LT3437 LINER [Linear Technology], LT3437 Datasheet - Page 20

• Manufacturer Part Number: LT3437
• Description: High Voltage 500mA, 200kHz Step-Down Switching Regulator with 100uA Quiescent Current
• Manufacturer: LINER [Linear Technology]

LT3437
APPLICATIO S I FOR ATIO
FREQUENCY COMPENSATION
Before starting on the theoretical analysis of frequency
response, the following should be remembered—the worse
the board layout, the more difficult the circuit will be to
stabilize. This is true of almost all high frequency analog
circuits. Read the Layout Considerations section first.
Common layout errors that appear as stability problems
are distant placement of input decoupling capacitor and/or
catch diode, and connecting the V
ground track carrying significant switch current. In addi-
tion, the theoretical analysis considers only first order
non-ideal component behavior. For these reasons, it is
important that a final stability check is made with produc-
tion layout and components.
The LT3437 uses current mode control. This alleviates
many of the phase shift problems associated with the
inductor. The basic regulator loop is shown in Figure 10.
The LT3437 can be considered as two g
amplifier and the power stage.
Figure 11 shows the overall loop response. At the V
the frequency compensation components used are:
R
capacitor used is a 100µF, 10V tantalum capacitor with
typical ESR of 100mΩ.
The ESR of the tantalum output capacitor provides a useful
zero in the loop frequency response for maintaining stabil-
ity. This ESR, however, contributes significantly to the
ripple voltage at the output (see Output Ripple Voltage in
the Applications Information section). It is possible to
reduce capacitor size and output ripple voltage by replac-
ing the tantalum output capacitor with a ceramic output
capacitor because of its very low ESR. The zero provided
by the tantalum output capacitor must now be reinserted
back into the loop. Alternatively, there may be cases
where, even with the tantalum output capacitor, an addi-
tional zero is required in the loop to increase phase margin
for improved transient response.
20
C
= 25k, C
C
= 1500pF and C
U
U
F
= 330pF. The output
W
C
compensation to a
m
blocks, the error
U
C
pin,
A zero can be added into the loop by placing a resistor (R
at the V
or by placing a capacitor (C
FB pin.
When using R
First, the combination of output capacitor ESR and R
stop the loop rolling off altogether. Second, if the loop gain
is not rolled off sufficiently at the switching frequency,
output ripple will perturb the V
unstable duty cycle switching, similar to subharmonic
oscillations. If needed, an additional capacitor (C
added across the R
to further suppress V
With a tantalum output capacitor, the LT3437 already
includes a resistor (R
pin (see Figures 10 and 11) to compensate the loop over
the entire V
high V
can still be used with a simple adjustment to the resistor R
for stable operation (see Ceramic Capacitors section for
stabilizing LT3430). If additional phase margin is required,
a capacitor (C
FB pin, but care must be taken for high output voltage
applications. Sudden shorts to the output can create
unacceptably large negative transients on the FB pin.
For V
possible by readjusting the frequency compensation com-
ponents at the V
When checking loop stability, the circuit should be oper-
ated over the application’s full voltage, current and tem-
perature range. Proper loop compensation may be obtained
by empirical methods, as described in Application Notes
19 and 76.
IN
IN
C
-to-V
-to-V
pin in series with the compensation capacitor, C
IN
OUT
OUT
range (to allow for stable pulse skipping for
FB
C
, the maximum value has two limitations.
) can be inserted between the output and
C
ratios < 10, higher loop bandwidths are
ratios ≥ 10). A ceramic output capacitor
pin.
C
/C
C
C
C
) and filter capacitor (C
ripple voltage.
network from the V
FB
) between the output and the
C
pin enough to cause
C
pin to ground
F
) at the V
F
) can be
C
may
3437f
C
C
C
C
)
,