LTC3786 LINER [Linear Technology], LTC3786 Datasheet - Page 24

• Manufacturer Part Number: LTC3786
• Description: Multi-Phase Current Mode Step-Up DC/DC Controller
• Manufacturer: LINER [Linear Technology]

LTC3862-2
The ITH series R
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
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
operaTion
24
C
• C
C
C
• C
filter values can typically be modified
C
filter sets the dominant pole-zero
500mA TO 1A
1A/DIV
2A/DIV
2A/DIV
1V/DIV
I
V
LOAD
OUT
I
I
L1
L2
Figure 17. Load Step Response of a Properly
Compensated Boost Converter
V
V
IN
OUT
= 24V
= 72V
400µs/DIV
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
of the loop will be increased by decreasing C
increased by the same factor that C
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.
38622 F17
C
C
and the bandwidth
is decreased, the
C
. If R
38622f
C
is