Datasheet

LTC3859
26
3859fa
APPLICATIONS INFORMATION
Alternatively, the TRACK/SS1 and TRACK/SS2 pins for the
two buck controllers can be used to track two (or more) sup-
plies during start-up, as shown qualitatively in Figures 7a
and 7b. To do this, a resistor divider should be connected
from the master supply (V
X
) to the TRACK/SS pin of the
slave supply (V
OUT
), as shown in Figure 8. During start-up
V
OUT
will track V
X
according to the ratio set by the resis-
tor divider:
V
X
V
OUT
=
R
A
R
TRACKA
•
R
TRACKA
+R
TRACKB
R
A
+R
B
For coincident tracking (V
OUT
= V
X
during start-up),
R
A
= R
TRACKA
R
B
= R
TRACKB
INTV
CC
Regulators
The LTC3859 features two separate internal P-channel
low dropout linear regulators (LDO) that supply power
at the INTV
CC
pin from either the V
BIAS
supply pin or the
EXTV
CC
pin depending on the connection of the EXTV
CC
pin. INTV
CC
powers the gate drivers and much of the
LTC3859’s internal circuitry. The V
BIAS
LDO and the EXTV
CC
LDO regulate INTV
CC
to 5.4V. Each of these can supply a
peak current of 50mA and must be bypassed to ground
with a minimum of 4.7µF ceramic capacitor. No matter
what type of bulk capacitor is used, an additional 1µF
ceramic capacitor placed directly adjacent to the INTV
CC
and PGND IC pins is highly recommended. Good bypassing
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
maximum junction temperature rating for the LTC3859 to
be exceeded. The INTV
CC
current, which is dominated by
the gate charge current, may be supplied by either the V
BIAS
LDO or the EXTV
CC
LDO. When the voltage on the EXTV
CC
pin is less than 4.7V, the V
BIAS
LDO is enabled. Power
dissipation for the IC in this case is highest and is equal to
V
BIAS
• I
INTVCC
. The gate charge current is dependent
on operating frequency as discussed in the Effi ciency
Considerations section. The junction temperature can be
estimated by using the equations given in Note 2 of the
Electrical Characteristics. For example, the LTC3859 INTV
CC
current is limited to less than 40mA from a 40V supply
when not using the EXTV
CC
supply at a 70°C ambient
temperature in the QFN package:
T
J
= 70°C + (40mA)(40V)(34°C/W) = 125°C
7a. Coincident Tracking 7b. Radiometric Tracking
Figure 7. Two Different Modes of Output Voltage Tracking
Figure 8. Using the TRACK/SS Pin for Tracking
3859 F07a
V
X(MASTER)
V
OUT(SLAVE)
OUTPUT (V
OUT
)
TIME
3859 F07b
V
X(MASTER)
V
OUT(SLAVE)
OUTPUT (V
OUT
)
TIME
3859 F08
LTC3859
V
FB1,2
TRACK/SS1,2
R
B
R
A
V
OUT
R
TRACKB
R
TRACKA
V
X