LTC3868 Linear Technology, LTC3868 Datasheet - Page 21

• Manufacturer Part Number: LTC3868
• Description: Low IQ Dual 2-Phase Synchronous Step-Down Controller
• Manufacturer: Linear Technology

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
the SS pin, allowing V
its final regulated value. The total soft-start time will be
approximately:
INTV
The LTC3868 features two separate internal P-channel
low dropout linear regulators (LDO) that supply power
at the INTV
EXTV
pin. INTV
LTC3868’s internal circuitry. The V
LDO regulate INTV
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
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
current, may be supplied by either the V
EXTV
than 4.7V, the V
IC in this case is highest and is equal to V
gate charge current is dependent on operating frequency
applicaTions inForMaTion
t
SS
Figure 7. Using the TRACK/SS Pin to Program Soft-Start
CC
FB
CC
CC
= C
pin (and hence V
pin depending on the connection of the EXTV
LDO. When the voltage on the EXTV
Regulators
CC
CC
SS
current, which is dominated by the gate charge
CC
powers the gate drivers and much of the
•
pin from either the V
IN
0.8V
1µA
LDO is enabled. Power dissipation for the
CC
to 5.1V. Each of these can supply a
OUT
C
SS
OUT
to rise smoothly from 0V to
SS
SGND
1/2 LTC3868
) according to the voltage on
3868 F07
IN
LDO and the EXTV
IN
supply pin or the
IN
IN
• I
CC
LDO or the
INTVCC
pin is less
. The
CC
CC
CC
as discussed in the Efficiency 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
than 45mA from a 28V supply when not using the EXTV
supply at 70°C ambient temperature:
T
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
When the voltage applied to EXTV
V
EXTV
EXTV
to regulate the INTV
is less than 5.1V, the LDO is in dropout and the INTV
voltage is approximately equal to EXTV
is greater than 5.1V, up to an absolute maximum of 14V,
INTV
Using the EXTV
control power to be derived from one of the LTC3868’s
switching regulator outputs (4.7V ≤ V
normal operation and from the V
put is out of regulation (e.g., start-up, short-circuit). If
more current is required through the EXTV
is specified, an external Schottky diode can be added
between the EXTV
not apply more than 6V to the EXTV
that EXTV
Significant efficiency and thermal gains can be realized
by powering INTV
rent resulting from the driver and control currents will be
scaled by a factor of (Duty Cycle)/(Switcher Efficiency).
For 5V to 14V regulator outputs, this means connecting
the EXTV
an 8.5V supply reduces the junction temperature in the
previous example from 125°C to:
T
However, for 3.3V and other low voltage outputs, addi-
tional circuitry is required to derive INTV
the output.
IN
LDO is turned off and the EXTV
J
J
= 70°C + (45mA)(28V)(43°C/W) = 125°C
CC
= 70°C + (45mA)(8.5V)(43°C/W) = 86°C
CC
CC
CC
is regulated to 5.1V.
LDO remains on as long as the voltage applied to
remains above 4.5V. The EXTV
) at maximum V
CC
CC
pin directly to V
≤ V
CC
IN
.
CC
LDO allows the MOSFET driver and
CC
CC
from the output, since the V
and INTV
voltage to 5.1V, so while EXTV
IN
.
OUT
CC
CC
. Tying the EXTV
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current is limited to less
CC
pins. In this case, do
CC
IN
CC
rises above 4.7V, the
LDO when the out-
LDO is enabled. The
pin and make sure
OUT
CC
LTC3868
CC
. When EXTV
CC
LDO attempts
≤ 14V) during
CC
power from
LDO than
CC

IN
pin to
cur-
3868fb
CC
CC
CC
CC