Datasheet

LTC3827
20
3827ff
APPLICATIONS INFORMATION
providing a linear ramping voltage at the TRACK/SS pin.
The LTC3827 will regulate the V
FB
pin (and hence V
OUT
)
according to the voltage on the TRACK/SS pin, allowing
V
OUT
to rise smoothly from 0V to its fi nal regulated value.
The total soft-start time will be approximately:
t
SS
= C
SS
•
0.8V
1μA
Alternatively, the TRACK/SS pin can be used to track two
(or more) supplies during start-up, as shown qualitatively
in Figures 6a and 6b. 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 7.
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 LTC3827 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, respectively, depending on the connection of the
EXTV
CC
pin. INTV
CC
powers the gate drivers and much of
the LTC3827’s internal circuitry. The V
IN
LDO regulates
the voltage at the INTV
CC
pin to 5.25V and the EXTV
CC
LDO regulates it to 7.5V. Each of these can supply a peak
current of 50mA and must be bypassed to ground with
a minimum of 4.7µF tantalum, 10µF special polymer, or
low ESR electrolytic capacitor. A ceramic capacitor with a
minimum value of 4.7µF can also be used if a 1 resistor
is added in series with the 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 LTC3827 to be
exceeded. The INTV
CC
current, which is dominated by the
Figure 7. Using the TRACK/SS Pin for Tracking
Figure 6. Two Different Modes of Output
Voltage Tracking
TIME
(6a) Coincident Tracking
V
X
(MASTER)
V
OUT
(SLAVE)
OUTPUT VOLTAGE
3827 F06a
V
X
(MASTER)
V
OUT
(SLAVE)
TIME
3827 F06b
(6b) Ratiometric Tracking
OUTPUT VOLTAGE
1/2 LTC3827
V
OUT
V
x
V
FB
TRACK/SS
3827 F07
R
B
R
A
R
TRACKA
R
TRACKB