LTC3619
14
3619fa
applicaTions inForMaTion
long wires, a load step at the output can induce ringing at
the input, V
IN
. At best, this ringing can couple to the output
and be mistaken as loop instability. At worst, a sudden
inrush of current through the long wires can potentially
cause a voltage spike at V
IN
, large enough to damage the
part. For more information, see Application Note 88.
When choosing the input and output ceramic capacitors,
choose the X5R or X7R dielectric formulations. These
dielectrics have the best temperature and voltage char-
acteristics of all the ceramics for a given value and size.
Setting the Output Voltage
The LTC3619 regulates the V
FB1
and V
FB2
pins to 0.6V
during regulation. Thus, the output voltage is set by a resis-
tive divider, Figure 2, according to the following formula:
V
OUT
= 0.6V 1+
R2
R1
(2)
Keeping the current small (<10µA) in these resistors
maximizes efficiency, but making it too small may allow
stray capacitance to cause noise problems or reduce the
phase margin of the error amp loop.
To improve the frequency response of the main control
loop, a feedback capacitor (C
F
) may also be used. Great
care should be taken to route the V
FB
line away from noise
sources, such as the inductor or the SW line.
Checking Transient Response
The regulator loop response can be checked by looking
at the load 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 DI
LOAD
• ESR, where ESR is the effective series
resistance of C
OUT
. DI
LOAD
also begins to charge or dis-
charge C
OUT
generating a feedback error signal used by the
regulator to return V
OUT
to its steady-state value. During
this recovery time, V
OUT
can be monitored for overshoot
or ringing that would indicate a stability problem.
The initial output voltage step may not be within the
bandwidth of the feedback loop, so the standard second
order overshoot/DC ratio cannot be used to determine the
phase margin. In addition, feedback capacitors (C
F1
and
C
F2
) can be added to improve the high frequency response,
as shown in Figure 2. Capacitor C
F
provides phase lead by
creating a high frequency zero with R2 which improves
the phase margin.
The output voltage settling behavior is related to the stability
of the closed-loop system and will demonstrate the actual
overall supply performance. For a detailed explanation of
optimizing the compensation components, including a
review of control loop theory, refer to Application Note 76.
In some applications, a more severe transient can be caused
by switching in loads with large (>1µF) 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 cur-
rent limiting, short-circuit protection, and soft-starting.
Efficiency Considerations
The percent efficiency of a switching
regulator is equal to
the
output power divided by the input power times 100%.
It is often useful to analyze individual losses to determine
what is limiting the efficiency and which change would
produce the most improvement. Percent efficiency can
be expressed as:
% Efficiency = 100% – (L1 + L2 + L3 + ...)
where L1, L2, etc., are the individual losses as a percent-
age of input power.
Although all dissipative elements in the circuit produce
losses, four sources usually account for the losses in
LTC3619 circuits: 1) V
IN
quiescent current, 2) switching
losses, 3) I
2
R losses, 4) other system losses.
1. The V
IN
current is the DC supply current given in the
Electrical Characteristics which excludes MOSFET
driver and control currents. V
IN
current results in a
small (<0.1%) loss that increases with V
IN
, even at
no load.
2. The switching current is the sum of the MOSFET driver
and control currents. The MOSFET driver current re-
