Datasheet

LTC3856
27
3856f
TIME
(11a) Coincident Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE
3856 F11a
V
OUT1
V
OUT2
TIME
3856 F11b
(11b) Ratiometric Tracking
OUTPUT VOLTAGE
Figure 11. Two Different Modes of Output Voltage Tracking
R3 R1
R4 R2
R3
V
OUT2
R4
(12a) Coincident Tracking Set-Up
TO
V
FB1
PIN
TO
TK/SS2
PIN
TO
V
FB2
PIN
V
OUT1
R1
R2
R3
V
OUT2
R4
3856 F12
(12b) Ratiometric Tracking Set-Up
TO
V
FB1
PIN
TO
TK/SS2
PIN
TO
V
FB2
PIN
V
OUT1
Figure 12. Set-Up and Coincident and Ratiometric Tracking
applicaTions inForMaTion
is needed to supply the high transient currents required
by the MOSFET gate drivers and to prevent interaction
between the channels.
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC3856 to be
exceeded. The INTV
CC
current, which is dominated by the
gate charge current, may be supplied by either the 5V LDO
or EXTV
CC
. When the voltage on the EXTV
CC
pin is less
than 4.7V, 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 3 of the Electrical Characteristics
table. For example, the LTC3856 INTV
CC
current is limited
to less than 42mA from a 38V supply in the UH package
and not using the EXTV
CC
supply:
T
J
= 70°C + (42mA)(38V)(34°C/W) = 125°C
To prevent the maximum junction temperature from be-
ing exceeded, the input supply current must be checked
while operating in continuous conduction mode (MODE
= SGND) at maximum V
IN
. When the voltage applied to
EXTV
CC
rises above 4.7V, the INTV
CC
LDO is turned off
and the EXTV
CC
is connected to the INTV
CC
. The EXTV
CC
remains on as long as the voltage applied to EXTV
CC
remains
above 4.5V. Using the EXTV
CC
allows the MOSFET driver
and control power to be derived from one of switching
regulator outputs during normal operation and from the
INTV
CC
when the output is out of regulation (e.g., start-up,