Vout

10 mV/Div

5V/Div

IL

200 mA/Div

1 Ps/DIV

0

10

20

30

40

0

50

100

150

200

250

22 PH

15PH

22PH

15PH

500 kHz

330 kHz

LM26001, LM26001Q

www.ti.com

SNVS430F – MAY 2006 – REVISED NOVEMBER 2010

Figure 17. Sleep Mode Threshold vs Vin Vout = 3.3V

Below the sleep threshold, decreasing load current results in longer sleep cycles, which can be quantified as

shown below:

Dwake = Iload/Isleep

(2)

Where Dwake is the percentage of time awake when the load current is below the sleep threshold.

Sleep mode combined with low IQ operation minimizes the input supply current. Input supply current in sleep

mode can be calculated based on the wake duty cycle, as shown below:

(3)

SW

And Io is the sum of Iload, Ibias, and current through the feedback resistors.

FPWM

Pulling the FPWM pin high disables sleep mode and forces the LM26001 to always operate in PWM mode. Light

load efficiency is reduced in PWM mode, but switching frequency remains stable. The FPWM pin can be

connected to the VDD pin to pull it high. In FPWM mode, under light load conditions, the regulator operates in

discontinuous conduction mode (DCM) . In discontinuous conduction mode, current through the inductor starts at

zero and ramps up to its peak, then ramps down to zero again. Until the next cycle, the inductor current remains

at zero. At nominal load currents, in FPWM mode, the device operates in continuous conduction mode, where

positive current always flows in the inductor. Typical discontinuous operation waveforms are shown in Figure 18.

Figure 18. Discontinuous Mode Waveforms 75mA Load, Vin = 12V

At very light load, in FPWM mode, the LM26001 may enter sleep mode. This is to prevent an over-voltage

condition from occurring. However, the FPWM sleep threshold is much lower than in normal operation.

Copyright © 2006–2010, Texas Instruments Incorporated

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