Datasheet

LM3488
LM3488-Q1
www.ti.com SNVS089M –JULY 2000–REVISED MARCH 2013
The LM3488 can be synchronized to an external clock. The external clock must be connected to the
FA/SYNC/SD pin through a resistor, R
SYNC
as shown in Figure 28. The value of this resistor is dependent on the
off time of the synchronization pulse, T
OFF(SYNC)
. Table 1 shows the range of resistors to be used for a given
T
OFF(SYNC)
.
Table 1.
T
OFF(SYNC)
(µsec) R
SYNC
range (kΩ)
1 5 to 13
2 20 to 40
3 40 to 65
4 55 to 90
5 70 to 110
6 85 to 140
7 100 to 160
8 120 to 190
9 135 to 215
10 150 to 240
It is also necessary to have the width of the synchronization pulse wider than the duty cycle of the converter
(when DR pin is high and the switching point is low). It is also necessary to have the synchronization pulse width
≥ 300nsecs.
The FA/SYNC/SD pin also functions as a shutdown pin. If a high signal (see Electrical Characteristics for
definition of high signal) appears on the FA/SYNC/SD pin, the LM3488 stops switching and goes into a low
current mode. The total supply current of the IC reduces to less than 10µA under these conditions.
Figure 29 and Figure 30 show implementation of shutdown function when operating in Frequency adjust mode
and synchronization mode respectively. In frequency adjust mode, connecting the FA/SYNC/SD pin to ground
forces the clock to run at a certain frequency. Pulling this pin high shuts down the IC. In frequency adjust or
synchronization mode, a high signal for more than 30µs shuts down the IC.
Figure 31 shows implementation of both frequency adjust with R
FA
resistor and frequency synchronization with
R
SYNC
. The switching frequency is defined by R
FA
when a synchronization signal is not applied. When sync is
applied it overrides the R
FA
setting.
Figure 27. Frequency Adjust
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