LT3430 Linear Technology, LT3430 Datasheet - Page 22

• Manufacturer Part Number: LT3430
• Description: High Voltage/ 3A/ 200kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

LT3430
APPLICATIO S I FOR ATIO
where, even with the tantalum output capacitor, an addi-
tional 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
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 LT3430 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.
22
LT3430
GND
CURRENT MODE
POWER STAGE
g
m
IN
= 2mho
C
-to-V
V
R
pin in series with the compensation capacitor, C
C
C
C
C
200k
R
IN
O
Figure 10. Model for Loop Response
2000 mho
OUT
range (to allow for stable pulse skipping for
FB
C
g
C
AMPLIFIER
m
F
, the maximum value has two limitations.
ERROR
) can be inserted between the output and
=
–
+
ratios 10). A ceramic output capacitor
C
U
/C
1.22V
C
C
V
C
SW
) and filter capacitor (C
FB
ripple voltage.
network from the V
U
FB
C
) between the output and the
FB
R1
R2
C
W
pin enough to cause
R
LOAD
+
C
TANTALUM
ESR
pin to ground
C1
OUTPUT
F
U
) at the V
F
) can be
CERAMIC
C
ESL
3430 F10
may
C1
C
C
C
C
)
,
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.
CONVERTER WITH BACKUP OUTPUT REGULATOR
In systems with a primary and backup supply, for ex-
ample, a battery powered device with a wall adapter input,
the output of the LT3430 can be held up by the backup
supply with the LT3430 input disconnected. In this condi-
tion, the SW pin will source current into the V
SHDN pin is held at ground, only the shut down current of
25 A will be pulled via the SW pin from the second supply.
With the SHDN pin floating, the LT3430 will consume its
quiescent operating current of 1.5mA. The V
source current to any other components connected to the
input line. If this load is greater than 10mA or the input
could be shorted to ground, a series Schottky diode must
be added, as shown in Figure 12. With these safeguards,
the output can be held at voltages up to the V
maximum rating.
IN
-to-V
OUT
–20
–40
Figure 11. Overall Loop Response
80
60
40
20
0
10
V
V
I
C
C
PHASE
ratios < 10, higher loop bandwidths are
LOAD
IN
OUT
OUT
= 42V
pin.
= 5V
= 100 F, 10V, 0.1
= 1A
100
GAIN
FREQUENCY (Hz)
1k
10k
R
C
C
C
F
C
= 220pF
= 3.3k
= 22nF
100k
3430 F11
1M
180
150
120
90
60
30
0
IN
IN
pin will also
IN
pin. If the
sn3430 3430is
absolute