10/13
Rev. B, Sep 2005
Applications Information (3)
Figure 8. Error Amplifier with Type 3 Compensation
Short-Circuit Improvements
The ML3440 is current limited to 2.7A peak to protect the
IC from damage. At input voltages above 4.5V a current
Limit condition may produce undesirable voltages to the IC
due to the series inductance of the package, as well as the
traces
and
external
components.
Following
the
recommendations for output voltage
> 4.3V and input
voltage > 4.5V will improve this condition. Addition short
-circuit protection can be accomplished with some external
circuitry.
In an overload or short-circuit condition, the ML3440
voltage loop opens and the error amp control voltage on
the Vc pin slams to the upper clamp level. This condition
forces boost mode operation in order to attempt to provide
more output voltage and the IC hits a peak switch current
limit of 2.7A. When switch current limit is reached switches
B and D turn on for the remainder of the cycle to reverse
the volts seconds on the inductor. Although this prevents
current run away, this condition produces four switch
operation producing a current foldback characteristic and
the average input current drops. The IC is trimmed to
guarantee greater than 1A average input current to meet
the maximum load demand, but in a short-circuit or
overload condition the foldback characteristic will occur
producing higher peak switch currents. To minimize this
effect during this condition the following circuits can be
utilized.
Restart Circuit
For a sustained short-circuit the circuit in figure 9 will force
a soft-start condition. The only design constraint is that
R2/C2 time constant must be longer than the soft-start
components R1/C1 to ensure start-up.
Figure 9
Soft-Start Reset Circuitry for a Sustained Short-Circuit
Simple Average Input Current Control
A simple average current limit circuit is show in figure 10.
Once the input current of the IC is above approximately 1A,
Q1 will start sourcing current into the FB pin and lower the
output voltage to maintain the average input current. Since
the voltage loop is utilized to perform average current limit,
the voltage control loop is maintained the Vc voltage does
not slam. The averaging function of current comes from the
fact that voltage loop compensation is also used with this
circuit.
Figure 10.
Simple Input Current Control Utilizing the Voltage
Loop