Datasheet

LTC3890
22
3890fc
EXTV
CC
V
IN
TG1
SW
BG1
PGND
1/2 LTC3890
R
SENSE
V
OUT
NDS7002
C
OUT
3890 F09
MBOT
MTOP
C
IN
L
BAT85 BAT85
BAT85
Figure 9. Capacitive Charge Pump for EXTV
CC
The following list summarizes the four possible connec-
tions for EXTV
CC
:
1. EXTV
CC
Grounded. This will cause INTV
CC
to be powered
from the internal 5.1V regulator resulting in an efficiency
penalty of up to 10% at high input voltages.
2. EXTV
CC
Connected Directly to V
OUT
. This is the normal
connection for a 5V to 14V regulator and provides the
highest efficiency.
3. EXTV
CC
Connected to an External Supply. If an external
supply is available in the 5V to 14V range, it may be
used to power EXTV
CC
providing it is compatible with the
MOSFET gate drive requirements. Ensure that EXTV
CC
< V
IN
.
4. EXTV
CC
Connected to an Output-Derived Boost Network.
For 3.3V and other low voltage regulators, efficiency
gains can still be realized by connecting EXTV
CC
to an
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
CC
< V
IN
.
APPLICATIONS INFORMATION
desired MOSFET. This enhances the top MOSFET switch
and turns it on. The switch node voltage, SW, rises to V
IN
and the BOOST pin follows. With the topside MOSFET
on, the boost voltage is above the input supply: V
BOOST
=
V
IN
+ V
INTVCC
. The value of the boost capacitor, C
B
, needs
to be 100 times that of the total input capacitance of the
topside MOSFET(s). The reverse breakdown of the external
Schottky diode must be greater than V
IN(MAX)
.
The external diode D
B
can be a Schottky diode or silicon
diode, but in either case it should have low-leakage and
fast recovery. Pay close attention to the reverse leakage
current specification for this diode, especially at high
temperatures where it generally increases substantially.
For applications with output voltages greater than ~5V
that are switching infrequently, a leaky diode D
B
can fully
discharge the bootstrap capacitor C
B
, creating a current
path from the output voltage to the BOOST pin to INTV
CC
.
Not only does this increase the quiescent current of the
converter, but it can cause INTV
CC
to rise to dangerous
levels if the leakage exceeds the current consumption on
INTV
CC
.
Particularly, this is a concern in Burst Mode operation at
no load or very light loads, where the part is switching
very infrequently and the current draw on INTV
CC
is very
low (typically about 35µA). Generally, pulse-skipping and
forced continuous modes are less sensitive to leakage,
since the more frequent switching keeps the bootstrap
capacitor C
B
charged, preventing a current path from the
output voltage to INTV
CC
.
However, in cases where the converter has been operat-
ing (in any mode) and then is shut down, if the leakage
of diode D
B
fully discharges the bootstrap capacitor C
B
before the output voltage discharges to below ~5V, then
the leakage current path can be created from the output
voltage to INTV
CC
. In shutdown, the INTV
CC
pin is able to
sink about 30µA. To accommodate diode leakage greater
than this amount in shutdown, INTV
CC
can be loaded
with an external resistor or clamped with a Zener diode.
Alternatively, the PGOOD resistor can be used to sink the
current (assuming the resistor pulls up to INTV
CC
) since
PGOOD is pulled low when the converter is shut down.
Nonetheless, using a low-leakage diode is the best choice
to maintain low quiescent current under all conditions.
Topside MOSFET Driver Supply (C
B
, D
B
)
External bootstrap capacitors, C
B
, connected to the BOOST
pins supply the gate drive voltages for the topside MOSFETs.
Capacitor C
B
in the Functional Diagram is charged though
external diode D
B
from INTV
CC
when the SW pin is low.
When one of the topside MOSFETs is to be turned on, the
driver places the C
B
voltage across the gate-source of the
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