LM3488QMM National Semiconductor, LM3488QMM Datasheet - Page 12

• Manufacturer Part Number: LM3488QMM
• Description: LM3488/LM3488Q High Efficiency Low-Side N-Channel Controller for Switching Regulators; Package: MINI SOIC; No of Pins: 8; Container: Reel
• Manufacturer: National Semiconductor

www.national.com
FREQUENCY ADJUST/SYNCHRONIZATION/SHUTDOWN
The switching frequency of LM3488 can be adjusted between
100kHz and 1MHz using a single external resistor. This re-
sistor must be connected between FA/SYNC/SD pin and
ground, as shown in Figure 6. Please refer to the typical per-
formance characteristics to determine the value of the resistor
required for a desired switching frequency.
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
this resistor is dependent on the off time of the synchroniza-
tion pulse, T
be used for a given T
OFF(SYNC)
FIGURE 5. ΔV
SYNC
. Table 1 shows the range of resistors to
OFF(SYNC)
as shown in Figure 7. The value of
.
SL
vs R
SL
FIGURE 6. Frequency Adjust
10138851
12
It is also necessary to have the width of the synchronization
pulse narrower than the duty cycle of the converter. 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 (refer to the 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 8 and Figure 9 show implementation of shutdown func-
tion when operating in Frequency adjust mode and synchro-
nization 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.
T
OFF(SYNC)
10
1
2
3
4
5
6
7
8
9
(µsec)
TABLE 1.
R
10138816
SYNC
100 to 160
120 to 190
135 to 215
150 to 240
70 to 110
85 to 140
20 to 40
40 to 65
55 to 90
5 to 13
range (kΩ)
≥