Datasheet

LTC3858-1
20
38581fd
APPLICATIONS INFORMATION
linear ramping voltage at the SS pin. The LTC3858-1 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
INTV
CC
Regulators
The LTC3858-1 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
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. Regardless of 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
maximum junction temperature rating for the LTC3858-1
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 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 Characteristics. For example, the LTC3858-1
INTV
CC
current is limited to less than 15mA from a 40V
supply when not using the EXTV
CC
supply at 70°C ambient
temperature in the SSOP package:
T
J
= 70°C + (15mA)(40V)(90°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 switching
regulator outputs (4.7V ≤ V
OUT
≤ 14V) during normal
operation and from the V
IN
LDO when the output 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
a 8.5V supply reduces the junction temperature in the
previous example from 125°C to:
T
J
= 70°C + (15mA)(8.5V)(90°C/W) = 82°C
However, for 3.3V and other low voltage outputs, addi-
tional circuitry is required to derive INTV
CC
power from
the output.
The following list summarizes the four possible connec-
tions for EXTV
CC
: