LTC1735 Linear Technology, LTC1735 Datasheet - Page 18

• Manufacturer Part Number: LTC1735
• Description: High Efficiency Synchronous Step-Down Switching Regulator
• Manufacturer: Linear Technology

LTC1735
APPLICATIO S I FOR ATIO
t
voltage and inductor value:
The resulting short-circuit current is:
The current foldback function is always active and is not
effected by the current latchoff function.
Fault Conditions: Output Overvoltage Protection
(Crowbar)
The output overvoltage crowbar is designed to blow a
system fuse in the input lead when the output of the
regulator rises much higher than nominal levels. This
condition causes huge currents to flow, much greater than
in normal operation. This feature is designed to protect
against a shorted top MOSFET; it does not protect against
a failure of the controller itself.
The comparator (OV in the Functional Diagram) detects
overvoltage faults greater than 7.5% above the nominal
output voltage. When this condition is sensed the top
MOSFET is turned off and the bottom MOSFET is forced
on. The bottom MOSFET remains on continuously for as
long as the 0V condition persists; if V
level, normal operation automatically resumes.
Note that dynamically changing the output voltage may
cause overvoltage protection to be momentarily activated
during programmed output voltage decreases. This will
not cause permanent latchoff nor will it disrupt the desired
voltage change. With soft-latch overvoltage protection,
dynamically changing the output voltage is allowed and
the overvoltage protection tracks the newly programmed
output voltage, always protecting the load.
Minimum On-Time Considerations
Minimum on-time t
that the LTC1735 is capable of turning the top MOSFET on
and off again. It is determined by internal timing delays and
18
ON(MIN)
I
SC
I
L(SC)
of the LTC1735 (approximately 200ns), the input
R
30
= t
SENSE
mV
ON(MIN)
ON(MIN)
U
2
1
V
IN
I
/L
L SC
(
U
is the smallest amount of time
)
W
OUT
returns to a safe
U
the gate charge required to turn on the top MOSFET. Low
duty cycle applications may approach this minimum on-
time limit and care should be taken to ensure that:
If the duty cycle falls below what can be accommodated by
the minimum on-time, the LTC1735 will begin to skip
cycles. The output voltage will continue to be regulated,
but the ripple current and voltage will increase.
The minimum on-time for the LTC1735 in a properly
configured application is generally less than 200ns. How-
ever, as the peak sense voltage decreases, the minimum
on-time gradually increases as shown in Figure 7. This is
of particular concern in forced continuous applications
with low ripple current at light loads. If the duty cycle drops
below the minimum on-time limit in this situation, a
significant amount of cycle skipping can occur with corre-
spondingly larger current and voltage ripple.
If an application can operate close to the minimum on-
time limit, an inductor must be chosen that is low enough
to provide sufficient ripple amplitude to meet the mini-
mum on-time requirement. As a general rule keep the
inductor ripple current equal or greater than 30% of
I
OUT(MAX)
t
ON MIN
(
at V
250
200
150
100
50
)
0
0
Figure 7. Minimum On-Time vs I
IN(MAX)
V f
V
IN
OUT
( )
10
.
I
L
/I
OUT(MAX)
20
(%)
30
1735 F07
40
L