lt3430-1 Linear Technology Corporation, lt3430-1 Datasheet - Page 22

• Manufacturer Part Number: lt3430-1
• Description: High Voltage, 3a, 100khz Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

LT3430/LT3430-1
APPLICATIONS INFORMATION
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
at the V
C
and the FB pin.
When using R
First, the combination of output capacitor ESR and R
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
cause 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 LT3430/LT3430-1
already includes a resistor (R
at the V
22
LT3430/LTC3430-1
C
GND
CURRENT MODE
, or by placing a capacitor (C
POWER STAGE
g
m
= 2mho
C
C
V
R
C
pin (see Figures 10 and 11) to compensate the
C
C
pin in series with the compensation capacitor,
200k
C
R
O
Figure 10. Model for Loop Response
C
2000µmho
, the maximum value has two limitations.
g
C
AMPLIFIER
m
F
ERROR
=
C
–
+
/C
C
C
ripple voltage.
network from the V
1.22V
V
SW
FB
C
) and fi lter capacitor (C
FB
C
FB
) between the output
R1
R2
C
R
C
LOAD
pin enough to
pin to ground
F
+
) can be
TANTALUM
ESR
C1
OUTPUT
C
F
C
)
)
CERAMIC
loop over the entire V
skipping for high V
capacitor can still be used with a simple adjustment to the
resistor R
section for stabilizing LT3430). If additional phase margin
is required, a capacitor (C
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
possible by readjusting the frequency compensation
components at the V
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
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
ESL
3430 F10
C1
IN
-to-V
IN
C
–20
–40
80
60
40
20
pin. If the ⎯ S ⎯ H ⎯ D ⎯ N pin is held at ground, only the
for stable operation (see Ceramic Capacitors
0
OUT
10
Figure 11. Overall Loop Response
V
V
I
C
PHASE
LOAD
IN
OUT
OUT
ratios < 10, higher loop bandwidths are
= 42V
IN
= 5V
= 100µF, 10V, 0.1Ω
= 1A
100
-to-V
C
GAIN
IN
pin.
FREQUENCY (Hz)
range (to allow for stable pulse
OUT
1k
FB
) can be inserted between the
ratios ≥ 10). A ceramic output
10k
R
C
C
C
F
C
= 220pF
= 22nF
= 3.3k
100k
3430 F11
1M
180
150
120
90
60
30
0
34301fa