LTC3811_15 Datasheet, PDF (22/48 Pages) Linear Technology – High Speed Dual, Multiphase Step-Down DC/DC Controller

LTC3811
OPERATION (Refer to the Functional Diagram)
PGOOD outputs for the unused error amplifiers should be
left open. Only the PGOOD output for the master control
error amplifier should be connected to the fault monitor.
Fault Conditions: Current Limit and Foldback
One of the main advantages of the LTC3811 is the fact that
the maximum inductor current is inherently limited due
to the use of peak current mode control. The maximum
sense voltage is controlled by the voltage on the RNG pins
and the maximum DC output current is:
I_LIMIT
=
V_{SENSE(MAX )}
R_SENSE
–
1
2
•
ΔI_L
The current limit value should be checked to ensure that
I_LIMIT(MIN) > I_OUT(MAX). The minimum value of the current
limit generally occurs with the largest V_IN at the highest
ambient temperature, conditions that cause the largest
power losses in the converter.
To further limit current in the event of an output short-circuit
to ground, the LTC3811 includes foldback current limiting.
When the FB pin falls below 0.3V (50% of its nominally
regulated value), the foldback circuit is activated, progres-
sively lowering the peak current limit in proportion to the
severity of the overcurrent or short circuit condition. If the
FB pin reaches 0V, the peak current sense threshold will
be reduced to 30% of its maximum value. The foldback
current limit transfer function is shown in Figure 8.
60
55
50
45
40
35
30
25
20
15
10
RNG = INTV_CC
5
0
0 100 200 300 400 500 600 700
FB VOLTAGE (mV)
3811 G31
Figure 8. Current Foldback Charactistic
Under short-circuit conditions with very low duty cycles,
the LTC3811 may begin to skip pulses in order to limit the
maximum current. In this situation the bottom MOSFET
will be dissipating most of the power; however this will
be less than in normal operation at maximum load. In this
case the short circuit ripple current is determined by the
minimum on-time, t_ON(MIN), of the LTC3811 (about 65ns),
the input voltage, and the inductor value, according to the
following equation:
ΔI_L(SC)
=
t_ON(MIN)
•
V_IN
L
The resulting short-circuit current is
I_SC
=
0.3 • V_SENSE(MAX)
R_SENSE
–
1
2
•
ΔI_L(SC)
Depending upon the ratio of the DC value of the current
limit to the maximum load current and the percentage
ripple current in the inductor, it is possible that the con-
verter will operate in discontinuous mode when V_FB = 0V
(a so-called “dead short”). In this case, the short-circuit
current of the converter will be:
I_SC
=
ΔI_L(SC)
2
=
t_ON(MIN)
•
V_IN
2•L
In order for the converter to start up properly with a non-
linear load, the foldback current limiting circuit in the
LTC3811 is disabled during the initial soft-start interval.
When the FB pin voltage reaches 0.54V, the soft-start in-
terval is terminated and the foldback circuit is enabled.
In the event the converter is turned on into a shorted load,
the foldback circuit will be disabled until the SS/TRACK
pin reaches 0.54V. This ensures that the converter will still
limit the maximum current to a safe level and reduce the
peak power dissipated with a shorted load.
3811f
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