Datasheet

11
LTC1775
A reasonable starting point is to choose a ripple current
that is about 40% of I
O(MAX)
. Note that the largest ripple
current occurs at the highest V
IN
. To guarantee that ripple
current does not exceed a specified maximum, the induc-
tor should be chosen according to:
L
V
fI
V
V
OUT
L MAX
OUT
IN MAX
≥












()( )
–
() ()
∆
1
Burst Mode Operation Considerations
The choice of R
DS(ON)
and inductor value also determines
the load current at which the LTC1775 enters Burst Mode
operation. When bursting, the controller clamps the peak
inductor current to approximately:
I
mV
R
BURST PEAK
DS ON
()
()
=
60
The corresponding average current depends on the amount
of ripple current. Lower inductor values (higher ∆I
L
) will
reduce the load current at which Burst Mode operation
begins.
The output voltage ripple can increase during Burst Mode
operation if ∆I
L
is substantially less than I
BURST
. This will
primarily occur when the duty cycle is very close to unity
(V
IN
is close to V
OUT
) or if very large value inductors are
chosen. This is generally only a concern in applications
with V
OUT
≥ 5V. At high duty cycles, a skipped cycle
causes the inductor current to quickly descend to zero.
However, it takes multiple cycles to ramp the current back
up to I
BURST(PEAK)
. During this interval, the output capaci-
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
L
comparable to I
BURST(PEAK)
.
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
sense resistance determine the maximum allowed peak
inductor current. The corresponding output current limit
is:
I
mV
R
I
LIMIT
DS ON T
L
=
()
()
300 1
2
()
–
ρ
∆
The current limit value should be checked to ensure that
I
LIMIT(MIN)
> I
O(MAX)
. The minimum value of current limit
generally occurs with the largest V
IN
at the highest ambi-
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
LIMIT
which heats the junction.
Caution should be used when setting the current limit
based upon R
DS(ON)
of the MOSFETs. The maximum
current limit is determined by the minimum MOSFET on-
resistance. Data sheets typically specify nominal and
maximum values for R
DS(ON)
, but not a minimum. A
reasonable, but perhaps overly conservative, assumption
is that the minimum R
DS(ON)
lies the same amount below
the typical value as the maximum R
DS(ON)
lies above it.
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
DS(ON)
will control the inductor current
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
ON(MIN)
of the LTC1775
(approximately 0.5µs), the input voltage, and inductor
value:
∆I
L(SC)
= t
ON(MIN)
V
IN
/L.
The resulting short-circuit current is:
I
mV
R
I
SC
DS ON BOT T
LSC
=
()
()
+
80 1
2
()( )
()
ρ
∆
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