Datasheet

LTC3835-1
16
38351fc
APPLICATIONS INFORMATION
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the
maximum junction temperature rating for the LTC3835-1
to be exceeded. The INTV
CC
current, which is dominated
by the gate charge current, is supplied by the 5.25V V
IN
LDO. Power dissipation for the IC in this case is equal
to V
IN
• I
INTVCC
. The gate charge current is dependent
on operating frequency as discussed in the Effi ciency
Considerations section. The junction temperature can be
estimated by using the equations given in Note 2 of the
Electrical Characteristics. For example, the LTC3835-1
INTV
CC
current is limited to less than 25mA from a 24V
supply when in the GN package:
T
J
= 70°C + (25mA)(24V)(90°C/W) = 125°C
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked while
operating in continuous conduction mode (PLLIN/MODE
= INTV
CC
) at maximum V
IN
.
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
one of 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(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
The LTC3835-1 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 maxi-
mum sense voltage is progressively lowered from 100mV
to 30mV. Under short-circuit conditions with very low duty
cycles, the LTC3835-1 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 deter-
mined by the minimum on-time t
ON(MIN)
of the LTC3835-1
(≈180ns), 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
=
10 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 controller is operating.
A comparator monitors the output for overvoltage condi-
tions. 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 OV 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.