LTC3714 Linear, LTC3714 Datasheet - Page 16

• Manufacturer Part Number: LTC3714
• Description: Wide Operating Range / Step-Down Controller with Internal Op Amp
• Manufacturer: Linear

APPLICATIO S I FOR ATIO
LTC3714
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 LTC3714, the maximum sense voltage is controlled by
the voltage on the V
the maximum sense voltage and the sense resistance
determine the maximum allowed inductor valley current.
The corresponding output current limit is:
The current limit value should be checked to ensure that
I
generally occurs with the lowest V
temperature. Note that it is important to check for self-
consistency between the assumed junction temperature
and the resulting value of I
switches.
Caution should be used when setting the current limit
based upon the R
current limit is determined by the minimum MOSFET on-
resistance. Data sheets typically specify nominal and
maximum values for R
reasonable assumption is that the minimum R
the same amount below the typical value as the maximum
lies above it. Consult the MOSFET manufacturer for further
guidelines.
16
LIMIT(MIN)
I
LIMIT
> I
*
V
R
OUT(MAX)
SNS MAX
DS ON T
(
(
DS(ON)
U
)
RNG
. The minimum value of current limit
)
DS(ON)
pin. With valley current control,
of the MOSFETs. The maximum
U
LIMIT
2
1
I
L
, but not a minimum. A
which heats the MOSFET
IN
W
at the highest ambient
U
DS(ON)
lies
To further limit current in the event of a short circuit to
ground, the LTC3714 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
time that the LTC3714 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
due to a dropping input voltage for example, then the
output will drop out of regulation. The minimum input
voltage to avoid dropout is:
Output Voltage Programming
The output voltage is digitally set to levels between 0.6V
and 1.75V using the voltage identification (VID) inputs
VID0-VID4. An internal 5-bit DAC configured as a preci-
sion resistive voltage divider sets the output voltage in
increments according to Table 1. The VID codes are
compatible with Intel Mobile Pentium
fications. Each VID input is pulled up by an internal 2.5 A
current source from the INTV
series diode to prevent damage from VID inputs that
exceed the supply.
*Use R
ON
V
/(t
IN MIN
SENSE
ON
(
value here if a sense resistor is connected between SENSE and PGND.
+ t
)
OFF(MIN)
V
OUT
). If the maximum duty cycle is reached,
t
ON
OFF(MIN)
t
t
ON
OFF MIN
(
CC
is the smallest amount of
)
supply and includes a
®
III processor speci-
3714f