LTC3709 Linear Technology, LTC3709 Datasheet - Page 16

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LTC3709

Manufacturer Part Number
LTC3709
Description
No RSENSE Synchronous DC/DC Controller
Manufacturer
Linear Technology
Datasheet

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LTC3709
APPLICATIO S I FOR ATIO
needs to store about 100 times the gate charge required by
the top MOSFET. In most applications 0.1µF to 0.47µF is
adequate.
Discontinuous Mode Operation and FCB Pin
The FCB pin determines whether the bottom MOSFET
remains on when current reverses in the inductor. Tying
this pin to V
bottom MOSFET turns off when inductor current reverses.
The load current at which inductor current reverses and
discontinuous operation begins depends on the amplitude
of the inductor ripple current. The ripple current depends
on the choice of inductor value and operating frequency as
well as the input and output voltages.
Tying the FCB pin to ground forces continuous synchro-
nous operation, allowing current to reverse at light loads
and maintaining high frequency operation.
Besides providing a logic input to force continuous opera-
tion, the FCB pin acts as the input for external clock syn-
chronization. Upon detecting a TTL level clock and the fre-
quency is higher than the minimum allowable, channel 1
will lock on to this external clock. This will be followed by
channel 2 (see PLL and Frequency Synchronization). The
LTC3709 will be forced to operate in forced continuous
mode in this situation.
Fault Conditions: Current Limit
The maximum inductor current is inherently limited in a
current mode controller by the maximum sense voltage. In
the LTC3709, 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
16
LIMIT(MIN)
I
LIMIT
=
> I
CC
R
OUT(MAX)
V
enables discontinuous operation where the
DS ON
SNS MAX
(
(
U
)
RNG
. The minimum value of current limit
ρ
)
T
pin. With valley current control,
U
+
2
1
I
L
W
2
U
generally occurs with the largest V
ent temperature, conditions which cause the largest power
loss in the converter. Note that it is important to check for
self-consistency between the assumed junction tempera-
ture and the resulting value of I
junction.
Caution should be used when setting the current limit based
upon 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.
For a more accurate current limiting, a sense resistor can
be used. Sense resistors in the 1W power range are easily
available with 5%, 2% or 1% tolerance. The temperature
coefficient of these resistors are very low, ranging from
±250ppm/°C to ±75ppm/°C. In this case, the denomina-
tor in the above equation can simply be replaced by the
R
Minimum Off-Time and Dropout Operation
The minimum off-time t
time that the LTC3709 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 in order to maintain the
duty cycle at its limit. The minimum input voltage to avoid
dropout is:
A plot of maximum duty cycle vs frequency is shown in
Figure 3.
ON
SENSE
V
/(t
IN MIN
DS(ON)
ON
(
value.
+ t
)
OFF(MIN)
DS(ON)
, but not a minimum. A reasonable assumption
=
V
OUT
of the MOSFETs. The maximum current
). If the maximum duty cycle is reached,
1
DS(ON)
t
OFF(MIN)
OFF MIN
1
(
lies the same amount below
)
is the smallest amount of
LIMIT
f
IN
at the highest ambi-
, which heats the
3709f

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