BLE18PS080SN1 datasheet(19/36 Pages) TI

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7.4.2 Low Duty Cycle Operation

For high input voltages or low output voltages, the 70-nsec minimum on-time limits the maximum input to output

voltage difference and the switching frequency selected. When the minimum on-time is reached, the output

voltage rises above the regulation point. Refer to Table 8-2 for detailed design recommendations.

7.4.3 High Duty Cycle Operation (100% Duty Cycle)

The device offers a low input-to-output voltage differential by entering 100% duty cycle mode. In this mode, the

high-side MOSFET switch is constantly turned on. The minimum input voltage to maintain output voltage

regulation, depending on the load current and the output voltage level, is calculated as:

(min)

(

)

(

)

OUT

DS ON

(2)

where

• VOUT(min) is the minimum output voltage the load can accept

• IOUT is the output current

• RDS(ON) is the RDS(ON) of the high-side MOSFET

• RL is the DC resistance of the inductor used

To maintain fixed frequency switching, the device requires a minimum off-time of 50 ns (typ), 60 ns (max). If this

limit is reached during a switching pulse, the device skips switching pulses to maintain output voltage regulation.

If the input voltage decreases further, the device enters 100% mode.

7.4.4 Second Stage L-C Filter Compensation (Optional)

Most low-noise and low-ripple applications use a ferrite bead and bypass capacitor before the load. Using a

second L-C filter is especially useful for low-noise and low-ripple applications with constant load current such as

ADCs, DACs, and Jitter Cleaner. The second stage L-C filter is optional, and the device can be used without this

filter. Without the filter, the device has a low output voltage noise of typically 18.3 μVRMS shown in Figure 6-32

with an output voltage ripple of 280 μVRMS shown in Figure 6-12. The second stage L-C filter attenuates the

output voltage ripple by another approximately 30 dB shown in Figure 6-14. To improve load regulation, the

device can remote sense the output voltage after the second stage L-C filter and is internally compensated for

the additional double pole generated by the L-C filter.

To keep the second stage L-C filter as small as possible, the internal compensation is optimized for a 10-nH to

50-nH inductance. A small ferrite bead or even a PCB trace provides sufficient inductance for output voltage

ripple filtering. See Section 8.2.2.2.4 for details.

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TPS62912, TPS62913

SLVSFP4 – AUGUST 2020

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BLE18PS080SN1 Datasheet(PDF) 19 Page - Texas Instruments