iL =

(VIN - VOUT) x D

L x fSW

IDCB =

(VIN - VOUT) x D

2 x L x fSW

CIN

&

350 kHz x 200mV

& 28µF

10A x

x 1 -

3.3V

12V

3.3V

12V

¹

·

©

§

¹

·

©

§

CIN

&

IOUT x D x (1 - D)

fSW x VIN

ICIN-RMS = IOUT x D(1-D)

LMZ22010

SNVS687H – MARCH 2011 – REVISED AUGUST 2015

www.ti.com

8.2.2.7 CIN Selection

The LMZ22010 module contains two internal ceramic input capacitors. 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:

where

•

D

≊ VOUT / 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 × VOUT.

Recommended minimum input capacitance is 30-µF X7R (or X5R) ceramic with a voltage rating at least 25%

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

voltage and temperature derating of the capacitor selected.

NOTE

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) to be

maintained then the following equation may be used.

(11)

If

ΔVIN is 200 mV or 1.66% of VIN for a 12-V input to 3.3-V output application and fSW = 350 kHz then:

(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 LMZ22010 typical applications schematic

and evaluation board include 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 Conduction and Continuous Conduction Modes Selection

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

(13)

The inductor internal to the module is 2.2

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 (

ΔiL). ΔiL can be calculated with:

where

•

VIN is the maximum input voltage

•

and fSW is typically 359 kHz.

(14)

If the output current IOUT is determined by assuming that IOUT = IL, the higher and lower peak of ΔiL can be

determined.

22

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