LT3430-1 Datasheet, PDF (22/28 Pages) Linear Technology – High Voltage, 3A,200kHz/100kHz Step-Down Switching Regulators

LT3430/LT3430-1
APPLICATIONS INFORMATION
stability. 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 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 sufficiently 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 filter 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 floating, the
LT3430/LTC3430-1
CURRENT MODE
POWER STAGE
g_m = 2mho
V_SW
ERROR
AMPLIFIER
g_m =
2000µmho
R_O
200k
FB
1.22V
GND
V_C
R_C
C_F
C_C
C_FB R1
OUTPUT
TANTALUM CERAMIC
ESR
ESL
R_{LOAD +}
C1
C1
R2
3430 F10
80
180
60
150
GAIN
40
120
20
90
PHASE
0
60
–20
30
–40
0
10
100
1k
10k 100k 1M
FREQUENCY (Hz)
3430 F11
V_IN = 42V
V_OUT = 5V
I_LOAD = 1A
C_OUT = 100µF, 10V, 0.1Ω
R_C = 3.3k
C_C = 22nF
C_F = 220pF
Figure 10. Model for Loop Response
22
Figure 11. Overall Loop Response
34301fa