LTC3786 LINER [Linear Technology], LTC3786 Datasheet - Page 19

• Manufacturer Part Number: LTC3786
• Description: Multi-Phase Current Mode Step-Up DC/DC Controller
• Manufacturer: LINER [Linear Technology]

Using the LTC3862-2 Transconductance (g
Amplifier in Multi-Phase Applications
The LTC3862-2 error amplifier is a transconductance, or g
amplifier, meaning that it has high DC gain but high output
impedance (the output of the error amplifier is a current
proportional to the differential input voltage). This style
of error amplifier greatly eases the task of implementing
a multi-phase solution, because the amplifiers from two
or more chips can be connected in parallel. In this case
the FB pins of multiple LTC3862-2s can be connected to-
gether, as well as the ITH pins, as shown in Figure 8. The
g
transconductance of one amplifier, or g
where n is the number of amplifiers connected in paral-
lel. The transfer function from the ITH pin to the current
comparator inputs was carefully designed to be accurate,
both from channel-to-channel and chip-to-chip. This way
the peak inductor current matching is kept accurate.
A buffered version of the output of the error amplifier deter-
mines the threshold at the input of the current comparator.
The ITH voltage that represents zero peak current is 0.4V
and the voltage that represents current limit is 1.2V (at
low duty cycle). During an overload condition, the output
of the error amplifier is clamped to 2.6V at low duty cycle,
in order to reduce the latency when the overload condition
terminates. A patented circuit in the LTC3862-2 is used
to recover the slope compensation signal, so that the
maximum peak inductor current is not a strong function
of the duty cycle.
In multi-phase applications that use more than one
LTC3862-2 controller, it is possible for ground currents
on the PCB to disturb the control lines between the ICs,
resulting in erratic behavior. In these applications the FB
pins should be connected to each other through 100Ω
resistors and each slave FB pin should be decoupled locally
with a 100pF capacitor to ground, as shown in Figure 8.
Soft-Start
The start-up of the LTC3862-2 is controlled by the volt-
age on the SS pin. An internal PNP transistor clamps the
current comparator sense threshold during soft-start,
thereby limiting the peak switch current. The base of the
PNP is connected to the SS pin and the emitter to an
operaTion
m
of the composite error amplifier is simply n times the
m(TOT)
m
= n • 660μS,
) Error
m
internal, buffered ITH node (please note that the ITH pin
voltage may not track the soft-start voltage during this time
period). An internal 5μA current source charges the SS
capacitor, and clamps the peak sense threshold until the
voltage on the soft-start capacitor reaches approximately
0.6V. The required amount of soft-start capacitance can
be estimated using the following equation:
The SS pin has an internal open-drain NMOS pull-down
transistor that turns on when the RUN pin is pulled low,
when the voltage on the INTV
voltage lockout threshold, or during an overtemperature
condition. In multi-phase applications that use more than
ALL ITH PINS
CONNECTED
ALL FB PINS
CONNECTED
TOGETHER
TOGETHER
C
V
SS
OUT
Figure 8. LTC3862-2 Error Amplifier Configuration
for Multi-Phase Operation
= 5µA
* R = 100
†
C
X
†
†
†
= 100pF
†
†

 
0.6V
t
SS
*
*

 
*
*
ITH
FREQ
ITH
FB
CLKOUT
SYNC
PLLFLTR
FREQ
ITH
FB
CLKOUT
SYNC
PLLFLTR
FB
CLKOUT
SYNC
PLLFLTR
LTC3862-2
LTC3862-2
LTC3862-2
MASTER
SLAVE
SLAVE
SGND
SGND
SGND
PHASEMODE
PHASEMODE
PHASEMODE
CC
INTV
INTV
INTV
RUN
RUN
RUN
pin is below its under-
SS
SS
SS
CC
CC
CC
LTC3862-2
38622 F08
ALL SS PINS
CONNNECTED
TOGETHER
ON/OFF
CONTROL
INDIVIDUAL
INTV
LOCALLY
DECOUPLED
ALL RUN PINS
CONNNECTED
TOGETHER
19
CC
PINS
38622f