lm3478-mdc National Semiconductor Corporation, lm3478-mdc Datasheet - Page 12

• Manufacturer Part Number: lm3478-mdc
• Description: High Efficiency Low-side N-channel Controller For Switching Regulator
• Manufacturer: National Semiconductor Corporation

www.national.com
Functional Description
FREQUENCY ADJUST/SHUTDOWN
The switching frequency of LM3478 can be adjusted be-
tween 100kHz and 1MHz using a single external resistor.
This resistor must be connected between FA/SD pin and
ground, as shown in Figure 6. Please refer to the typical
performance characteristics to determine the value of the
resistor required for a desired switching frequency.
The FA/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/SD pin, the LM3478 stops
FIGURE 4. Increasing the Slope of the Compensation Ramp
(Continued)
FIGURE 5. ∆V
12
SL
switching and goes into a low current mode. The total supply
current of the IC reduces to less than 10 µA under these
conditions.
Figure 7 shows implementation of shutdown function when
operating in frequency adjust mode. In frequency adjust
mode, connecting the FA/SD pin to ground forces the clock
to run at a certain frequency. Pulling this pin high shuts down
the IC. In frequency adjust mode, a high signal for more than
30µs shuts down the IC.
vs R
SL
10135551
10135513