LM27402 datasheet(16/32 Pages) NSC

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FIGURE 7. Soft-Start Timing

TRACKING

The SS/TRACK pin also functions as a tracking pin when ex-

ternal power supply tracking is needed. Tracking is achieved

by simply dividing down the external supply voltage with a

simple resistor network shown in Figure 8. With the correct

resistor divider configuration, the LM27402 can track an ex-

ternal voltage source to obtain a coincident or ratiometric

startup behavior.

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FIGURE 8. Tracking an External Power Supply

Since the soft-start charging current I

SS is sourced from the

SS/TRACK pin, the size of R

2 should be less than 10 kΩ to

minimize errors in the tracking output. Once a value for R

2 is

selected, the value for R

1 can be calculated using the appro-

priate equation in Figure 9 to give the desired startup se-

quence. Figure 9 shows two common startup sequences; the

top waveform shows a coincidental startup while the bottom

waveform illustrates a ratiometric startup. A coincidental con-

figuration provides a robust startup sequence for certain ap-

plications since it avoids turning on any parasitic conduction

paths that may exist between loads. A ratiometric configura-

tion is preferred in applications where both supplies need to

be at the final value at the same time.

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FIGURE 9. Tracking Startup Sequences

Similar to the soft-start function, the fastest possible startup

time is 1.28 ms regardless of the rise time of the tracking volt-

age. When using the track feature, the final voltage seen by

the SS/TRACK pin should exceed 0.8V to provide sufficient

overdrive and transient immunity.

SETTING THE SWITCHING FREQUENCY

There are two options for setting the switching frequency of

the LM27402. The frequency can be adjusted by an external

resistor from FADJ to GND, or the user can synchronize the

LM27402 to an external clock signal through SYNC. The

LM27402 will only synchronize to frequencies above the fre-

quency set by the R

FADJ resistor. The clock signal must range

from less than 0.8V to greater than 2.0V to ensure proper op-

eration. If the clock signal ceases, the switching frequency will

reduce to the frequency set by the FADJ resistor. The fre-

quency range is 200 kHz to 1.2 MHz. The sync-in clock can

synchronize a maximum of 400 kHz above the frequency set

by the resistor. To find the value of resistance needed for a

given frequency use the following equation: (f

SW (kHz),

FADJ (kΩ))

SETTING THE CURRENT LIMIT THRESHOLD

The LM27402 exploits the filter inductor DCR (DC resistance)

to detect over current events. If desired, the user can employ

inductors with low tolerance DCR to increase the accuracy of

the current limit threshold. The most common topology for

sensing the DCR is shown in Figure 10.

LM27402 Datasheet(PDF) 16 Page - National Semiconductor (TI)

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