LMZ22005 datasheet(21/34 Pages) TI

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LMZ22005

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SNVS686J – MARCH 2011 – REVISED AUGUST 2015

For applications with dynamic load steps; the following equation provides a good first pass approximation of CO

for load transient requirements. Where VO-Tran is 100 mV on a 3.3-V output design.

CO ≥ IO-Tran / (VO-Tran – ESR × IO-Tran) × (Fsw / VO)

(8)

Solving:

CO ≥ 4.5 A / (0.1 V – 0.007 × 4.5 A) × ( 800000 Hz / 3.3 V) ≥ 271 μF

(9)

NOTE

The stability requirement for 200-µF minimum output capacitance will take precedence.

One recommended output capacitor combination is a 220-µF, 7-m

Ω ESR specialty polymer cap in parallel with a

100-µF, 6.3-V X5R ceramic. This combination provides excellent performance that may exceed the requirements

of certain applications. Additionally some small ceramic capacitors can be used for high-frequency EMI

suppression.

8.2.2.7 CIN Selection

The LMZ22005 module contains only a small amount of input capacitance. Additional input capacitance is

required external to the module to handle the input ripple current of the application. The input capacitor can be

several capacitors in parallel. This input capacitance must be located in very close proximity to the module. Input

capacitor selection is generally directed to satisfy the input ripple current requirements rather than by

capacitance value. Input ripple current rating is dictated by the equation:

I(CIN(RMS)) ≊ 1 / 2 × IO × SQRT (D / 1 – D)

where

•

≊ VO / VIN

(10)

As a point of reference, the worst case ripple current will occur when the module is presented with full load

current and when VIN = 2 × VO.

Recommended minimum input capacitance is 22-µF X7R (or X5R) ceramic with a voltage rating at least 25%

higher than the maximum applied input voltage for the application. TI recommends to pay attention to the voltage

and temperature derating of the capacitor selected. The 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 (

ΔVIN) be maintained

then the following equation may be used.

CIN ≥ IO × D × (1 – D) / fSW-CCM × ΔVIN

(11)

ΔVIN is 1% of VIN for a 12-V input to 3.3-V output application this equals 120 mV and fSW = 812 kHz.

CIN ≥ 5 A × 3.3 V / 12 V × (1 – 3.3 V / 12 V) / (812000 × 0.120 V) ≥ 10.2 μF

(12)

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 50-V aluminum capacitor for this function. There are many situations where this capacitor

is not necessary.

8.2.2.8 Discontinuous And Continuous Conduction Modes Selection

The approximate formula for determining the DCM/CCM boundary is as follows:

IDCB ≊ VO × (VIN– VO) / (2 × 3.3 μH × fSW(CCM) × VIN)

(13)

The inductor internal to the module is 3.3

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 (ILR). ILR can be calculated with:

ILR P-P = VO× (VIN– VO) / (3.3 µH × fSW × VIN)

where

•

VIN is the maximum input voltage

•

And fSW is typically 812 kHz.

(14)

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