LTC3619B_15 Datasheet, PDF (9/20 Pages) Linear Technology – 400mA/800mA Synchronous Step-Down DC/DC with Average Input Current Limit

LTC3619B
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
91.6k
110k
137.5k
I_DC
600mA
500mA
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.
3619bfb
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