LTC3619
10
3619fa
operaTion
When the output is loaded heavily, for example, with
millifarad of capacitance, it may take longer than 950µs
to charge the output to regulation. If the output is still low
after the soft-start time, the LTC3619 will try to quickly
charge the output capacitor. In this case, the input current
limit (after it engages) can prevent excessive amount of
instantaneous current that is required to quickly charge
the output. See the Channel 2 Start-Up from Shutdown
curve in the Typical Performance Characteristics section.
After input current limit is engaged, the output slowly
ramps up to regulation while limited by its 500mA of
input current.
Short-Circuit Protection
When either regulator output is shorted to ground, the
corresponding internal N-channel switch is forced on for
a longer time period for each cycle in order to allow the
inductor to discharge, thus preventing inductor current
runaway. This technique has the effect of decreasing
switching frequency. Once the short is removed, normal
operation resumes and the regulator output will return to
its nominal voltage.
Input Current Limit
Internal current sense circuitry in each channel measures
the inductor current through the voltage drop across the
power PFET switch and forces the
same voltage across
the small sense PFET. The voltage across the small sense
PFET generates a current representing 1/55,000th of the
inductor current during the on-cycle. The current out of
RLIM pin is the summed representation of the inductor
currents from both channels, which can be expressed in
the following equation.
I
RLIM
= I
OUT1
• D1 • K1 + I
OUT2
• D2 • K2,
where D1 = V
OUT1
/V
IN
and D2 = V
OUT2
/V
IN
are the duty
cycle of channel 1 and 2, respectively.
K1 is the ratio R
DS(ON)
(power PFET)/R
DS(ON)
(sense PFET)
of channel 1, and K2 is the ratio R
DS(ON)
(power PFET)/
R
DS(ON)
(sense PFET) of channel 2. The ratio of the power
PFET to the sense PFET is trimmed to within 2%.
Given that both PFETs are carefully laid out and matched,
their temperature and voltage coefficient effects will be
similar and their terms be canceled out in the equation. In
that case, the constants K1 and K2 will only be dependent
on area scaling, which is trimmed to within 2%. Thus, the
I
RLIM
current will track the input current very well over
varying temperature and V
IN
.
The RLIM pin can be grounded to disable input
current
limit function.
Programming Input Current Limit
Selection of one external R
LIM
resistor will program the
input current limit. The current limit can be programmed
from 200mA up to I
PEAK
current. As the input current
increases, R
LIM
voltage will follow. When R
LIM
reaches
the internal comparator threshold of 1V, channel 2’s
power PFET on-time will be shortened, thereby, limiting
the input current.
Use the following equation to select the R
LIM
resistance
that corresponds to the input current limit.
R
LIM
= 55k / I
DC
I
DC
is the input current (at V
IN
) to be limited. The following
are some R
LIM
values with the corresponding current limit.
R
LIM
I
DC
91.6k 600mA
110k 500mA
137.5k 400mA
Selection of C
LIM
Capacitance
Since I
RLIM
current is a function of the inductor current,
its dependency on the duty cycle cannot be ignored. Thus,
a C
LIM
capacitor is needed to integrate the I
RLIM
current
and smooth out transient currents. The LTC3619 is stable
with any size capacitance >100pF at the RLIM pin.
Each application input current limit will call for different
C
LIM
value to optimize its response time. Using a large C
LIM
capacitor requires longer time for the RLIM pin voltage to
charge. For example, consider the application 500mA input
current limit, 5V input and 1A, 2.5V output with a 50% duty