Datasheet

LTC3890-3
21
38903f
For more information www.linear.com/3890-3
applicaTions inForMaTion
(7a) Coincident Tracking
(7b) Ratiometric Tracking
Figure 8. Using the TRACK/SS Pin for Tracking
Figure 7. Two Different Modes of Output Voltage Tracking
TIME
V
X(MASTER)
V
OUT(SLAVE)
OUTPUT VOLTAGE
38903 F07a
TIME
38903 F07b
V
X(MASTER)
V
OUT(SLAVE)
OUTPUT VOLTAGE
1/2 LTC3890-3
V
OUT
V
x
V
FB
TRACK/SS
38903 F08
R
B
R
A
R
TRACKA
R
TRACKB
For coincident tracking (V
OUT
= V
X
during start-up):
R
A
= R
TRACKA
R
B
= R
TRACKB
INTV
CC
Regulators
The LTC3890-3 features two separate internal P-channel
low dropout linear regulators (LDO) that supply power at
the INTV
CC
pin from either the V
IN
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 LTC3890-3’s
internal circuitry. The V
IN
LDO and the EXTV
CC
LDO regulate
INTV
CC
to 5.1V. 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 maxi
-
mum junction temperature rating for the LTC3890-3 to be
exceeded. The INTV
CC
current, which is dominated by the
gate charge current, may be supplied by either the V
IN
LDO or the EXTV
CC
LDO. When the voltage on the EXTV
CC
pin is less than 4.7V, the V
IN
LDO is enabled. Power dis-
sipation 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 Con
-
siderations section. The junction temperature can be
estimated by using the equations given in Note 3 of the
Electrical Characteristics. For example, the LTC3890E-3
INTV
CC
current is limited to less than 15mA from a 40V
supply when not using the EXTV
CC
supply at 70°C ambi-
ent temperature:
T
J
= 70°C + (15mA)(40V)(90°C/W) = 125°C