LM43602 datasheet(16/44 Pages) TI2

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Enable

Internal SS Ramp

Ext Tracking Signal to SS pin

VOUT

Enable

Internal SS Ramp

Ext Tracking Signal to SS pin

VOUT

SS/TRK

RTRT

RTRB

EXT RAMP

LM43602-Q1

SNVSA83C – APRIL 2015 – REVISED OCTOBER 2017

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Product Folder Links: LM43602-Q1

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Feature Description (continued)

When an external voltage ramp is applied to the SS/TRK pin, the LM43602-Q1 FB voltage follows the ramp if the

ramp magnitude is lower than the internal soft-start ramp. A resistor divider pair can be used on the external

control ramp to the SS/TRK pin to program the tracking rate of the output voltage. The final voltage seen by the

SS/TRK pin should not fall below 1.2 V to avoid abnormal operation.

Figure 28. Soft Start Tracking External Ramp

VOUT tracked to external voltage ramps has options of ramping up slower or faster than the internal voltage ramp.

VFB always follows the lower potential of the internal voltage ramp and the voltage on the SS/TRK pin. Figure 29

shows the case when VOUT ramps slower than the internal ramp, while Figure 30 shows when VOUT ramps faster

than the internal ramp. Faster start-up time may result in inductor current tripping current protection during start-

up. Use with special care.

Figure 29. Tracking with Longer Start-up Time than the Internal Ramp

Figure 30. Tracking with Shorter Start-up Time than the Internal Ramp

7.3.7 Switching Frequency (RT) and Synchronization (SYNC)

The switching frequency of the LM43602-Q1 can be programmed by the impedance RT from the RT pin to

ground. The frequency is inversely proportional to the RT resistance. The RT pin can be left floating and the

LM43602-Q1 operates at 500-kHz default switching frequency. The RT pin is not designed to be shorted to

ground. For a desired frequency, typical RT resistance can be found by Equation 3. Table 1 gives typical RT

values with a given FS.

RT(kΩ) = 40200 / Freq (kHz) – 0.6

(3)

LM43602 Datasheet(PDF) 16 Page - Texas Instruments