LT3430/LT3430-1
22
34301fa
stability. This ESR, however, contributes signifi cantly 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 additional
zero is required in the loop to increase phase margin for
improved transient response.
A zero can be added into the loop by placing a resistor (R
C
)
at the V
C
pin in series with the compensation capacitor,
C
C
, or by placing a capacitor (C
FB
) between the output
and the FB pin.
When using R
C
, the maximum value has two limitations.
First, the combination of output capacitor ESR and R
C
may stop the loop rolling off altogether. Second, if the
loop gain is not rolled off suffi ciently at the switching
frequency, output ripple will perturb the V
C
pin enough to
cause unstable duty cycle switching similar to subharmonic
oscillations. If needed, an additional capacitor (C
F
) can be
added across the R
C
/C
C
network from the V
C
pin to ground
to further suppress V
C
ripple voltage.
With a tantalum output capacitor, the LT3430/LT3430-1
already includes a resistor (R
C
) and fi lter capacitor (C
F
)
at the V
C
pin (see Figures 10 and 11) to compensate the
loop over the entire V
IN
range (to allow for stable pulse
skipping for high V
IN
-to-V
OUT
ratios ≥ 10). A ceramic output
capacitor can still be used with a simple adjustment to the
resistor R
C
for stable operation (see Ceramic Capacitors
section for stabilizing LT3430). If additional phase margin
is required, a capacitor (C
FB
) can be inserted between the
output and 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
IN
-to-V
OUT
ratios < 10, higher loop bandwidths are
possible by readjusting the frequency compensation
components at the V
C
pin.
When checking loop stability, the circuit should be operated
over the application’s full voltage, current and tempera-
ture range. Proper loop compensation may be obtained
by empirical methods as described in Application Notes
19 and 76.
CONVERTER WITH BACKUP OUTPUT REGULATOR
In systems with a primary and backup supply, for example,
a battery powered device with a wall adapter input, the
output of the LT3430/LT3430-1 can be held up by the
backup supply with the LT3430/LT3430-1 input discon-
nected. In this condition, the SW pin will source current
into the V
IN
pin. If the
⎯
S
⎯
H
⎯
D
⎯
N pin is held at ground, only the
shut down current of 30µA will be pulled via the SW pin
from the second supply. With the
⎯
S
⎯
H
⎯
D
⎯
N pin fl oating, the
APPLICATIONS INFORMATION
–
+
1.22V
V
SW
V
C
LT3430/LTC3430-1
GND
3430 F10
R1
OUTPUT
ESR
C
F
C
C
R
C
R
O
200k
ERROR
AMPLIFIER
FB
R2
C1
R
LOAD
CURRENT MODE
POWER STAGE
g
m
= 2mho
g
m
=
2000µmho
+
TANTALUM
ESL
C1
CERAMIC
C
FB
FREQUENCY (Hz)
GAIN (dB)
80
60
40
20
0
–20
–40
PHASE (DEG)
180
150
120
90
60
30
0
3430 F11
GAIN
PHASE
10
V
IN
= 42V
V
OUT
= 5V
I
LOAD
= 1A
C
OUT
= 100µF, 10V, 0.1Ω
1k 10k 1M100 100k
R
C
= 3.3k
C
C
= 22nF
C
F
= 220pF
Figure 10. Model for Loop Response Figure 11. Overall Loop Response
