Datasheet

LTC3612
21
3612fb
For more information www.linear.com/LTC3612
APPLICATIONS INFORMATION
For coincident start-up, the voltage value at the TRACK/SS
pin for the slave channel needs to reach the final reference
value after the internal soft-start time (around 1ms). The
master start-up time needs to be adjusted with an external
capacitor and resistor to ensure this.
External Reference Input (DDR Mode)
If the DDR pin is tied to SV
IN
(DDR mode), the run state is
entered when V
TRACK/SS
exceeds 0.3V and tracking down
behavior is possible if the V
TRACK/SS
voltage is below 0.6V.
This allows TRACK/SS to be used as an external reference
between 0.3V and 0.6V if desired. During the run state in
DDR mode, the power good window moves in relation
to the actual TRACK/SS pin voltage if the voltage value
is between 0.3V and 0.6V. Note: if TRACK/SS voltage is
0.6V, either the tracking circuit or the internal reference
can be used.
During up/down tracking the output current foldback is
disabled and the PGOOD pin is always pulled down (see
Figure 8).
Figure 6a. Set-Up for Coincident Tracking
Figure 6b. Set-Up for Ratiometric Tracking
V
FB2
R4
R2
R4
R2
R3
R2
R4 ≤ R3
V
OUT2
V
OUT1
LTC3612
TRACK/SS2
V
FB1
V
IN
LTC3612
LTC3612 CHANNEL 2
SLAVE
LTC3612 CHANNEL 1
MASTER
TRACK/SS1
3612 F06a
V
FB2
R1
R2
R5
R6
R3 R1/R2 < R5/R6
R4
V
OUT2
V
OUT1
LTC3612
TRACK/SS2
V
FB1
V
IN
3612 F06b
LTC3612
LTC3612 CHANNEL 2
SLAVE
LTC3612 CHANNEL 1
MASTER
TRACK/SS1
Efficiency Considerations
The efficiency of a switching regulator is equal to the output
power divided by the input power times 100%. It is often
useful to analyze individual losses to determine what is
limiting the efficiency and which change would produce
the most improvement. Efficiency can be expressed as:
Efficiency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc. are the individual losses as a percent
-
age of input power.
Although
all
dissipative elements in the circuit produce
losses, two main sources usually account for most of
the losses: V
IN
quiescent current and I
2
R losses. The V
IN
quiescent current loss dominates the efficiency loss at
very low load currents whereas the I
2
R loss dominates
the efficiency loss at medium to high load currents. In a
typical efficiency plot, the efficiency curve at very low load
currents can be misleading since the actual power lost is
usually of no consequence.
1. The V
IN
quiescent current is due to two components: the
DC bias current as given in the Electrical Characteristics
and the internal main switch and synchronous switch
gate charge currents. The gate charge current results
from switching the gate capacitance of the internal power
MOSFET switches. Each time the gate is switched from
low to high to low again, a packet of charge dQ moves
from V
IN
to ground. The resulting dQ/dt is the current
out of V
IN
due to gate charge, and it is typically larger
than the DC bias current. Both the DC bias and gate
charge losses are proportional to V
IN
; thus, their effects
will be more pronounced at higher supply voltages.
2. I
2
R losses are calculated from the resistances of the
internal switches, R
SW
, and external inductor, R
L
. In
continuous mode the average output current flowing
through inductor L is “chopped” between the main
switch and the synchronous switch. Thus, the series
resistance looking into the SW pin is a function of both
top and bottom MOSFET R
DS(ON)
and the duty cycle
(DC) as follows:
R
SW
= (R
DS(ON)TOP
)(DC) + (R
DS(ON)BOT
)(1 – DC)