Datasheet
LTC3862
18
3862fc
For more information www.linear.com/LTC3862
operaTion
Table 1
PHASEMODE
CH-1 to CH-2
PHASE
CH-1 to CLKOUT
PHASE
APPLICATION
SGND 180° 90° 2-Phase, 4-Phase
Float 180° 60° 6-Phase
3V8 120° 240° 3-Phase
Using the LTC3862 Transconductance (g
m
) Error
Amplifier in Multi-Phase Applications
The LTC3862 error amplifier is a transconductance, or g
m
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 LTC3862s can be connected together,
as well as the ITH pins, as shown in Figure 8. The g
m
of the
composite error amplifier is simply n times the transcon-
ductance of one amplifier, or g
m(TOT)
= n • 660μS, where
n is the number of amplifiers connected in parallel. 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
determines the threshold at the input of the current com
-
parator. 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 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 multiphase applications that use more than one LTC3862
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.
Figure 8. LTC3862 Error Amplifier Configuration
for Multi-Phase Operation
Soft-Start
The start-up of the LTC3862 is controlled by the voltage 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 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:
CµA
t
V
SS
SS
=
5
06.
FREQ
FB
CLKOUT
SYNC
PLLFLTR
LTC3862
MASTER
SGND
ITH
V
OUT
INTV
CC
SS
RUN
ON/OFF
CONTROL
ALL RUN PINS
CONNNECTED
TOGETHER
INDIVIDUAL
INTV
CC
PINS
LOCALLY
DECOUPLED
FREQ
FB
100pF
CLKOUT
SYNC
PLLFLTR
LTC3862
SLAVE
SGND
ITH
INTV
CC
SS
RUN
SLAVE
3862 F08
ALL SS PINS
CONNNECTED
TOGETHER
FREQ
FB
ALL FB PINS
CONNECTED
TOGETHER
ALL ITH PINS
CONNECTED
TOGETHER
100Ω
CLKOUT
SYNC
PLLFLTR PHASEMODE
PHASEMODE
PHASEMODE
LTC3862
SGND
ITH
INTV
CC
SS
RUN
100Ω 100Ω
100Ω
100pF
100pF
100pF