LTC1430IS Linear Technology, LTC1430IS Datasheet - Page 11

• Manufacturer Part Number: LTC1430IS
• Description: IC SW REG CNTRLR STEP-DWN 16SOIC
• Manufacturer: Linear Technology

Specifications of LTC1430IS

• Applications: Controller, Intel Pentium®
• Voltage - Input: 4 ~ 8 V
• Number Of Outputs: 1
• Voltage - Output: 3.3V, Adjustable
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIO S I FOR ATIO
Compensation and Transient Response
The LTC1430 voltage feedback loop is compensated at the
COMP pin; this is the output node of the internal g
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
empirically tweak the R
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 330 F AVX TPS series surface mount tantalum
capacitors as the output capacitor.
Table 1. Recommended Compensation Network for 5V to 3.3V
Application Using Multiple 330 F AVX Output Capacitors
Output transient response is set by three major factors: 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-
L1 ( H)
2.7
2.7
2.7
2.7
5.6
5.6
5.6
5.6
10
10
10
10
1
1
1
1
C
OUT
1980
4950
9900
1980
4950
9900
1980
4950
9900
1980
4950
9900
990
990
990
990
( F)
U
C
values until the transient recovery
R
U
C
180
1.8
3.6
9.1
3.6
7.5
9.1
18
18
39
18
47
91
18
39
91
(k )
W
C
0.0047
0.0022
0.0047
0.0022
0.0047
0.0022
0.0022
820pF
470pF
470pF
220pF
0.022
0.001
0.001
C
0.01
0.01
( F)
U
C1 (pF)
m
820
470
150
470
220
150
82
82
39
82
33
15
82
39
15
10
error
pensation components. In general, a smaller value induc-
tor will improve transient response at the expense of ripple
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 LTC1430 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 8-lead versions. SS
requires an external capacitor to GND with the value
determined by the required soft-start time. An internal
12 A 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 V
increase until the error amplifier takes over and begins to
regulate the output. When SS reaches 1V below V
LTC1430 will be in full operation. An internal switch shorts
the SS pin to GND during shutdown.
The LTC1430 detects the output current by watching the
voltage at I
this voltage to the voltage at I
state, M1 has a known resistance; by calculating back-
wards, the voltage generated at I
output current in M1 can be determined. As I
I
MAX
, I
LIM
CC
will begin to sink current from the soft-start pin,
. As SS continues to rise, the duty cycle will
FB
Figure 8. Compensation Pin Hook-Up
while M1 is ON. The I
GND
R
C
C
LTC1430
C
COMP
MAX
SGND
LIM
(Figure 10). In the ON
FB
C1
LTC1430 • F08
amplifier compares
by the maximum
LTC1430
FB
falls below
11
CC
the