f

D

L

V

I

IN

L

×

=

I

= I

VALLEY

LIM

IL

f

Rectifier

Current

Inductorr

Current

Increased

Load Current

Current Limit

Threshold

IN

L

V

D

I =

L

f

D

g

IN

OUT

V

D

1

V

h

=

-

g

-

D

g

OUT(CL)

VALLEY

L

1

I

= (1 D)

(I

+

I )

2

16

TPS61253, TPS61254, TPS61256, TPS61258, TPS61259, TPS612592

SLVSAG8G – SEPTEMBER 2011 – REVISED JUNE 2016

www.ti.com

Product Folder Links: TPS61253 TPS61254 TPS61256 TPS61258 TPS61259 TPS612592

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Copyright © 2011–2016, Texas Instruments Incorporated

9.3 Feature Description

9.3.1 Current Limit Operation

The TPS6125x device employs a valley current limit sensing scheme. Current limit detection occurs during the

off-time by sensing of the voltage drop across the synchronous rectifier.

The output voltage is reduced as the power stage of the device operates in a constant current mode. The

Equation 1.

(1)

The duty cycle (D) can be estimated by Equation 2

(2)

and the peak-to-peak current ripple (

(3)

increased such that the lower peak is above the current limit threshold, the off-time is increased to allow the

current to decrease to this threshold before the next on-time begins (so called frequency fold-back mechanism).

When the current limit is reached the output voltage decreases during further load increase.

Figure 26 illustrates the inductor and rectifier current waveforms during current limit operation.

Figure 26. Inductor/Rectifier Currents in Current Limit Operation

9.3.2 Enable

The TPS6125x device starts operation when EN is set high and starts up with the soft-start sequence. For proper

operation, the EN pin must be terminated and must not be left floating.

Pulling the EN and BP pins low forces the device in shutdown, with a shutdown current of typically 1 µA. In this

Pulling the EN pin low and the BP pin high forces the device in standby mode, refer to the Standby Mode section

for more details.