LTC3815_15 Datasheet, PDF (23/42 Pages) Linear Technology – 6A Monolithic Synchronous DC/DC Step-Down Converter with Digital Power System Management

LTC3815
Applications Information
When choosing the input and output ceramic capacitors,
choose the X5R and X7R dielectric formulations. These
dielectrics have the best temperature and voltage char-
acteristics of all the ceramics for a given value and size.
Since the ESR of a ceramic capacitor is so low, the input
and output capacitor must instead fulfill a charge storage
requirement. During a load step, the output capacitor must
instantaneously supply the current to support the load
until the feedback loop raises the switch current enough
to support the load. The time required for the feedback
loop to respond is dependent on the compensation and the
output capacitor size. Typically, 3 to 4 cycles are required
to respond to a load step, but only in the first cycle does
the output drop linearly. The output droop, V_DROOP, is
usually about 2 to 3 times the linear drop of the first cycle.
Thus, a good place to start with the output capacitor value
is approximately:
C_OUT
≈
2.5
f_O
ΔI_OUT
• V_DROOP
More capacitance may be required depending on the duty
cycle and load step requirements.
In most applications, the input capacitor is merely required
to supply high frequency bypassing, since the impedance to
the supply is very low. A 22µF ceramic capacitor is usually
enough for these conditions. Place this input capacitor as
close to the PV_IN pins as possible.
Checking Transient Response
The regulator loop response can be checked by looking at
the load current transient response. Switching regulators
take several cycles to respond to a step in load current.
When a load step occurs, V_OUT immediately shifts by an
amount equal to ∆I_LOAD • ESR, where ESR is the effective
series resistance of C_OUT. ∆I_LOAD also begins to charge or
discharge C_OUT generating a feedback error signal used by
the regulator to return V_OUT to its LTC3815 steady-state
value. During this recovery time, V_OUT can be monitored
for overshoot or ringing that would indicate a stability
problem.
The OPTI-LOOP compensation allows the transient
response to be optimized for a wide range of output
capacitors. The availability of the I_TH pin not only allows
optimization of the control loop behavior but also provides
a DC-coupled and AC-filtered closed-loop response test
point. The DC step, rise time and settling at this test point
truly reflects the closed-loop response. Assuming a pre-
dominantly second order system, phase margin and/or
damping factor can be estimated using the percentage of
overshoot seen at this pin.
The I_TH external components (R_C1, C_C1, C_C2) shown in
the Figure 8 circuit provides an adequate starting point
for most applications. The values can 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 and value have been
determined. The output capacitors need to be selected
because their various types and values determine the
loop feedback factor gain and phase. An output current
pulse of 20% to 100% of full load current having a rise
time of 1µs to 10µs will produce output voltage and I_TH
pin waveforms that will give a sense of the overall loop
stability without breaking the feedback loop.
In some applications, a more severe transient can be caused
by switching in loads with large (>10µF) input capacitors.
The discharged input capacitors are effectively put in paral-
lel with C_OUT, causing a rapid drop in V_OUT. No regulator
can deliver enough current to prevent this problem, if the
switch connecting the load has low resistance and is driven
quickly. The solution is to limit the turn-on speed of the
load switch driver. A Hot Swap™ controller is designed
specifically for this purpose and usually incorporates
current limiting, short-circuit protection and soft-starting.
Calculating Compensation Values
If the “trial and error” approach described in the previous
section doesn’t result in adequate transient performance,
the procedure described in this section can be used to
calculate more precise compensation component values
to achieve a desired bandwidth and phase margin. This
procedure is also helpful if the output capacitor type is
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3815p
23