Datasheet

VIN
GND
V
IN
V
O
C
in1
C
O1
Loop 1
Loop 2
VOUT
High
di/dt
LMZ22005
SNVS686I –MARCH 2011–REVISED OCTOBER 2013 www.ti.com
If ΔV
IN
is 1% of V
IN
for a 12V input to 3.3V output application this equals 120 mV and f
SW
= 812 kHz.
C
IN
≥ 5A * 3.3V/12V * (1– 3.3V/12V) / (812000 * 0.120 V)
≥ 10.2μF
Additional bulk capacitance with higher ESR may be required to damp any resonant effects of the input
capacitance and parasitic inductance of the incoming supply lines. The LMZ22005 typical applications schematic
recommends a 150 μF 50V aluminum capacitor for this function. There are many situations where this capacitor
is not necessary.
POWER DISSIPATION AND BOARD THERMAL REQUIREMENTS
When calculating module dissipation use the maximum input voltage and the average output current for the
application. Many common operating conditions are provided in the characteristic curves such that less common
applications can be derived through interpolation. In all designs, the junction temperature must be kept below the
rated maximum of 125°C.
For the design case of V
IN
= 12V, V
O
= 3.3V, I
O
= 5A, and T
AMB(MAX)
= 85°C, the module must see a thermal
resistance from case to ambient of less than:
θ
CA
< (T
J-MAX
– T
A-MAX
) / P
IC-LOSS
- θ
JC
(10)
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 4.3W.
θ
CA
= (125 – 85) / 4.3W – 1.9 = 7.4 (11)
To reach θ
CA
= 7.4, the PCB is required to dissipate heat effectively. With no airflow and no external heat-sink, a
good estimate of the required board area covered by 2 oz. copper on both the top and bottom metal layers is:
Board_Area_cm
2
= 500°C x cm
2
/W / θ
CA
(12)
As a result, approximately 67square cm of 2 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 sixty, 8mils thermal vias
spaced 39 mils (1.0 mm) apart connect the top copper to the bottom copper. For an example of a high thermal
performance PCB layout for SIMPLE SWITCHER© power modules, refer to AN-2085, AN-2125, 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. A good example layout is shown in
Figure 51.
Figure 48.
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