LTC3879
16
3879f
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 LTC3879, 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
Δ
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 effi ciency.
Worst-case effi ciency 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 LTC3879 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 LTC3879. 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 LTC3879
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 LTC3879 in the MSE16 is
limited to less than:
T
J
= 70°C + (46mA)(30)(40°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 LTC3879 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
CC
takes
away from the drive available to source gate charge during
high frequency transient load steps.
Soft-Start with the TRACK/SS Pin
The LTC3879 has the ability to either soft-start by itself
with a capacitor or track the output of another channel
or external supply. When the LTC3879 is confi gured to
soft-start by itself, a capacitor should be connected to the
TRACK/SS pin. The LTC3879 is in the shutdown state when
the RUN pin is below 0.7V. When RUN is greater than 0.7V
but less than 1.5V, all internal circuitry is enabled while
M
T
and M
B
are forced off. The TRACK/SS pin is actively
pulled to ground when RUN is less than 1.5V.
When the RUN pin voltage is greater than 1.5V, the LTC3879
powers up. When not tracking, a soft-start current of 1μA
is used to charge a soft-start capacitor placed on the
TRACK/SS pin. Note that soft-start or tracking is achieved
not by limiting the maximum output current of the control-
ler, but instead by controlling the output voltage according
to the ramp rate on the TRACK/SS pin. Current foldback
is disabled during start-up to ensure smooth soft-start or
tracking. The soft-start or tracking range is determined to be
the voltage range from 0V to 0.6V on the TRACK/SS pin.
t
C
μA
SOFT START
SS
−
= 06
1
.•
