LTC3811_15 Datasheet, PDF (36/48 Pages) Linear Technology – High Speed Dual, Multiphase Step-Down DC/DC Controller

LTC3811
APPLICATIONS INFORMATION
Measuring the Loop’s Transient Response
Once the compensation components have been chosen, the
AC performance of the power supply should be verified in
the lab. The two most common ways of checking the AC
response of the circuit are with load and line steps, and
by measuring the loop gain using a network analyzer or
Venable measurement system. Both of these measurement
techniques should be performed on the final design to en-
sure adequate correlation between the two, and to identify
and correct potential regions of marginal stability. These
measurements should be performed over all of the load,
line, temperature and components tolerance variations the
system will experience in a practical application.
Figure 25 illustrates a typical load step response for the
LTC3811. When a positive load step occurs, the output
voltage immediately drops by ΔI_LOAD • ESR, where ESR
is the equivalent series resistance of the output capacitor.
The increased load current then begins to discharge the
output capacitor, generating a feedback error signal that
forces the regulator to adapt to the current change and
return V_OUT to its steady-state regulated value. During
this recovery time V_OUT can be monitored for excessive
overshoot or ringing which would indicate a stability
problem. Assuming a second order system, the phase
margin and/or damping factor can be estimated using the
percentage overshoot seen at the output.
An output current pulse of 20% to 100% of full load having
a rise time of 0.1μs to 1μs will produce an output voltage
waveform that will give an indication of the loop stabil-
V_OUT
50mV/DIV
AC COUPLED
I_L
5A/DIV
V_IN = 12V
20μs/DIV
V_OUT = 1.5V
I_LOAD = 0.5A TO 8A
3811 G02
Figure 25. Load Step Response for the
LTC3811 Circuit in Figure 33
ity without having to break the feedback loop. Placing a
power MOSFET directly across the output capacitor and
driving the gate with an appropriate signal generator or
gate driver is a practical way to produce a realistic load
step condition.
Voltage Positioning for Single Output,
Multiphase Applications
The output voltage load line can be programmed with the
LTC3811 using one external resistor, allowing the user to
reduce the total output capacitance required for a given
error budget. The inductor current information is sensed
using the SENSE⁺ and SENSE^– inputs for both channels
and fed into a transconductance amplifier with two input
stages, as shown in Figure 26. The output current of the
transconductance amplifier, along with one external resis-
tor (R_AVP), allows the user to inject a load-current-related
error signal into the voltage feedback loop. Please note that
because the g_m amplifier mixes the signals from both chan-
nels, voltage positioning is only possible for multiphase,
single output applications; dual output applications with
voltage positioning are not possible.
The internal mixing of the current sense signals within
the voltage positioning amplifier, combined with the fact
that the g_m amplifier output signal is a current, allows the
user to connect the CSOUT pins of several LTC3811 chips
together in multiphase applications. The transconductance
(g_m) of the voltage positioning amplifier is 2.5mS/phase,
and the load slope is:
V_OUT
=
0.6
•
⎡⎣⎢₁₊
R2⎤
R1_⎦⎥
–
⎡⎣⎢_IOUT
•
R_SENSE
n
•
5m
•
R
AVP
⎤
⎦⎥
where n is the number of phases.
The input common mode range of the voltage position
g_m amplifier is 0.6V to 3.5V, comfortably allowing output
voltages up to 3.3V. In addition, the output voltage range
of the g_m amplifier for linear operation is limited to volt-
ages above 0.6V, due to the headroom requirements of
the NMOS sink transistors in the output stage. And finally,
the maximum differential input voltage for linear operation
is ±100mV.
3811f
36