ltc3890 Linear Technology Corporation, ltc3890 Datasheet - Page 21

• Manufacturer Part Number: ltc3890
• Description: 60v Low Iq, Dual, 2-phase Synchronous Step-down Dc/dc Controller
• Manufacturer: Linear Technology Corporation

INTV
50mA and must be bypassed to ground with a minimum
of 4.7μF ceramic capacitor. No matter 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
maximum junction temperature rating for the LTC3890
to be exceeded. The INTV
by the gate charge current, may be supplied by either the
V
EXTV
dissipation for the IC in this case is highest and is equal
to V
on operating frequency 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 LTC3890
INTV
supply when not using the EXTV
ent temperature:
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
trol power to be derived from one of the LTC3890’s switch-
ing regulator outputs (4.7V ≤ V
APPLICATIONS INFORMATION
IN
IN
T
LDO is turned off and the EXTV
J
LDO or the EXTV
IN
CC
CC
CC
CC
CC
CC
= 70°C + (32mA)(40V)(43°C/W) = 125°C
CC
• I
to 5.1V. Each of these can supply a peak current of
is regulated to 5.1V.
pin is less than 4.7V, the V
current is limited to less than 32mA from a 40V
LDO remains on as long as the voltage applied to
remains above 4.5V. The EXTV
) at maximum V
INTVCC
CC
. The gate charge current is dependent
LDO allows the MOSFET driver and con-
CC
CC
voltage to 5.1V, so while EXTV
LDO. When the voltage on the
IN
CC
.
current, which is dominated
OUT
CC
IN
CC
CC
supply at a 70°C ambi-
LDO is enabled. Power
≤ 14V) during normal
CC
rises above 4.7V, the
LDO is enabled. The
and PGND pins is
CC
CC
. When EXTV
LDO attempts
CC
CC
CC
operation and from the V
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
an 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
1. EXTV
2. EXTV
3. EXTV
4. EXTV
T
to be powered from the internal 5.1V regulator result-
ing in an effi ciency penalty of up to 10% at high input
voltages.
connection for a 5V to 14V regulator and provides the
highest effi ciency.
supply is available in the 5V to 14V range, it may be
used to power EXTV
MOSFET gate drive requirements. Ensure that EXTV
< V
For 3.3V and other low voltage regulators, effi ciency
gains can still be realized by connecting EXTV
output-derived voltage that has been boosted to greater
than 4.7V. This can be done with the capacitive charge
pump shown in Figure 9. Ensure that EXTV
J
= 70°C + (32mA)(8.5V)(43°C/W) = 82°C
IN
.
CC
CC
CC
CC
CC
CC
Connected to an Output-Derived Boost Network.
Left Open (or Grounded). This will cause INTV
Connected to an External Supply. If an external
Connected Directly to V
CC
pins. In this case, do not apply more than 6V
pin directly to V
pin and make sure that EXTV
CC
:
CC
from the output, since the V
CC
providing it is compatible with the
IN
LDO when the output is out of
OUT
CC
. Tying the EXTV
LDO than is specifi ed, an
OUT
. This is the normal
LTC3890
CC
CC
power from
CC
≤ V
CC
CC
< V
21
IN
IN
pin to
.
to an
IN
cur-
3890f
.
CC
CC
CC