Datasheet

LTC3890-1
20
38901fb
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
Alternatively, the TRACK/SS pin can be used to track two
(or more) supplies 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 resistor 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 LTC3890-1 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 LTC3890-1’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 LTC3890-1 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
TIME
V
X(MASTER)
V
OUT(SLAVE)
OUTPUT VOLTAGE
38901 F07a
TIME
38901 F07b
V
X(MASTER)
V
OUT(SLAVE)
OUTPUT VOLTAGE
1/2 LTC3890-1
V
OUT
V
x
V
FB
TRACK/SS
38901 F08
R
B
R
A
R
TRACKA
R
TRACKB
1/2 LTC3890-1
TRACK/SS
C
SS
SGND
38901 F06
Figure 6. Using the TRACK/SS Pin to Program Soft-Start