Datasheet

Vout
10 mV/Div
V
SW
5V/Div
IL
200 mA/Div
1 Ps/DIV
0
10 20 30 40
V
IN
(V)
0
50
100
150
200
250
I
DC
(mA)
22 PH
15 PH
22 PH
15 PH
500 kHz
330 kHz
LM26001, LM26001Q
www.ti.com SNVS430G –MAY 2006–REVISED MAY 2006
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:
Iin = Iq + (I
QG
x Dwake) + (Io x D) (3)
Where I
QG
is the gate drive current, calculated as:
I
QG
= (4.6 x 10
-9
) x f
SW
And Io is the sum of Iload, Ibias, and current through the feedback resistors.
Because this calculation applies only to sleep mode, use the I
q_Sleep_VB
and I
BIAS_SLEEP
values from the Electrical
Characteristics. If VBIAS is connected to ground, use the same equation with Ibias equal to zero and I
q_Sleep_VDD
.
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.
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