LTC1430
U
W U U
APPLICATIO S I FOR ATIO
Compensation and Transient Response
pensation components. In general, a smaller value induc-
torwillimprovetransientresponseattheexpenseofripple
and inductor core saturation rating. Minimizing output
capacitor ESR will also help optimize output transient
response. See Input and Output Capacitors for more
information.
TheLTC1430voltagefeedbackloopiscompensatedatthe
COMP pin; this is the output node of the internal gm error
amplifier. The loop can generally be compensated prop-
erly with an RC network from COMP to GND and an
additional small C from COMP to GND (Figure 8). 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
empiricallytweaktheRC valuesuntilthetransientrecovery
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.
LTC1430
COMP
R
C
C
C1
C
GND
SGND LTC1430 • F08
Figure 8. Compensation Pin Hook-Up
Table 1. Recommended Compensation Network for 5V to 3.3V
Application Using Multiple 330µF AVX Output Capacitors
Soft-Start and Current Limit
L1 (µH)
1
C
(µF)
R (kΩ)
C (µF)
C1 (pF)
820
470
150
82
OUT
C
C
990
1.8
3.6
9.1
18
0.022
0.01
The 16-lead versions of the LTC1430 include a soft-start
circuitattheSSpin;thiscircuitisusedbothforinitialstart-
up and during current limit operation. The soft-start and
current limit circuitry is disabled in 8-lead versions. 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 9). The LTC1430 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
LTC1430willbeinfulloperation. Aninternalswitchshorts
the SS pin to GND during shutdown.
1
1980
4950
9900
990
1
0.0047
0.0022
0.01
1
2.7
2.7
2.7
2.7
5.6
5.6
5.6
5.6
10
3.6
7.5
18
470
220
82
1980
4950
9900
990
0.0047
0.0022
0.001
0.0047
0.0022
820pF
470pF
0.0022
0.001
470pF
220pF
39
39
9.1
18
150
82
1980
4950
9900
990
47
33
91
15
18
82
10
1980
4950
9900
39
39
10
91
15
10
180
10
The LTC1430 detects the output current by watching the
voltage at IFB while M1 is ON. The ILIM amplifier compares
this voltage to the voltage at IMAX (Figure 10). In the ON
state, M1 has a known resistance; by calculating back-
wards, the voltage generated at IFB by the maximum
output current in M1 can be determined. As IFB falls below
IMAX, ILIM will begin to sink current from the soft-start pin,
Outputtransientresponseissetbythreemajorfactors:the
time constant of the inductor and the output capacitor, the
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-
11
Linear [ Linear ]
