LTC3858-1 Linear Technology Corporation, LTC3858-1 Datasheet - Page 20

• Manufacturer Part Number: LTC3858-1
• Description: Dual 2-Phase Synchronous Step-Down Controller
• Manufacturer: Linear Technology Corporation

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LTC3858-1
linear ramping voltage at the SS pin. The LTC3858-1 will
regulate the V
voltage on the SS pin, allowing V
0V to its fi nal regulated value. The total soft-start time will
be approximately:
INTV
The LTC3858-1 features two separate internal P-channel
low dropout linear regulators (LDO) that supply power at
the INTV
V
INTV
circuitry. The V
TV
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
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 LTC3858-1 to be
exceeded. The INTV
gate charge current, may be supplied by either the V
or the EXTV
is less than 4.7V, the V
tion for the IC in this case is highest and is equal to V
I
frequency as discussed in the Effi ciency Considerations
section. The junction temperature can be estimated by
using the equations given in Note 2 of the Electrical Char-
acteristics. For example, the LTC3858-1 INTV
is limited to less than 32mA from a 40V supply when not
using the EXTV
APPLICATIONS INFORMATION
20
INTVCC
CC
CC
T
t
SS
J
pin depending on the connection of the EXTV
CC
CC
to 5.1V. Each of these can supply a peak current of
= 70°C + (32mA)(40V)(43°C/W) = 125°C
. The gate charge current is dependent on operating
=
Regulators
powers the gate drivers and much of the internal
CC
C
SS
pin from either the V
CC
•
FB
LDO. When the voltage on the EXTV
. 0 8
IN
CC
1
μA
pin (and hence V
LDO and the EXTV
supply:
V
CC
current, which is dominated by the
IN
LDO is enabled. Power dissipa-
OUT
IN
CC
supply pin or the EXT-
OUT
to rise smoothly from
CC
and PGND IC pins is
) according to the
LDO regulate IN-
CC
current
CC
IN
CC
LDO
pin.
IN
pin
•
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
Signifi cant effi ciency 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 Effi ciency).
For 5V to 14V regulator outputs, this means connecting
the EXTV
a 8.5V supply reduces the junction temperature in the
previous example from 125°C to:
However, for 3.3V and other low voltage outputs, addi-
tional circuitry is required to derive INTV
the output.
The following list summarizes the four possible connec-
tions for EXTV
IN
T
LDO is turned off and the EXTV
J
CC
CC
CC
= 70°C + (32mA)(8.5V)(43°C/W) = 82°C
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
CC
pins. In this case, do not apply more than 6V
pin directly to V
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
.
OUT
LDO when the output is out
CC
OUT
. Tying the EXTV
LDO than is specifi ed, an
CC
≤ 14V) during normal
CC
rises above 4.7V, the
LDO is enabled. The
CC
CC
. When EXTV
CC
LDO attempts
CC
power from
≤ V
CC
IN
IN
pin to
.
38581f
cur-
CC
CC
CC
CC