LTC3878 Linear Technology Corporation, LTC3878 Datasheet - Page 15

• Manufacturer Part Number: LTC3878
• Description: Wide Operating Range No RSENSE Step-Down Controller
• Manufacturer: Linear Technology Corporation

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Fault Conditions: Current Limit and Foldback
The maximum inductor current is inherently limited in a
current mode controller by the maximum sense voltage.
In the LTC3878, the maximum sense voltage is controlled
by the voltage on the V
control, the maximum sense voltage and the sense re-
sistance determine the maximum allowed inductor valley
current. The corresponding output current limit is:
The current limit value should be checked to ensure that
I
be greater than the inductor current required to produce
maximum output power at the worst-case effi ciency.
Worst-case effi ciency typically occurs at the highest V
and highest ambient temperature. It is important to check
for consistency between the assumed MOSFET junction
temperatures and the resulting value of I
the MOSFET switches.
Caution should be used when setting the current limit based
on the R
limit is determined by the minimum MOSFET on-resistance.
Data sheets typically specify nominal and maximum values
for R
is that the minimum R
the typical value as the maximum lies above it. Consult the
MOSFET manufacturer for further guidelines.
To further limit current in the event of a short circuit to
ground, the LTC3878 includes foldback current limiting.
If the output fails by more than 50%, then the maximum
sense voltage is progressively lowered to about one-sixth
of its full value.
INTV
An internal P-channel low dropout regulator produces the
5.3V supply that powers the drivers and internal circuitry
within the LTC3878. The INTV
RMS and must be bypassed to ground with a minimum of
1μF low ESR tantalum or ceramic capacitor (10V, X5R or
X7R). Output capacitance greater than 10μF is discouraged.
Good bypassing is necessary to supply the high transient
currents required by the MOSFET gate drivers.
APPLICATIONS INFORMATION
LIMIT(MIN)
I
LIMIT
DS(ON)
CC
Regulator
DS(ON)
=
R
> I
but not a minimum. A reasonable assumption
V
DS ON
SNS MAX
OUT(MAX)
(
of the MOSFETs. The maximum current
(
)
•
ρ
DS(ON)
)
RNG
T
. The current limit value should
+
2
pin. With valley current mode
1
CC
lies the same amount below
•
Δ
pin can supply up to 50mA
I
L
LIMIT
which heats
IN
Applications using large MOSFETs with a high input voltage
and high frequency of operation may cause the LTC3878
to exceed its maximum junction temperature rating or
RMS current rating. In continuous mode operation, this
current is I
temperature can be estimated from the equations given
in Note 2 of the Electrical Characteristics. For example,
with a 30V input supply, the LTC3878 is limited to less
than 16.5mA:
Using the INTV
than 5mA is discouraged. INTV
the LTC3878 with minimal external loading. When using
the regulator to supply larger external loads, carefully
consider all operating load conditions. During load steps
and soft-start, transient current requirements signifi cantly
exceed the RMS values. Additional loading on INTV
away from the drive available to source gate charge during
high frequency transient load steps.
Soft-Start with the RUN/SS Pin
The RUN/SS pin both enables the LTC3878 and provides a
means of programmable current limited soft-start. Pulling
the RUN/SS pin below 0.7V puts the LTC3878 into a low
quiescent current shutdown (I
pin allows an internal 1.2μA current source to charge up
the external timing capacitor C
pulled all the way to ground, there is a delay before start-
ing. This delay is created by charging C
to 1.5V through a 1.2μA current source.
When the voltage on RUN/SS reaches 1.5V, the LTC3878
begins to switch. I
RUN/SS – 0.6V, and the device begins switching when
I
clamp on I
limit after an additional delay of 1.3s/μF . During this time,
the soft-start current limit is set to:
TH
T
I
t
exceeds 0.9V. As the RUN/SS voltage rises to 3V, the
LIMIT SS
J
DELAY
= 70°C + (16.5mA)(30)(110°C/W) = 125°C
(
=
TH
GATECHG
)
1 2
=
increases until it reaches the full-scale 2.4V
1 5
.
CC
I
.
LIMIT
μA
regulator to supply external loads greater
V
= f
TH
•
C
•
OP
SS
is clamped to be no greater than
(
RUN SS
(Q
=
g(TOP)
(
/
1 3
2 4
. /
.
Q
CC
SS
s μF C
– .
V
+ Q
< 15μA). Releasing the
. If RUN/SS has been
0 6
– .
is designed to supply
g(BOT)
0 8 8 V
)
V
SS
LTC3878
)
SS
– .
). The junction
0 8
from ground
V
CC
15
takes
3878f