LTC3709 Datasheet, PDF (16/24 Pages) Linear Technology – Fast 2-Phase, No RSENSE Synchronous DC/DC Controller with Tracking/Sequencing

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_CC enables discontinuous operation where the
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_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_LIMIT
=
⎛ V_SNS(MAX)
⎝⎜ ^RDS(ON) ^{• ρ}T
+
1
2
•
⎞
∆I_{L ⎠⎟}
•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 largest V_IN at the highest ambi-
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_LIMIT, which heats the
junction.
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.
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_SENSE value.
Minimum Off-Time and Dropout Operation
The minimum off-time t_OFF(MIN) is the smallest amount of
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_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 in order to maintain the
duty cycle at its limit. The minimum input voltage to avoid
dropout is:
V_IN(MIN)
=
V_OUT
1–
1
t_OFF(MIN)
•f
A plot of maximum duty cycle vs frequency is shown in
Figure 3.
3709f
16