Datasheet

LTC3834
18
3834fb
APPLICATIONS INFORMATION
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 de-
sired 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 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
The LTC3834 includes current foldback to help limit load
current when the output is shorted to ground. If the out-
put 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 LTC3834 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 LTC3834 (≈200ns), the input voltage and inductor
value:
ΔI
L(SC)
= t
ON(MIN)
(V
IN
/L)
The resulting short-circuit current is:
I
mV
R
I
SC
SENSE
LSC
=
30 1
2
–
()
Δ
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 con-
ditions. 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 accom-
modate the leakage.
EXTV
CC
V
IN
TG1
BG1
PGND
LTC3834
R
SENSE
V
OUT
VN2222LL
+
C
OUT
3834 F06
N-CH
N-CH
+
C
IN
1μF
V
IN
L1
BAT85 BAT85
BAT85
0.22μF
SW
Figure 6. Capacitive Charge Pump for EXTV
CC