LTC3606B_15 Datasheet, PDF (8/20 Pages) Linear Technology – 800mA Synchronous Step-Down DC/DC with Average Input Current Limit

LTC3606B
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 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
representation of the inductor current, which can be
expressed in the following equation.
I_RLIM = I_OUT • D1 • K1
where D1 = V_OUT1/V_IN is the duty cycle.
K1 is the ratio R_DS(ON) (power PFET)/R_DS(ON)(sense PFET).
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 constant K1 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, the 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
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 LTC3606B 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 LTC3606B will start input current
limiting. Any current within this time must be considered
in each application to determine if it is tolerable.
3606bfb
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