LTC3835 Linear Technology, LTC3835 Datasheet - Page 21

• Manufacturer Part Number: LTC3835
• Description: Low IQ Synchronous Step-Down Controller
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
4. Transition losses apply only to the topside MOSFET, and
Other “hidden” losses such as copper trace and internal
battery resistances can account for an additional 5% to
10% effi ciency 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
the switching frequency. A 25W supply will typically
require a minimum of 20μF to 40μF of capacitance hav-
ing 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
an amount equal to ΔI
fective series resistance of C
charge or discharge C
signal that forces the regulator to adapt to the current
change and return V
this recovery time V
overshoot or ringing, which would indicate a stability
IN
a 5V output, or a 4% to 20% loss for a 3.3V output.
Effi ciency varies as the inverse square of V
same external components and output power level. The
combined effects of increasingly lower output voltages
and higher currents required by high performance digital
systems is not doubling but quadrupling the importance
of loss terms in the switching regulator system!
become signifi cant only when operating at high input
voltages (typically 15V or greater). Transition losses
can be estimated from:
has adequate charge storage and very low ESR at
Transition Loss = (1.7) V
OUT
OUT
OUT
LOAD
to its steady-state value. During
can be monitored for excessive
generating the feedback error
(ESR), where ESR is the ef-
OUT
IN
. ΔI
2 I
LOAD
O(MAX)
also begins to
OUT
C
OUT
RSS
shifts by
for the
f
problem. OPTI-LOOP compensation allows the transient
response to be optimized over a wide range of output
capacitance and ESR values. The availability of the I
not only allows optimization of control loop behavior but
also provides a DC coupled and AC fi ltered closed loop
response test point. The DC step, rise time and settling
at this test point truly refl ects 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
Application circuit will provide an adequate starting point
for most applications.
The I
loop compensation. The values can be modifi ed slightly
(from 0.5 to 2 times their suggested values) to optimize
transient response once the fi nal 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 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
feedback loop and is the fi ltered and compensated control
loop response. The gain of the loop will be increased
by increasing R
increased by decreasing C
factor that C
the same, thereby keeping the phase shift the same in the
TH
series RC-CC fi lter sets the dominant pole-zero
TH
C
external components shown in the Typical
is decreased, the zero frequency will be kept
C
and the bandwidth of the loop will be
C
. If R
TH
TH
C
pin signal which is in the
pin waveforms that will
is increased by the same
LTC3835
21
TH
3835fc
pin