LM3478
LM3478-Q1
SNVS085V –JULY 2000–REVISED FEBRUARY 2013
www.ti.com
Q
DR
LM3478
I
SEN
R
SEN
R
SL
Figure 27. Adding External Slope Compensation
FREQUENCY ADJUST/SHUTDOWN
Figure 28. External Slope Compensation
ΔVSL vs RSL
The switching frequency of the LM3478 can be adjusted between 100kHz and 1MHz using a single external
resistor. This resistor must be connected between FA/SD pin and ground, as shown in Figure 29. To determine
the value of the resistor required for a desired switching frequency refer to Typical Performance Characteristics
or use the following equation:
- 1.26
RFA = 4.503 x 1011 x fS
(7)
Figure 29. Frequency Adjust
The FA/SD pin also functions as a shutdown pin. If a high signal (>1.35V) appears on the FA/SD pin, the
LM3478 stops 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 30 shows implementation of the shutdown function when operating in
frequency adjust mode. In this mode a high signal for more than 30us shuts down the IC. However, the voltage
on the FA/SD pin should be always less than the absolute maximum of 7V to avoid any damage to the device.
12
Submit Documentation Feedback
Copyright © 2000–2013, Texas Instruments Incorporated
Product Folder Links: LM3478 LM3478-Q1
TI [ TEXAS INSTRUMENTS ]
