Datasheet

LMZ12003EXT
www.ti.com SNVS663G –JUNE 2010–REVISED OCTOBER 2013
This equation can be used to select R
ON
if a certain operating frequency is desired so long as the minimum on-
time of 150 ns is observed. The limit for R
ON
can be calculated as follows:
R
ON
≥ V
IN(MAX)
* 150 nsec / (1.3 * 10
-10
) (11)
If R
ON
calculated in Equation 8 is less than the minimum value determined in Equation 11 a lower frequency
should be selected. Alternatively, V
IN(MAX)
can also be limited in order to keep the frequency unchanged.
Additionally note, the minimum off-time of 260 ns limits the maximum duty ratio. Larger R
ON
(lower F
SW
) should
be selected in any application requiring large duty ratio.
Discontinuous Conduction and Continuous Conduction Modes
At light load the regulator will operate in discontinuous conduction mode (DCM). With load currents above the
critical conduction point, it will operate in continuous conduction mode (CCM). When operating in DCM the
switching cycle begins at zero amps inductor current; increases up to a peak value, and then recedes back to
zero before the end of the off-time. Note that during the period of time that inductor current is zero, all load
current is supplied by the output capacitor. The next on-time period starts when the voltage on the at the FB pin
falls below the internal reference. The switching frequency is lower in DCM and varies more with load current as
compared to CCM. Conversion efficiency in DCM is maintained since conduction and switching losses are
reduced with the smaller load and lower switching frequency. Operating frequency in DCM can be calculated as
follows:
f
SW(DCM)
≊V
O
*(V
IN
-1)*6.8μH*1.18*10
20
*I
O
/(V
IN
–V
O
)*R
ON
2
(12)
In CCM, current flows through the inductor through the entire switching cycle and never falls to zero during the
off-time. The switching frequency remains relatively constant with load current and line voltage variations. The
CCM operating frequency can be calculated using Equation 7 above.
Following is a comparison pair of waveforms of the showing both CCM (upper) and DCM operating modes.
Figure 24. CCM and DCM Operating Modes
V
IN
= 12V, V
O
= 3.3V, I
O
= 3A/0.4A 2 μsec/div
The approximate formula for determining the DCM/CCM boundary is as follows:
I
DCB
≊V
O
*(V
IN
–V
O
)/(2*6.8 μH*f
SW(CCM)
*V
IN
) (13)
Following is a typical waveform showing the boundary condition.
Figure 25. Transition Mode Operation
V
IN
= 12V, V
O
= 3.3V, I
O
= 0.5 A 2 μsec/div
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