Datasheet

14
LTC3778
3778f
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 LTC3778, the maximum sense voltage is controlled by
the voltage on the V
RNG
pin. With valley current control,
the maximum sense voltage and the sense resistance
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 minimum value of current limit
generally occurs with the lowest V
IN
at the highest ambient
temperature. Note that it is important to check for self-
consistency between the assumed MOSFET junction tem-
perature and the resulting value of I
LIMIT
which heats the
MOSFET switches.
Caution should be used when setting the current limit
based upon 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 LTC3778 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.
Minimum Off-time and Dropout Operation
The minimum off-time t
OFF(MIN)
is the smallest amount of
time that the LTC3778 is capable of turning on the bottom
MOSFET, tripping the current comparator and turning the
MOSFET back off. This time is generally about 250ns. The
minimum off-time limit imposes a maximum duty cycle of
t
ON
/(t
ON
+ t
OFF(MIN)
). If the maximum duty cycle is reached,
due to a dropping input voltage for example, then the
output will drop out of regulation. The minimum input
voltage to avoid dropout is:
VV
tt
t
IN MIN OUT
ON OFF MIN
ON
()
()
=
+
INTV
CC
Regulator
An internal P-channel low dropout regulator produces the
5V supply that powers the drivers and internal circuitry
within the LTC3778. The INTV
CC
pin can supply up to
50mA RMS and must be bypassed to ground with a
minimum of 4.7µF tantalum or other low ESR capacitor.
Good bypassing is necessary to supply the high transient
currents required by the MOSFET gate drivers. Applica-
tions using large MOSFETs with a high input voltage and
high frequency of operation may cause the LTC3778 to
exceed its maximum junction temperature rating or RMS
current rating. Most of the supply current drives the
MOSFET gates unless an external EXTV
CC
source is used.
In continuous mode operation, this current is I
GATECHG
=
f(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, the LTC3778EF is
limited to less than 15mA from a 30V supply:
T
J
= 70°C + (15mA)(30V)(110°C/W) = 120°C
For larger currents, consider using an external supply with
the DRV
CC
pin.
APPLICATIO S I FOR ATIO
WUUU
Figure 4. Secondary Output Loop and EXTV
CC
Connection
V
IN
LTC3778
SGND
FCB
EXTV
CC
TG
SW
OPTIONAL
EXTV
CC
CONNECTION
5V < V
OUT2
< 7V
R3
R4
3778 F04
T1
1:N
BG
PGND
+
C
SEC
1µF
V
OUT1
V
OUT2
V
IN
+
C
IN
1N4148
•
•
+
C
OUT
*Use R
SENSE
value here if a sense resistor is connected between SENSE
+
and SENSE
–
.