ltc3869gn-2 Linear Technology Corporation, ltc3869gn-2 Datasheet - Page 25

• Manufacturer Part Number: ltc3869gn-2
• Description: Ltc3869/ltc3869-2 - Dual, 2-phase Synchronous Step-down Dc/dc Controllers
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
losses can be minimized by making sure that C
adequate charge storage and very low ESR at the switch-
ing frequency. A 25W supply will typically require a
minimum of 20µF to 40µF of capacitance having
a maximum of 20mΩ to 50mΩ of ESR. The LTC3869
2-phase architecture typically halves this input capacitance
requirement over competing solutions. Other losses
including Schottky conduction losses during dead time
and inductor core losses generally account for less than
2% total additional loss.
Modest improvements in Burst Mode efficiency may be
realized by using a smaller inductor value, a lower switch-
ing frequency or for DCR sensing applications, making the
DCR filter’s time constant smaller than the L/DCR time
constant for the inductor. A small Schottky diode with a
current rating equal to about 20% of the maximum load
current or less may yield minor improvements, too.
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 DC (resistive)
load current. When a load step occurs, V
amount equal to ∆I
series resistance of C
discharge C
forces the regulator to adapt to the current change and
return V
time V
ringing, which would indicate a stability problem. The
availability of the I
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 predominantly second
order system, phase margin and/or damping factor can be
estimated using the percentage of overshoot seen at this
pin. The bandwidth can also be estimated by examining the
rise time at the pin. The I
in the Typical Application circuit will provide an adequate
starting point for most applications.
OUT
OUT
can be monitored for excessive overshoot or
OUT
to its steady-state value. During this recovery
generating the feedback error signal that
TH
LOAD
OUT
pin not only allows optimization of
(ESR), where ESR is the effective
. ∆I
TH
LOAD
external components shown
also begins to charge or
OUT
shifts by an
IN
has
The I
loop compensation. 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 the various types and values determine 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 I
give a sense of the overall loop stability without break-
ing the feedback loop. Placing a power MOSFET directly
across the output capacitor and driving the gate with an
appropriate signal generator is a practical way to produce
a realistic load step condition. The initial output voltage
step resulting from the step change in 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 I
the feedback loop and is the filtered and compensated
control loop response. The gain of the loop will be in-
creased by increasing R
will be increased by decreasing C
the same factor that C
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.
A second, more severe transient is caused by switching
in loads with large (>1µF) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel
with C
alter its delivery of current quickly enough to prevent this
sudden step change in output voltage if the load switch
resistance is low and it is driven quickly. If the ratio of
C
should be controlled so that the load rise time is limited
to approximately 25 • C
require a 250µs rise time, limiting the charging current
to about 200mA.
LOAD
TH
OUT
to C
series R
, causing a rapid drop in V
OUT
is greater than 1:50, the switch rise time
LTC3869/LTC3869-2
C
-C
C
C
filter sets the dominant pole-zero
LOAD
is decreased, the zero frequency
C
and the bandwidth of the loop
. Thus a 10µF capacitor would
TH
TH
pin waveforms that will
C
. If R
pin signal which is in
OUT
. No regulator can
C
is increased by
25
3869f