LTC3862 Datasheet, PDF (23/40 Pages) Linear Technology – Multi-Phase Current Mode Step-Up DC/DC Controller

LTC3862
OPERATION
The ITH series R_C • C_C filter sets the dominant pole-zero
loop compensation. The transfer function for boost and
flyback converters contains a right half plane zero that
normally requires the loop crossover frequency to be
reduced significantly in order to maintain good phase
margin. The R_C • C_C filter values can typically be modified
slightly (from 0.5 to 2 times their suggested values) to
optimize transient response once the final PC layout is done
and the particular output capacitor type(s) and value(s)
have been determined. The output capacitor configuration
needs to be selected in advance because the effective ESR
and bulk capacitance have a significant effect on the loop
gain and phase. An output current pulse of 20% to 80%
of full-load current having a rise time of 1μs to 10μs will
produce output voltage and ITH pin waveforms that will
give a sense of the overall loop stability without breaking
the feedback loop. Placing a power MOSFET and load
resistor directly across the output capacitor and driving
the gate with an appropriate signal generator is a practi-
cal way to produce a fast load step condition. The initial
output voltage step resulting from the step change in the
output current may not be within the bandwidth of the
feedback loop, so this signal cannot be used to determine
phase margin. This is why it is better to look at the ITH
pin signal which is in the feedback loop and is the filtered
and compensated control loop response. The gain of the
loop will be increased by increasing R_C and the bandwidth
of the loop will be increased by decreasing C_C. If R_C is
increased by the same factor that C_C is decreased, the
zero frequency will be kept the same, thereby keeping the
phase shift the same in the most critical frequency range
of the feedback loop. The output voltage settling behavior
is related to the stability of the closed-loop system and
will demonstrate the actual overall supply performance.
Figure 17 illustrates the load step response of a properly
compensated boost converter.
I_LOAD
5A/DIV
1A TO 5A
I_L1
5A/DIV
I_L2
5A/DIV
V_OUT
500mV/DIV
V_IN = 24V
V_OUT = 48V
500μs/DIV
3862 F17
Figure 17. Load Step Response of a Properly
Compensated Boost Converter
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