LMZ23605_14 Datasheet, PDF (17/25 Pages) Texas Instruments – 5A SIMPLE SWITCHER® Power Module with 36V Maximum Input Voltage

Tracking option input detail
FIGURE 2.
30116915
C_O SELECTION
None of the required C_O output capacitance is contained with-
in the module. A minimum value of 200 μF is required based
on the values of internal compensation in the error amplifier.
Low ESR tantalum, organic semiconductor or specialty poly-
mer capacitor types are recommended for obtaining lowest
ripple. The output capacitor C_O may consist of several ca-
pacitors in parallel placed in close proximity to the module.
The output capacitor assembly must also meet the worst case
minimum ripple current rating of 0.5 * I_{LR P-P}, as calculated in
equation (14) below. Beyond that, additional capacitance will
reduce output ripple so long as the ESR is low enough to per-
mit it. Loop response verification is also valuable to confirm
closed loop behavior.
For applications with dynamic load steps; the following equa-
tion provides a good first pass approximation of C_O for load
transient requirements. Where V_O-Tran is 100mV on a 3.3V
output design.
C_O≥I_O-Tran*/((V_O-Tran – ESR * I_O-Tran)*(Fsw / V_O)(6)
Solving:
C_O≥ 4.5A / ((0.1V – .007*4.5) * ( 800000 / 3.3) ≥ 271μF (7)
Note that the stability requirement for 200 µF minimum output
capacitance will take precedence.
One recommended output capacitor combination is a 220uF,
7 milliohm ESR specialty polymer cap in parallel with a 100
uF 6.3V X5R ceramic. This combination provides excellent
performance that may exceed the requirements of certain ap-
plications. Additionally some small ceramic capacitors can be
used for high frequency EMI suppression.
C_IN SELECTION
The LMZ23605 module contains a small amount of internal
ceramic input capacitors. Additional input capacitance is re-
quired external to the module to handle the input ripple current
of the application. The input capacitor can be several capac-
itors in parallel. This input capacitance should be located in
very close proximity to the module. Input capacitor selection
is generally directed to satisfy the input ripple current require-
ments rather than by capacitance value. Input ripple current
rating is dictated by the equation:
I(C_IN(RMS)) ≊ 1 /2 * I_O * SQRT (D / 1-D) (8)
where D ≊ V_O / V_IN
(As a point of reference, the worst case ripple current will oc-
cur when the module is presented with full load current and
when V_IN = 2 * V_O).
Recommended minimum input capacitance is 22uF X7R (or
X5R) ceramic with a voltage rating at least 25% higher than
the maximum applied input voltage for the application. It is
also recommended that attention be paid to the voltage and
temperature derating of the capacitor selected. It should be
noted that ripple current rating of ceramic capacitors may be
missing from the capacitor data sheet and you may have to
contact the capacitor manufacturer for this parameter.
If the system design requires a certain minimum value of
peak-to-peak input ripple voltage (ΔV_IN) be maintained then
the following equation may be used.
C_IN ≥ I_O * D * (1–D) / f_SW-CCM * ΔV_IN(9)
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 * 0.12 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
LMZ23605 typical applications schematic and evaluation
board include a 150 μF 50V aluminum capacitor for this func-
tion. 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 char-
acteristic curves such that less common applications can be
derived through interpolation. In all designs, the junction tem-
perature must be kept below the rated maximum of 125°C.
For the design case of V_IN = 24V, 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 5.5W. (Note that for package dissipations above 5W air
flow or external heat sinking may be required.)
θ_CA = (125 – 85) / 5.5W – 1.9 = 5.37 (11)
To reach θ_CA = 5.37., 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_cm² = 500°C x cm²/W / θ_CA (12)
As a result, approximately 93 square 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, 10mil (254 μm) 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.
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