LMZ12001EXT Datasheet, PDF (15/23 Pages) Texas Instruments – 1A SIMPLE SWITCHER® Power Module with 20V Maximum Input Voltage for Military and Rugged Applications

V_IN = 12V, V_O = 3.3V, I_O = 0.29A
LMZ12001EXT
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The inductor internal to the module is 10 μH. 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*(V_IN- V_O)/(10µH*f_SW*V_IN) (17)
Where V_IN is the maximum input voltage and f_SW is determined from equation 10.
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 the design case of V_IN = 12V, V_O = 1.8V, I_O = 1A, T_AMB(MAX) = 85°C , and T_JUNCTION = 125°C, the device must see a thermal
resistance from case to ambient of:
θ_CA< (T_J-MAX — T_AMB(MAX)) / P_IC-LOSS - θ_JC (18)
Given the typical thermal resistance from junction to case to be 1.9 °C/W .Use the 85°C power dissipation curves in the Typical
Performance Characteristics section to estimate the P_IC-LOSS for the application being designed. In this application it is 0.4W
θ_CA< (125 — 85) / 0.4W —1.9 = 98.1
To reach θ_CA = 98.1, the PCB is required to dissipate heat effectively. With no airflow and no external heat, a good estimate of the
required board area covered by 1 oz. copper on both the top and bottom metal layers is:
Board Area_cm² = 500°C x cm²/W / θ_JC (19)
As a result, approximately 5 square cm of 1 oz copper on top and bottom layers is required for the PCB design. The PCB copper
heat sink must be connected to the exposed pad. Approximately thirty six, 10 mils (254 μm) thermal vias spaced 59 mils (1.5 mm)
apart must connect the top copper to the bottom copper. For an example of a high thermal performance PCB layout, refer to the
Evaluation Board application note AN–2024. For more information on thermal design see AN–2020 and AN–2026.
PC BOARD LAYOUT GUIDELINES
PC board layout is an important part of DC-DC converter design. Poor board layout can disrupt the performance of a DC-DC
converter and surrounding circuitry by contributing to EMI, ground bounce and resistive voltage drop in the traces. These can send
erroneous signals to the DC-DC converter resulting in poor regulation or instability. Good layout can be implemented by following
a few simple design rules.
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