Datasheet

LTC3878
15
3878fa
applicaTions inForMaTion
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
RNG
pin. With valley current mode
control, the maximum sense voltage and the sense re-
sistance determine
the maximum allowed inductor valley
current. The corresponding output current limit is:
I
V
R
I
LIMIT
SNS MAX
DS ON T
L
= +
( )
( )
•
•
ρ
1
2
∆
The current limit value should be checked to ensure that
I
LIMIT(MIN)
> I
OUT(MAX)
. The current limit value should
be greater than the inductor current required to produce
maximum output power at the worst-case efficiency.
Worst-case efficiency typically occurs at the highest V
IN
and highest ambient temperature. It is important to check
for consistency between the assumed MOSFET junction
temperatures and the resulting value of I
LIMIT
which heats
the MOSFET switches.
Caution should be used when setting the current limit based
on the 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 assumption
is that the minimum R
DS(ON)
lies the same amount below
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 falls by more than 50%, then the maximum
sense voltage is progressively lowered to about one-sixth
of its full value.
INTV
CC
Regulator
An internal P-channel low dropout regulator produces the
5.3V supply that powers the drivers and internal circuitry
within the LTC3878. The INTV
CC
pin can supply up to 50mA
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 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
GATECHG
= f
OP
(Q
g(TOP)
+ Q
g(BOT)
). The junction
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:
T
J
= 70°C + (16.5mA)(30)(110°C/W) = 125°C
Using the INTV
CC
regulator to supply external loads greater
than 5mA is discouraged. INTV
CC
is designed to supply
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 significantly
exceed the RMS values. Additional loading on INTV
CC
takes
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
Q
< 15µA). Releasing the
pin allows an internal 1.2µA current source to charge up
the external timing capacitor C
SS
. If RUN/SS has been
pulled all the way to ground, there is a delay before start-
ing. This
delay is created by charging C
SS
from ground
to 1.5V through a 1.2µA current source.
t
V
µA
C s µF C
DELAY SS SS
= =
( )
1 5
1 2
1 3
.
.
• . /
When the voltage on RUN/SS reaches 1.5V, the LTC3878
begins to switch. I
TH
is clamped to be no greater than
RUN/SS – 0.6V, and the device begins switching when
I
TH
exceeds 0.9V. As the RUN/SS voltage rises to 3V, the
clamp on I
TH
increases until it reaches the full-scale 2.4V
limit after an additional delay of 1.3s/µF. During this time,
the soft-start current limit is set to:
I I
RUN SS V V
V
LIMIT SS LIMIT( )
•
/ – . – .
. – .
=
( )
0 6 0 8
2 4 0
88V