Datasheet

LMZ14202H
SNVS691E –JANUARY 2011–REVISED OCTOBER 2013 www.ti.com
The selection of R
ON
and f
SW(CCM)
must be confined by limitations in the on-time and off-time for the COT Control
Circuit Overview section.
The on-time of the LMZ14202H timer is determined by the resistor R
ON
and the input voltage V
IN
. It is calculated
as follows:
t
ON
= (1.3 x 10
-10
x R
ON
) / V
IN
(14)
The inverse relationship of t
ON
and V
IN
gives a nearly constant switching frequency as V
IN
is varied. R
ON
should
be selected such that the on-time at maximum V
IN
is greater than 150 ns. The on-timer has a limiter to ensure a
minimum of 150 ns for t
ON
. This limits the maximum operating frequency, which is governed by Equation 15:
f
SW(MAX)
= V
O
/ (V
IN(MAX)
x 150 nsec) (15)
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)
x 150 nsec / (1.3 x 10
-10
) (16)
If R
ON
calculated in Equation 13 is less than the minimum value determined in Equation 16 a lower frequency
should be selected. Alternatively, V
IN(MAX)
can also be limited in order to keep the frequency unchanged.
Additionally, the minimum off-time of 260 ns (typ) 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 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
x (V
IN
-1) x 15μH x 1.18 x 10
20
x I
O
/ (V
IN
–V
O
) x R
ON
2
(17)
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 12 above.
The approximate formula for determining the DCM/CCM boundary is as follows:
I
DCB
≊V
O
x (V
IN
–V
O
) / ( 2 x 15μH x f
SW(CCM)
x V
IN
) (18)
The inductor internal to the module is 15μH. This value was chosen as a good balance between low and high
input voltage applications. The main parameter affected by the inductor is the amplitude of the inductor ripple
current (I
LR
). I
LR
can be calculated with:
I
LR P-P
=V
O
x (V
IN
- V
O
) / (15µH x f
SW
x V
IN
)
where
• V
IN
is the maximum input voltage and f
SW
is determined from Equation 12. (19)
If the output current I
O
is determined by assuming that I
O
= I
L
, the higher and lower peak of I
LR
can be
determined. Be aware that the lower peak of I
LR
must be positive if CCM operation is required.
POWER DISSIPATION AND BOARD THERMAL REQUIREMENTS
For a design case of V
IN
= 24V, V
OUT
= 12V, I
OUT
= 2A, T
AMB
(MAX) = 85°C , and T
JUNCTION
= 125°C, the device
must see a maximum junction-to-ambient thermal resistance of:
θ
JA-MAX
< (T
J-MAX
- T
AMB(MAX)
) / P
D
This θ
JA-MAX
will ensure that the junction temperature of the regulator does not exceed T
J-MAX
in the particular
application ambient temperature.
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