ltc3890-1 Linear Technology Corporation, ltc3890-1 Datasheet - Page 21

• Manufacturer Part Number: ltc3890-1
• Description: 60v Low Iq, Dual, 2-phase Synchronous Step-down Dc/dc Controller
• Manufacturer: Linear Technology Corporation

LDO regulate INTV
peak current of 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
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 maxi-
mum junction temperature rating for the LTC3890-1 to
be exceeded. The INTV
the gate charge current, may be supplied by either the V
LDO or the EXTV
V
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 LTC3890-1
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
control power to be derived from one of the LTC3890-1’s
APPLICATIONS INFORMATION
CC
IN
T
LDO is turned off and the EXTV
J
pin is less than 4.7V, the V
IN
CC
CC
= 70°C + (15mA)(40V)(90°C/W) = 125°C
CC
CC
• I
CC
is regulated to 5.1V.
current is limited to less than 15mA from a 40V
LDO remains on as long as the voltage applied to
remains above 4.5V. The EXTV
) at maximum V
INTVCC
. The gate charge current is dependent
CC
CC
CC
LDO allows the MOSFET driver and
CC
LDO. When the voltage on the EXT-
to 5.1V. Each of these can supply a
CC
voltage to 5.1V, so while EXTV
IN
current, which is dominated by
.
IN
CC
CC
LDO is enabled. Power
CC
supply at 70°C ambi-
rises above 4.7V, the
LDO is enabled. The
CC
CC
. When EXTV
LDO attempts
CC
CC
CC
CC
IN
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 specifi ed, an external Schottky diode can be added
between the EXTV
not apply more than 6V to the EXTV
that 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
INTV
resulting 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.
nal supply is available in the 5V to 14V range, it may
be used to power EXTV
with the MOSFET gate drive requirements. Ensure that
EXTV
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 + (15mA)(8.5V)(90°C/W) = 82°C
CC
CC
CC
CC
CC
CC
CC
CC
to be powered from the internal 5.1V regulator
Connected to an Output-Derived Boost Network.
Connected directly to V
Connected to an External supply. If an exter-
< V
Left Open (or Grounded). This will cause
pin directly to V
≤ V
CC
IN
IN
.
:
CC
.
CC
from the output, since the V
and INTV
CC
OUT
providing it is compatible
. Tying the EXTV
CC
IN
OUT
pins. In this case, do
LTC3890-1
CC
LDO when the out-
. This is the normal
OUT
pin and make sure
CC
≤ 14V) during
CC
power from
CC
LDO than
CC
CC
< V
21
IN
pin to
to an
IN
38901f
cur-
.