Datasheet

0.0 0.5 1.0 1.5 2.0 2.5 3.0
0
1
2
3
4
5
POWER DISSIPATION (W)
OUTPUT CURRENT (A)
VIN = 15V
VIN = 24V
VIN = 30V
VIN = 36V
VIN = 42V
LMZ14203H
SNVS692D –JANUARY 2011–REVISED OCTOBER 2013 www.ti.com
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 10μH x 1.18 x 10
20
x I
O
/ (V
IN
–V
O
) x R
ON
2
(21)
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 16.
The approximate formula for determining the DCM/CCM boundary is as follows:
I
DCB
≊V
O
x (V
IN
–V
O
) / ( 2 x 10μH x f
SW(CCM)
x V
IN
) (22)
The inductor internal to the module is 10μH. 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
) / (10µH x f
SW
x V
IN
) (23)
Where V
IN
is the maximum input voltage and f
SW
is determined from Equation 16.
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
= 3A, T
AMB
(MAX) = 65°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.
To calculate the required θ
JA-MAX
we need to get an estimate for the power losses in the IC. The following graph
is taken form the TYPICAL PERFORMANCE CHARACTERISTICS section (Figure 22) and shows the power
dissipation of the LMZ14203H for V
OUT
= 12V at 85°C T
AMB
.
Figure 53. Power Dissipation V
OUT
= 12V T
AMB
= 85°C
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