ
–90
–180
–270
–380
3806 F12
Figure 12. Type 3 Amplifier Compensation
3860fc
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