Datasheet

LTC3891
21
3891fa
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 8. Ensure that EXTV
CC
< V
IN
.
APPLICATIONS INFORMATION
Topside MOSFET Driver Supply (C
B
, D
B
)
An external bootstrap capacitor, C
B
, connected to the
BOOST pin supplies the gate drive voltage for the topside
MOSFET. Capacitor C
B
in the Functional Diagram is charged
though external diode D
B
from INTV
CC
when the SW pin
is low. When the topside MOSFET is to be turned on, the
driver places the C
B
voltage across the gate-source of
the 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 top-
side MOSFET(s). The reverse breakdown of the external
Schottky diode must be greater than V
IN(MAX)
.
When adjusting the gate drive level, the final arbiter is the
total input current for the regulator. If a change is made
and the input current decreases, then the efficiency has
improved. If there is no change in input current, then there
is no change in efficiency.
EXTV
CC
V
IN
TG
SW
BG
PGND
LTC3891
R
SENSE
V
OUT
NDS7002
C
OUT
3891 F08
MBOT
MTOP
C
IN
L
BAT85 BAT85
BAT85
Figure 8. Capacitive Charge Pump for EXTV
CC
Fault Conditions: Current Limit and Current Foldback
The LTC3891 includes current foldback to help limit load
current when the output is shorted to ground. If the output
voltage falls below 70% of its nominal output level, then
the maximum sense voltage is progressively lowered from
100% to 45% of its maximum selected value. Under short-
circuit conditions with very low duty cycles, the LTC3891
will begin cycle skipping in order to limit the short-circuit
current. In this situation the bottom MOSFET will be dis-
sipating most of the power but less than in normal opera-
tion. The short-circuit ripple current is determined by the
minimum on-time, t
ON(MIN)
, of the LTC3891 (≈95ns), the
input voltage and inductor value:
ΔI
L(SC)
= t
ON(MIN)
V
IN
L






The resulting average short-circuit current is:
I
SC
= 45% • I
LIM(MAX)
–
1
2
DI
L(SC)
Fault Conditions: Overvoltage Protection (Crowbar)
The overvoltage crowbar is designed to blow a system
input fuse when the output voltage of the regulator rises
much higher than nominal levels. The crowbar causes huge
currents to flow, that blow the fuse to protect against a
shorted top MOSFET if the short occurs while the control-
ler is operating.
A comparator monitors the output for overvoltage condi-
tions. The comparator detects faults greater than 10%
above the nominal output voltage. When this condition
is sensed, the top MOSFET is turned off and the bottom
MOSFET is turned on until the overvoltage condition is
cleared. The bottom MOSFET remains on continuously
for as long as the overvoltage condition persists; if V
OUT
returns to a safe level, normal operation automatically
resumes.
A shorted top MOSFET will result in a high current condition
which will open the system fuse. The switching regulator
will regulate properly with a leaky top MOSFET by altering
the duty cycle to accommodate the leakage.