LT6657
applicaTions inForMaTion
Turn-On and Line Transient Response
Increasing the output load speeds up the response
(Figure 3).
The turn-on time is slew-limited and determined by the
short-circuit current, the output capacitor, and the output
voltage value as determined by the equation:
V
IN
COUT
ISC
0.2V/DIV
3V/DC
tON = VOUT
•
For example, the LT6657-2.5V, with a 1µF output capaci-
tor and a typical current limit of 15mA the turn-on time
V
OUT
2mV/DIV
2.5V/DC
would be:
1µF
15mA
The resulting turn-on time is shown in Figure 1.
C
= 1µF, I
= 1mA SOURCING
50µs/DIV
OUT
OUT
6657 F03
tON = 2.5V •
= 167µs
Figure 3. Line Transient Response with VIN = 0.4VP-P
A larger output capacitor lowers the amplitude response
with a longer time response trade-off (Figure 4).
V
IN
1V/DIV
V
IN
0.5V/DIV
3V/DC
GND
V
OUT
1V/DIV
V
OUT
GND
2mV/DIV
2.5V/DC
C
= 1µF
OUT
6657 F01
50µs/DIV
C
= 10µF, I
= 0mA
OUT
OUT
6657 F04
50µs/DIV
Figure 1. 2.5V Turn-On Characteristics
Figure 4. Line Transient Response with VIN = 1VP-P
Line transient response with a 1µF output capacitor and
no output current is shown in Figure 2. The peak voltage
output response is less than 1mV.
Load Transient Response
ThetestcircuitofFigure5isusedtomeasureloadtransient
response with various currents.
V
IN
0.2V/DIV
3V/DC
2
V
IN
= 3V
IN
V
OUT
1k
6
V
LT6657
GND
OUT
GEN
3
V
OUT
SHDN
C
C
OUT
2mV/DIV
2.5V/DC
IN
0.1µF
1µF
C
= 1µF, I
= 0mA
6657 F05
OUT
OUT
4
6657 F02
50µs/DIV
Figure 5. Transient Load Test Circuit
Figure 2. Line Transient Response with VIN = 0.4VP-P
6657fb
10
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Linear Systems [ Linear Systems ]
