Datasheet

LTC3868-1
22
38681fd
APPLICATIONS INFORMATION
The following list summarizes the four possible connec-
tions for EXTV
CC
:
1. EXTV
CC
Left Open (or Grounded). This will cause INTV
CC
to be powered from the internal 5.1V regulator result-
ing in an effi ciency 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 effi ciency.
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
. Ensure that EXTV
CC
< V
IN
.
4. EXTV
CC
Connected to an Output-Derived Boost Network.
For 3.3V and other low voltage regulators, effi ciency
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
.
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)
.
When adjusting the gate drive level, the fi nal arbiter is the
total input current for the regulator. If a change is made
and the input current decreases, then the effi ciency has
improved. If there is no change in input current, then there
is no change in effi ciency.
Fault Conditions: Current Limit and Current Foldback
When the output current hits the current limit, the output
voltage begins to drop. If the output voltage falls below
70% of its nominal output level, then the maximum sense
voltage is progressively lowered to about one-half of its
maximum selected value. Under short-circuit conditions
with very low duty cycles, the LTC3868-1 will begin cycle
skipping in order to limit the short-circuit current. In this
situation the bottom MOSFET will be dissipating most of
the power but less than in normal operation. The short-
circuit ripple current is determined by the minimum on-
time, t
ON(MIN)
, of the LTC3868-1 (≈90ns), the input voltage
and inductor value:
ΔI
L(SC)
= t
ON(MIN)
V
IN
L
⎛
⎝
⎜
⎞
⎠
⎟
The resulting average short-circuit current is:
I
SC
=
50% •I
LIM(MAX)
R
SENSE
–
1
2
ΔI
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 fl ow, 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%
Figure 8. Capacitive Charge Pump for EXTV
CC
EXTV
CC
V
IN
TG1
SW
BG1
PGND
1/2 LTC3868-1
R
SENSE
V
OUT
VN2222LL
C
OUT
38681 F08
MBOT
MTOP
C
IN
L
D
BAT85 BAT85
BAT85
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
desired MOSFET. This enhances the top MOSFET switch
and turns it on. The switch node voltage, SW, rises to V
IN