LTC2927

14

2927fb

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.

Figure 20. Layout Considerations

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:

R

VV

I

V

V

RR

SERIES

FB

MAX

FB

FA

FB

=

−

=−

⎛

⎝⎜

⎞

15

15

1

..

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.

2927 F20

LTC2927

FB

GND

DC/DC

FB

OUT

0.1μF

MINIMIZE

TRACE

LENGTH

APPLICATIO S I FOR ATIO

Figure 18. Power Supply Start-Up Delayed

Figure 19. ON Pin Delayed

2927 F18

1V/DIV

20ms/DIV

MASTER

SLAVE

ON

2927 F19

1V/DIV

5ms/DIV

MASTER

SLAVE

ON