Datasheet

LTC3866
22
3866fb
TIME
(8a) Coincident Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE
V
OUT1
V
OUT2
TIME
3866 F08
(8b) Ratiometric Tracking
OUTPUT VOLTAGE
APPLICATIONS INFORMATION
some trade-offs exist. The ratiometric mode saves a pair
of resistors, but the coincident mode offers better output
regulation. Under ratiometric tracking, when the master
controller’s output experiences dynamic excursion (under
load transient, for example), the slave controller output
will be affected as well. For better output regulation, use
the coincident tracking mode instead of ratiometric.
INTV
CC
(LDO) and EXTV
CC
The LTC3866
features a true PMOS LDO that supplies
power to INTV
CC
from the V
IN
supply. INTV
CC
powers the
gate drivers and much of the LTC3866’s internal circuitry.
The LDO regulates the voltage at the INTV
CC
pin to 5.5V
when V
IN
is greater than 6V. EXTV
CC
connects to INTV
CC
through a P-channel MOSFET and can supply the needed
power when its voltage is
higher than 4.7V. Either of these
can supply a peak current of 100mA and must be bypassed
to ground with a minimum of 4.7µF ceramic capacitor or
low ESR electrolytic capacitor. No matter what type of bulk
capacitor is used, an additional 0.1µF ceramic capacitor
placed directly adjacent to the INTV
CC
and PGND pins is
highly recommended. Good bypassing is needed to sup-
ply the high transient
currents required by the MOSFET
gate drivers. High input voltage applications in which
large MOSFETs are being driven at high frequencies may
cause the maximum junction temperature rating for the
LTC3866 to be exceeded. The INTV
CC
current, which is
dominated by the gate charge current, may be supplied by
either the 5.5V LDO or EXTV
CC
. When the voltage on the
EXTV
CC
pin is less than 4.5V, the LDO is enabled. Power
dissipation for the IC in this case is highest and is equal
to V
IN
• I
INTVCC
. The gate charge current is dependent
on operating frequency as discussed in the Efficiency
Considerations section. The junction temperature can be
estimated by using the equations given in Note 2 of the
Electrical Characteristics tables. For example, the LTC3866
INTV
CC
current is limited to less than 39mA from a 38V
supply in the UF package and not using the EXTV
CC
supply
with a 70°C ambient temperature:
T
J
= 70°C + (39mA)(38V)(37°C/W) ≅ 125°C
Figure 8. Tw o Different Modes of Output Voltage Tracking
Figure 9. Setup and Coincident and Ratiometric Tracking
R3 R1
R4 R2
R3
V
OUT2
R4
(9a) Coincident Tracking Setup
TO
V
FB1
PIN
TO
TK/SS2
PIN
TO
V
FB2
PIN
V
OUT1
R1
R2
R3
V
OUT2
R4
3866 F09
(9b) Ratiometric Tracking Setup
TO
V
FB1
PIN
TO
TK/SS2
PIN
TO
V
FB2
PIN
V
OUT1