Datasheet

LM2588
www.ti.com SNVS117D –APRIL 1998–REVISED APRIL 2013
The device will shut down when 3V or greater is applied on the ON /OFF pin, sourcing current into pin 1. In shut
down mode, the device will draw typically 56 μA of supply current (16 μA to V
IN
and 40 μA to the ON /OFF pin).
To turn the device back on, leave pin 1 floating, using an (isolation) diode, as shown in Figure 54 (for normal
operation, do not source or sink current to or from this pin—see the next section).
FREQUENCY ADJUSTMENT
The switching frequency of the LM2588 can be adjusted with the use of an external resistor. This feature allows
the user to optimize the size of the magnetics and the output capacitor(s) by tailoring the operating frequency. A
resistor connected from pin 1 (the Freq. Adj. pin) to ground will set the switching frequency from 100 kHz to 200
kHz (maximum). As shown in Figure 54, the pin can be used to adjust the frequency while still providing the shut
down function. A curve in the Performance Characteristics Section graphs the resistor value to the corresponding
switching frequency. The table in Table 4 shows resistor values corresponding to commonly used frequencies.
However, changing the LM2588's operating frequency from its nominal value of 100 kHz will change the
magnetics selection and compensation component values.
Table 4. Frequency Setting Resistor Guide
R
SET
(kΩ) Frequency (kHz)
Open 100
200 125
47 150
33 175
22 200
Figure 55. Frequency Synchronization
FREQUENCY SYNCHRONIZATION
Another feature of the LM2588 is the ability to synchronize the switching frequency to an external source, using
the sync pin (pin 6). This feature allows the user to parallel multiple devices to deliver more output power.
A negative falling pulse applied to the sync pin will synchronize the LM2588 to an external oscillator (see
Figure 55 and Figure 56).
Use of this feature enables the LM2588 to be synchronized to an external oscillator, such as a system clock. This
operation allows multiple power supplies to operate at the same frequency, thus eliminating frequency-related
noise problems.
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