LTC3890
applicaTions inForMaTion
The following list summarizes the four possible connec-
desired MOSFET. This enhances the top MOSFET switch
and turns it on. The switch node voltage, SW, rises to V
tions for EXTV :
CC
IN
and the BOOST pin follows. With the topside MOSFET
1. EXTV Grounded.ThiswillcauseINTV tobepowered
CC
CC
on, the boost voltage is above the input supply: V
=
BOOST
fromtheinternal5.1Vregulatorresultinginanefficiency
V + V
. The value of the boost capacitor, C , needs
IN
INTVCC
B
penalty of up to 10% at high input voltages.
to be 100 times that of the total input capacitance of the
2. EXTV Connected Directly to V . This is the normal
topsideMOSFET(s).Thereversebreakdownoftheexternal
CC
OUT
connection for a 5V to 14V regulator and provides the
Schottky diode must be greater than V
.
IN(MAX)
highest efficiency.
The external diode D can be a Schottky diode or silicon
B
3. EXTV Connected to an External Supply. If an external
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
CC
supply is available in the 5V to 14V range, it may be
usedtopowerEXTV providingitiscompatiblewiththe
CC
MOSFET gate drive requirements. Ensure that EXTV
CC
< V .
IN
that are switching infrequently, a leaky diode D can fully
B
4. EXTV ConnectedtoanOutput-DerivedBoostNetwork.
CC
discharge the bootstrap capacitor C , creating a current
B
For 3.3V and other low voltage regulators, efficiency
path from the output voltage to the BOOST pin to INTV .
CC
gains can still be realized by connecting EXTV to an
CC
Not only does this increase the quiescent current of the
output-derivedvoltagethathasbeenboostedtogreater
converter, but it can cause INTV to rise to dangerous
CC
than 4.7V. This can be done with the capacitive charge
levels if the leakage exceeds the current consumption on
pump shown in Figure 9. Ensure that EXTV < V .
CC
IN
INTV .
CC
Particularly, this is a concern in Burst Mode operation at
no load or very light loads, where the part is switching
C
IN
very infrequently and the current draw on INTV is very
CC
BAT85
BAT85
V
IN
low (typically about 35µA). Generally, pulse-skipping and
forced continuous modes are less sensitive to leakage,
since the more frequent switching keeps the bootstrap
MTOP
MBOT
BAT85
NDS7002
TG1
1/2 LTC3890
L
R
SENSE
capacitor C charged, preventing a current path from the
B
V
OUT
EXTV
SW
CC
output voltage to INTV .
CC
C
BG1
OUT
However, in cases where the converter has been operat-
ing (in any mode) and then is shut down, if the leakage
of diode D fully discharges the bootstrap capacitor C
PGND
3890 F09
B
B
before the output voltage discharges to below ~5V, then
the leakage current path can be created from the output
Figure 9. Capacitive Charge Pump for EXTVCC
voltage to INTV . In shutdown, the INTV pin is able to
CC
CC
sink about 30µA. To accommodate diode leakage greater
Topside MOSFET Driver Supply (C , D )
B
B
than this amount in shutdown, INTV can be loaded
CC
with an external resistor or clamped with a Zener diode.
Externalbootstrapcapacitors,C ,connectedtotheBOOST
B
Alternatively, the PGOOD resistor can be used to sink the
pinssupplythegatedrivevoltagesforthetopsideMOSFETs.
current (assuming the resistor pulls up to INTV ) since
Capacitor C in the Functional Diagram is charged though
CC
B
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.
external diode D from INTV when the SW pin is low.
B
CC
When one of the topside MOSFETs is to be turned on, the
driver places the C voltage across the gate-source of the
B
3890fb
22
Linear Systems [ Linear Systems ]
