V

=3.6V

I

=150mA

IN

OUT

V

=5V

OUT

EN=5V/div

I

=200mA/div

COIL

V

=2V/div

OUT

t-TimeBase-100

s/div

m

V

=3.6V

I

=150mA

IN

OUT

V

=5V

OUT

EN=5V/div

I

=200mA/div

COIL

V

=1V/div

OUT

V

=1V/div

IN

t-TimeBase-200

s/div

m

DETAILED DESCRIPTION

OPERATION

CURRENT LIMIT OPERATION

-

-

´

D

D

´

»

OUT

IN

IN

OUT(CL)

VALLEY

L

L

OUT

V

V

1

V

D

I

= (1

D)

(I

+

I )

with I =

and D

2

L

f

V

(1)

TPS61240, TPS61241

Figure 15. Startup After Enable – With Load

Figure 16. Startup and Shutdown

The TPS6124x Boost Converter operates with typically 3.5MHz fixed frequency pulse width modulation (PWM) at

moderate to heavy load currents. At light load currents the converter will automatically enter Power Save Mode

and operates then in PFM (Pulse Frequency Modulation) mode. During PWM operation the converter uses a

unique fast response quasi-constant on-time valley current mode controller scheme which allows “Best in Class”

line and load regulation allowing the use of small ceramic input and output capacitors.

cycle, the low-side N-MOS switch is turned-on and the inductor current ramps up to a defined peak current. In

the second phase, once the peak current is reached, the current comparator trips, the on-timer is reset turning off

the switch, and the current through the inductor then decays to an internally set valley current limit. Once this

occurs, the on-timer is set to turn the boost switch back on again and the cycle is repeated.

The current limit circuit employs a valley current sensing scheme. Current limit detection occurs during the off

time through 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 as shown.

Figure 17 illustrates the inductor and rectifier current waveforms during current limit operation. The output

the lower peak is above the current limit threshold, the off time is lengthened to allow the current to decrease to

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

10

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