Datasheet

LTC3619B
9
3619bfb
operaTion
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
=
55kΩ − A
I
DC
(A)
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 LTC3619B 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 cycle. When an instantaneous 1A output pulse is
applied, the current out of the RLIM pin becomes 1A/55k
= 18.2µA during the 50% on-time or 9.1µA full duty cycle.
With a C
LIM
capacitor of 1µF, R
LIM
of 116k, and using I =
CdV/dt, it will take 110ms for C
LIM
to charge from 0V to
1V. This is the time after which the LTC3619B will start
input current limiting. Any current within this time must be
considered in each application to determine if it is
tolerable.