LTC3835 Linear Technology, LTC3835 Datasheet - Page 18

• Manufacturer Part Number: LTC3835
• Description: Low IQ Synchronous Step-Down Controller
• Manufacturer: Linear Technology

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LTC3835
APPLICATIONS INFORMATION
4. EXTV
Topside MOSFET Driver Supply (C
External bootstrap capacitors C
pins supply the gate drive voltages for the topside MOSFET.
Capacitor C
though external diode D
is low. When the topside MOSFET is to be turned on,
the driver places the C
of the desired MOSFET. This enhances the MOSFET and
turns on the topside switch. The switch node voltage, SW,
rises to V
MOSFET on, the boost voltage is above the input supply:
V
C
of the topside MOSFET. The reverse breakdown of the
external Schottky diode must be greater than V
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.
18
BOOST
B
For 3.3V and other low voltage regulators, effi ciency
gains can still be realized by connecting EXTV
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.
needs to be 100 times that of the total input capacitance
EXTV
CC
= V
CC
LTC3835
Figure 6. Capacitive Charge Pump for EXTV
IN
Connected to an Output-Derived Boost Network.
IN
and the BOOST pin follows. With the topside
B
+ V
PGND
BG1
TG1
in the Functional Diagram is charged
V
SW
C
IN
IN
INTVCC
+
N-CH
N-CH
V
IN
B
. The value of the boost capacitor
B
voltage across the gate-source
from INTV
BAT85
L1
B
connected to the BOOST
VN2222LL
B
, D
CC
R
SENSE
0.22μF
B
when the SW pin
)
+
CC
CC
3835 F06
BAT85
BAT85
IN(MAX)
1μF
C
V
OUT
OUT
to an
.
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
LTC3835 (≈180ns), the input voltage and inductor value:
The resulting short-circuit current is:
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
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
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.
ΔI
I
SC
L(SC)
=
R
10
= t
SENSE
OUT
mV
ON(MIN)
returns to a safe level, normal operation
–
2
1
(V
Δ
IN
I
L SC
/L)
(
)
ON(MIN)
of the
3835fc