Datasheet

LTC3835
18
3835fd
APPLICATIONS INFORMATION
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 6.
Fault Conditions: Current Limit and Current Foldback
The LTC3835 includes current foldback to help limit load
current when the output is shorted to ground. If the output
falls below 70% of its nominal output level, then the
maximum sense voltage is progressively lowered from
100mV to 30mV. Under short-circuit conditions with very
low duty cycles, the LTC3835 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 cur-
rent is determined by the minimum on-time t
ON(MIN)
of the
LTC3835 (≈180ns), the input voltage and inductor value:
∆I
L(SC)
= t
ON(MIN)
(V
IN
/L)
The resulting short-circuit current is:
I
SC
=
10mV
R
SENSE
–
1
2
∆I
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 controller
is operating.
A comparator monitors the output for overvoltage
conditions. The comparator (OV) detects overvoltage 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.
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 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 desired MOSFET. This enhances the MOSFET and
turns on the topside switch. 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. 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.
Figure 6. Capacitive Charge Pump for EXTV
CC
EXTV
CC
V
IN
TG1
SW
BG1
PGND
LTC3835
R
SENSE
V
OUT
VN2222LL
+
C
OUT
3835 F06
N-CH
N-CH
+
C
IN
1µF
V
IN
L1
BAT85 BAT85
BAT85
0.22µF