LTC2927CDDB View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC2927CDDB Datasheet PDF : 16 Pages
| |||
LTC2927
APPLICATIO S I FOR ATIO
Start-Up Delays
Often power supplies do not start-up immediately when
their input supplies are applied. If the LTC2927 tries to
ramp-up these power supplies as soon as the input sup-
ply is present, the start-up of the outputs may be delayed,
defeating the tracking circuit (Figure 18). Often this delay is
intentionally configured by a soft-start capacitor. This can
be remedied either by reducing the soft-start capacitor on
the slave supply or by including a capacitor in the ON pin’s
resistive divider to delay the ramp up. See Figure 19.
MASTER
1V/DIV
SLAVE
ON
20ms/DIV
2927 F18
Figure 18. Power Supply Start-Up Delayed
1V/DIV
MASTER
SLAVE
Layout Considerations
Be sure to place a 0.1μF bypass capacitor as near as pos-
sible to the supply pin of the LTC2927.
To minimize the noise on the slave supply’s output, keep
the trace connecting the FB pin of the LTC2927 and the
feedback node of the slave supply as short as possible.
In addition, do not route this trace next to signals with
fast transition times. In some circumstances it might be
advantageous to add a resistor near the feedback node of
the slave supply in series with the FB pin of the LTC2927.
This resistor must not exceed:
( ) RSERIES
=
1.5V − VFB
IMAX
=
⎛
⎝⎜
1.5V
VFB
− 1⎞⎠⎟
RFA
RFB
This resistor is most effective if there is already a ca-
pacitor at the feedback node of the slave supply (often a
compensation component). Increasing the capacitance on
a slave supply’s feedback node will further improve the
noise immunity, but could affect the stability and transient
response of the supply.
VCC
LTC2927
FB
GND
RSERIES
MINIMIZE
TRACE
LENGTH
0.1μF
2927 F20
DC/DC
FB OUT
RFA RFB
Figure 20. Layout Considerations
ON
5ms/DIV
2927 F19
Figure 19. ON Pin Delayed
2927fb
14
Share Link: 