LTC1430A Просмотр технического описания (PDF) - Linear Technology
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LTC1430A Datasheet PDF : 24 Pages
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LTC1430A
APPLICATI S I FOR ATIO
properly with an RC network from COMP to GND and an
additional small C from COMP to GND (Figure 11). Loop
stability is affected by inductor and output capacitor
values and by other factors. Optimum loop response can
be obtained by using a network analyzer to find the loop
poles and zeros; nearly as effective and a lot easier is to
empirically tweak the RC values until the transient recovery
looks right with an output load step. Table 1 shows
recommended compensation components for 5V to 3.3V
applications based on the inductor and output capacitor
values. The values were calculated using multiple paral-
leled 330µF AVX TPS series surface mount tantalum
capacitors as the output capacitor.
Table 1. Recommended Compensation Network for 5V to 3.3V
Application Using Multiple 330µF AVX Output Capacitors
L1 (µH)
1
1
1
1
2.7
2.7
2.7
2.7
5.6
5.6
5.6
5.6
10
10
10
10
COUT (µF)
990
1980
4950
9900
990
1980
4950
9900
990
1980
4950
9900
990
1980
4950
9900
RC (kΩ)
1.8
3.6
9.1
18
3.6
7.5
18
39
9.1
18
47
91
18
39
91
180
CC (µF)
0.022
0.01
0.0047
0.0022
0.01
0.0047
0.0022
0.001
0.0047
0.0022
820pF
470pF
0.0022
0.001
470pF
220pF
C1 (pF)
820
470
150
82
470
220
82
39
150
82
33
15
82
39
15
10
Output transient response is set by three major factors: the
time constant of the inductor and the output capacitor, the
ESR of the output capacitor, and the loop compensation
components. The first two factors usually have much
more impact on overall transient recovery time than the
third; unless the loop compensation is way off, more
improvement can be had by optimizing the inductor and
the output capacitor than by fiddling with the loop com-
pensation components. In general, a smaller value induc-
tor will improve transient response at the expense of ripple
LTC1430A
COMP
RC
CC
C1
GND
SGND 1430 F11
Figure 11. Compensation Pin Hook-Up
and inductor core saturation rating. Minimizing output
capacitor ESR will also help optimize output transient
response. See Input and Output Capacitors for more
information.
Soft Start and Current Limit
The 16-lead versions of the LTC1430A include a soft start
circuit at the SS pin; this circuit is used both for initial start-
up and during current limit operation. The soft start and
current limit circuitry is disabled in the 8-lead version. SS
requires an external capacitor to GND with the value
determined by the required soft start time. An internal
12µA current source is included to charge the external
capacitor. Soft start functions by clamping the maximum
voltage that the COMP pin can swing to, thereby control-
ling the duty cycle (Figure 12). The LTC1430A will begin to
operate at low duty cycle as the SS pin rises to about 2V
below VCC. As SS continues to rise, the duty cycle will
increase until the error amplifier takes over and begins to
regulate the output. When SS reaches 1V below VCC the
COMP
LTC1430A
FB
VCC
SS
CSS
12µA
1430 F12
Figure 12. Soft Start Clamps COMP Pin
14
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