LTC1775 LINER [Linear Technology], LTC1775 Datasheet - Page 11

• Manufacturer Part Number: LTC1775
• Description: High Power No RSENSE TM Current Mode Synchronous Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
A reasonable starting point is to choose a ripple current
that is about 40% of I
current occurs at the highest V
current does not exceed a specified maximum, the induc-
tor should be chosen according to:
Burst Mode Operation Considerations
The choice of R
the load current at which the LTC1775 enters Burst Mode
operation. When bursting, the controller clamps the peak
inductor current to approximately:
The corresponding average current depends on the amount
of ripple current. Lower inductor values (higher I
reduce the load current at which Burst Mode operation
begins.
The output voltage ripple can increase during Burst Mode
operation if I
primarily occur when the duty cycle is very close to unity
(V
chosen. This is generally only a concern in applications
with V
causes the inductor current to quickly descend to zero.
However, it takes multiple cycles to ramp the current back
up to I
tor must supply the load current and enough charge may
be lost to cause significant droop in the output voltage. It
is a good idea to keep I
Otherwise, one might need to increase the output capaci-
tance in order to reduce the voltage ripple or else disable
Burst Mode operation by forcing continuous operation
with the FCB pin.
Fault Conditions: Current Limit and Output Shorts
The LTC1775 current comparator can accommodate a
maximum sense voltage of 300mV. This voltage and the
IN
L
I
BURST PEAK
is close to V
BURST(PEAK)
OUT
( )(
f
(
V
I
OUT
L MAX
L
5V. At high duty cycles, a skipped cycle
)
(
DS(ON)
is substantially less than I
OUT
. During this interval, the output capaci-
R
U
60
)
DS ON
) or if very large value inductors are
)
O(MAX)
and inductor value also determines
mV
(
1
U
)
–
L
comparable to I
. Note that the largest ripple
V
IN MAX
IN
V
OUT
(
. To guarantee that ripple
W
)
BURST
BURST(PEAK)
U
. This will
L
) will
.
sense resistance determine the maximum allowed peak
inductor current. The corresponding output current limit
is:
The current limit value should be checked to ensure that
I
generally occurs with the largest V
ent temperature, conditions which cause the highest power
dissipation in the top MOSFET. Note that it is important to
check for self-consistency between the assumed junction
temperature of the top MOSFET and the resulting value of
I
Caution should be used when setting the current limit
based upon R
current limit is determined by the minimum MOSFET on-
resistance. Data sheets typically specify nominal and
maximum values for R
reasonable, but perhaps overly conservative, assumption
is that the minimum R
the typical value as the maximum R
Consult the MOSFET manufacturer for further guidelines.
The LTC1775 includes current foldback to help further
limit load current when the output is shorted to ground. If
the output falls by more than half, then the maximum
sense voltage is progressively lowered from 300mV to
about 80mV. Under short-circuit conditions with very low
duty cycle, the LTC1775 will begin skipping cycles in order
to limit the short-circuit current. In this situation the
bottom MOSFET R
valley rather than the top MOSFET controlling the inductor
current peak. The short-circuit ripple current is deter-
mined by the minimum on-time t
(approximately 0.5 s), the input voltage, and inductor
value:
The resulting short-circuit current is:
LIMIT(MIN)
LIMIT
I
I
SC
LIMIT
I
L(SC)
which heats the junction.
R
= t
> I
DS ON BOT
R
ON(MIN)
O(MAX)
(
DS ON
300
DS(ON)
80
(
)(
mV
DS(ON)
mV
)
. The minimum value of current limit
V
)
DS(ON)
IN
of the MOSFETs. The maximum
T
/L.
DS(ON)
will control the inductor current
T
–
lies the same amount below
2
1
, but not a minimum. A
2
1
I
ON(MIN)
L
IN
I
L SC
(
at the highest ambi-
DS(ON)
)
LTC1775
of the LTC1775
lies above it.
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