LTC3867
30
3867f
APPLICATIONS INFORMATION
4. Transition losses apply only to the topside MOSFET(s),
and become significant only when operating at high
input voltages (typically 15V or greater). Transition
losses can be estimated from:
Transition Loss = (1.7) V
IN
2
• I
O(MAX)
• C
RSS
• f
Other hidden losses such as copper trace and internal
battery resistances can account for an additional 5%
to 10% efficiency degradation in portable systems. It
is very important to include these system level losses
during the design phase. The internal battery and fuse
resistance losses can be minimized by making sure that
C
IN
has adequate charge storage and very low ESR at
the switching frequency. A 25W supply will typically
require a minimum of 20µF to 40µF of capacitance
having a maximum of 20mΩ to 50mΩ of ESR. Other
losses including Schottky conduction losses during
dead time and inductor core losses generally account
for less than 2% total additional loss.
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
OUT
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 the feedback error signal that
forces the regulator to adapt to the current change and
return V
OUT
to its steady-state value. During this recovery
time V
OUT
can be monitored for excessive overshoot or
ringing, which would indicate a stability problem. The
availability of the I
TH
pin not only allows optimization of
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
TH
external components shown
in the Typical Application circuit will provide an adequate
starting point for most applications. The I
TH
series R
C
-C
C
filter sets the dominant pole-zero 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
TH
pin waveforms that will give a sense of the overall loop
stability without breaking 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
TH
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.
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
OUT
, causing a rapid drop in V
OUT
. No regulator can
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
LOAD
to C
OUT
is greater than 1:50, the switch rise time
should be controlled so that the load rise time is limited
to approximately 25 • C
LOAD
. Thus a 10µF capacitor would
require a 250µs rise time, limiting the charging current
to about 200mA.
