LTC3858 Linear Technology, LTC3858 Datasheet - Page 21

• Manufacturer Part Number: LTC3858
• Description: 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 LTC3858 will regulate
the V
the SS pin, allowing V
its final regulated value. The total soft-start time will be
approximately:
INTV
The LTC3858 features two separate internal P-channel low
dropout linear regulators (LDO) that supply power at the
INTV
pin depending on the connection of the EXTV
powers the gate drivers and much of the internal circuitry.
The V
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. Regardless of what type of bulk capacitor is
used, an additional 1µF ceramic capacitor placed directly
adjacent to the INTV
mended. 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 LTC3858 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
CC
FB
CC
IN
=
pin (and hence V
pin from either the V
LDO and the EXTV
LDO. When the voltage on the EXTV
Regulators
CC
C
SS
current, which is dominated by the gate charge
•
IN
. 0 8
1
LDO is enabled. Power dissipation for the
µA
V
CC
and PGND pins is highly recom-
OUT
C
SS
OUT
CC
to rise smoothly from 0V to
SS
SGND
1/2 LTC3858
) according to the voltage on
LDO regulate INTV
IN
supply pin or the EXTV
3858 F07
IN
IN
CC
• I
CC
LDO or the
pin. INTV
INTVCC
pin is less
CC
to 5.1V.
. The
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 LTC3858 INTV
than 32mA from a 40V 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 switching
regulator outputs (4.7V ≤ V
operation and from the V
of regulation (e.g., start-up, short-circuit). If more current
is required through the EXTV
external Schottky diode can be added between the EXTV
and INTV
to the 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 + (32mA)(40V)(43°C/W) = 125°C
CC
= 70°C + (32mA)(8.5V)(43°C/W) = 82°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
pins. In this case, do not apply more than 6V
CC
pin and make sure that EXTV
CC
CC
LDO allows the MOSFET driver and
CC
from the output, since the V
voltage to 5.1V, so while EXTV
IN
IN
.
LDO when the output is out
OUT
CC
OUT
CC
LDO than is specified, an
. Tying the EXTV
≤ 14V) during normal
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current is limited to less
CC
CC
rises above 4.7V, the
LDO is enabled. The
CC
LTC3858
CC
. When EXTV
CC
LDO attempts
power from
CC
≤ V
CC

IN
pin to
IN
cur-
.
3858fa
CC
CC
CC
CC
CC