Datasheet

LTC3868
21
3868fd
APPLICATIONS INFORMATION
Figure 7. Using the TRACK/SS Pin to Program Soft-Start
Soft-start is enabled by simply connecting a capacitor from
the SS pin to ground, as shown in Figure 7. An internal 1µA
current source charges the capacitor, providing a linear
ramping voltage at the SS pin. The LTC3868 will regulate
the V
FB
pin (and hence V
OUT
) according to the voltage on
the 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
1µ A
as discussed in the Effi ciency Considerations section.
The junction temperature can be estimated by using the
equations given in Note 3 of the Electrical Characteristics.
For example, the LTC3868 INTV
CC
current is limited to less
than 45mA from a 28V supply when not using the EXTV
CC
supply at 70°C ambient temperature:
T
J
= 70°C + (45mA)(28V)(43°C/W) = 125°C
To prevent the maximum junction temperature from be-
ing exceeded, the input supply current must be checked
while operating in forced continuous mode (PLLIN/MODE
= INTV
CC
) at maximum V
IN
.
When the voltage applied to EXTV
CC
rises above 4.7V, the
V
IN
LDO is turned off and the EXTV
CC
LDO is enabled. The
EXTV
CC
LDO remains on as long as the voltage applied to
EXTV
CC
remains above 4.5V. The EXTV
CC
LDO attempts
to regulate the INTV
CC
voltage to 5.1V, so while EXTV
CC
is less than 5.1V, the LDO is in dropout and the INTV
CC
voltage is approximately equal to EXTV
CC
. When EXTV
CC
is greater than 5.1V, up to an absolute maximum of 14V,
INTV
CC
is regulated to 5.1V.
Using the EXTV
CC
LDO allows the MOSFET driver and
control power to be derived from one of the LTC3868’s
switching regulator outputs (4.7V ≤ V
OUT
≤ 14V) during
normal operation and from the V
IN
LDO when the out-
put is out of regulation (e.g., start-up, short-circuit). If
more current is required through the EXTV
CC
LDO than
is specifi ed, an external Schottky diode can be added
between the EXTV
CC
and INTV
CC
pins. In this case, do
not apply more than 6V to the EXTV
CC
pin and make sure
that EXTV
CC
≤ V
IN
.
Signifi cant effi ciency and thermal gains can be realized
by powering INTV
CC
from the output, since the V
IN
cur-
rent resulting from the driver and control currents will be
scaled by a factor of (Duty Cycle)/(Switcher Effi ciency).
For 5V to 14V regulator outputs, this means connecting
the EXTV
CC
pin directly to V
OUT
. Tying the EXTV
CC
pin to
an 8.5V supply reduces the junction temperature in the
previous example from 125°C to:
T
J
= 70°C + (45mA)(8.5V)(43°C/W) = 86°C
However, for 3.3V and other low voltage outputs, addi-
tional circuitry is required to derive INTV
CC
power from
the output.
1/2 LTC3868
SS
C
SS
SGND
3868 F07
INTV
CC
Regulators
The LTC3868 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
LTC3868’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 low ESR capacitor. No matter
what type of bulk capacitor is used, an additional 1µF
ceramic capacitor placed directly adjacent to the INTV
CC
and PGND 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 LTC3868 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 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