Datasheet

LTC3860
22
3860fc
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
V
OUT2
=
R2A
R
TRACKA
•
R
TRACKA
+R
TRACKB
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 fi nal
value is:
t
SS2
= t
SS1
•
0.6
V
OUT1F
•
R
TRACKA
+R
TRACKB
R
TRACKA
For coincident tracking,
t
SS2
= t
SS1
•
V
OUT2F
V
OUT1F
where V
OUT1F
and V
OUT2F
are the fi nal, 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 fi nal 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.
APPLICATIONS INFORMATION
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
amplifi ers sense the output voltage at V
OUT
.
The positive input of the error amplifi er 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
)
amplifi er, the LTC3860 is designed to use an inverting
summing amplifi er topology with the FB pin confi gured
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
amplifi er. In addition, the voltage
feedback amplifi er allows fl exibility 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 signifi cantly improves the control loop
phase margin.
–
+
V
OUT
V
REF
R1 R3
C3
R2
C1
GAIN (dB)
C2
FB
R
B
COMP
FREQ
–1
–1+1
GAIN
PHASE
BOOST
0
PHASE (DEG)
–90
–180
–270
–380
3806 F12
Figure 12. Type 3 Amplifi er Compensation