LMZ14202HTZXSLASHNOPB datasheet(18/28 Pages) TI

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LMZ14202H

SNVS691D – JANUARY 2011 – REVISED FEBRUARY 2013

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The selection of RON and fSW(CCM) must be confined by limitations in the on-time and off-time for the COT Control

Circuit Overview section.

The on-time of the LMZ14202H timer is determined by the resistor RON and the input voltage VIN. It is calculated

as follows:

tON = (1.3 x 10

-10 x R

ON) / VIN

(14)

The inverse relationship of tON and VIN gives a nearly constant switching frequency as VIN is varied. RON should

be selected such that the on-time at maximum VIN is greater than 150 ns. The on-timer has a limiter to ensure a

minimum of 150 ns for tON. This limits the maximum operating frequency, which is governed by Equation 15:

fSW(MAX) = VO / (VIN(MAX) x 150 nsec)

(15)

This equation can be used to select RON if a certain operating frequency is desired so long as the minimum on-

time of 150 ns is observed. The limit for RON can be calculated as follows:

RON ≥ VIN(MAX) x 150 nsec / (1.3 x 10

-10)

(16)

If RON calculated in Equation 13 is less than the minimum value determined in Equation 16 a lower frequency

should be selected. Alternatively, VIN(MAX) can also be limited in order to keep the frequency unchanged.

Additionally, the minimum off-time of 260 ns (typ) limits the maximum duty ratio. Larger RON (lower FSW) should

be selected in any application requiring large duty ratio.

Discontinuous Conduction and Continuous Conduction Modes

At light load the regulator will operate in discontinuous conduction 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 current is zero, all load

current is supplied by the output capacitor. The next on-time period starts when the voltage on the FB pin falls

below the internal reference. The switching frequency is lower in DCM and varies more with load current as

compared to CCM. Conversion efficiency in DCM is maintained since conduction and switching losses are

reduced with the smaller load and lower switching frequency. Operating frequency in DCM can be calculated as

follows:

fSW(DCM)≊VO x (VIN-1) x 15μH x 1.18 x 10

20 x I

O / (VIN–VO) x RON

(17)

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 current and line voltage variations. The

CCM operating frequency can be calculated using Equation 12 above.

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

IDCB≊VOx (VIN–VO) / ( 2 x 15μH x fSW(CCM) x VIN)

(18)

The inductor internal to the module is 15

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 x (VIN- VO) / (15µH x fSW x VIN)

where

•

VIN is the maximum input voltage and fSW is determined from Equation 12.

(19)

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

determined. Be aware that the lower peak of ILR must be positive if CCM operation is required.

POWER DISSIPATION AND BOARD THERMAL REQUIREMENTS

For a design case of VIN = 24V, VOUT = 12V, IOUT = 2A, TAMB (MAX) = 85°C , and TJUNCTION = 125°C, the device

must see a maximum junction-to-ambient thermal resistance of:

θJA-MAX < (TJ-MAX - TAMB(MAX)) / PD

This

θJA-MAX will ensure that the junction temperature of the regulator does not exceed TJ-MAX in the particular

application ambient temperature.

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