LMZ14203 datasheet(11/18 Pages) NSC

Nome de Peças	LMZ14203

Descrição Electrónicos	3A SIMPLE SWITCHER짰Power Module with 42V Maximum Input Voltage

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Fabricante Electrônico	NSC [National Semiconductor (TI)]

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natively, V

IN(MAX) can also be limited in order to keep the

frequency unchanged.

Additionally note, the minimum off-time of 260 ns limits the

maximum duty ratio. Larger R

ON (lower FSW) should be se-

lected in any application requiring large duty ratio.

Discontinuous Conduction and Continuous Conduction

Modes

At light load the regulator will operate in discontinuous con-

duction mode (DCM). With load currents above the critical

conduction point, it will operate in continuous conduction

mode (CCM). When operating in DCM the switching cycle

begins at zero amps inductor current; increases up to a peak

value, and then recedes back to zero before the end of the

off-time. Note that during the period of time that inductor cur-

rent is zero, all load current is supplied by the output capacitor.

The next on-time period starts when the voltage on the at the

FB pin falls below the internal reference. The switching fre-

quency is lower in DCM and varies more with load current as

compared to CCM. Conversion efficiency in DCM is main-

tained since conduction and switching losses are reduced

with the smaller load and lower switching frequency. Operat-

ing frequency in DCM can be calculated as follows:

f

SW(DCM)≊VO*(VIN-1)*6.8μH*1.18*10

20*I

O/(VIN–VO)*RON

2 (15)

In CCM, current flows through the inductor through the entire

switching cycle and never falls to zero during the off-time. The

switching frequency remains relatively constant with load cur-

rent and line voltage variations. The CCM operating frequen-

cy can be calculated using equation 7 above.

Following is a comparison pair of waveforms of the showing

both CCM (upper) and DCM operating modes.

CCM and DCM Operating Modes

V

IN = 24V, VO = 3.3V, IO = 3A/0.4A 2 μsec/div

30107012

The approximate formula for determining the DCM/CCM

boundary is as follows:

I

DCB≊VO*(VIN–VO)/(2*6.8 μH*fSW(CCM)*VIN) (16)

Following is a typical waveform showing the boundary condi-

tion.

Transition Mode Operation

V

IN = 24V, VO = 3.3V, IO = 0.5 A 2 μsec/div

30107014

The inductor internal to the module is 6.8

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). ILR can be

calculated with:

I

LR P-P=VO*(VIN- VO)/(6.8µH*fSW*VIN) (17)

Where V

IN is the maximum input voltage and fSW is deter-

mined from equation 10.

If the output current I

O is determined by assuming that IO =

I

L, the higher and lower peak of ILR can be determined. Be

aware that the lower peak of I

LR must be positive if CCM op-

eration is required.

POWER DISSIPATION AND BOARD THERMAL

REQUIREMENTS

For the design case of V

IN = 24V, VO = 3.3V, IO = 3A, TAMB

(MAX) = 85°C , and TJUNCTION = 125°C, the device must see a

thermal resistance from case to ambient of:

θ

CA< (TJ-MAX — TAMB(MAX)) / PIC-LOSS - θJC (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 2.25W.

θ

CA <(125 — 85) / 2.25W — 1.9 = 15.8

To reach

θ

CA = 15.8, the PCB is required to dissipate heat

effectively. With no airflow and no external heat, a good esti-

mate 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

CA (19)

As a result, approximately 31.5 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, 10mils (254

μm) thermal vias spaced

59mils (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.

PC BOARD LAYOUT GUIDELINES

PC board layout is an important part of DC-DC converter de-

sign. 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

11

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