LMZ14203HTZXSLASHNOPB datasheet(19/31 Pages) TI

19 / 31 page

LMZ14203H

www.ti.com

SNVS692G – JANUARY 2011 – REVISED OCTOBER 2015

Typical Application (continued)

Worst-case input ripple current rating is dictated by Equation 12.

I(CIN(RMS)) ≊ 1 / 2 × IO × √ (D / 1-D)

where

•

≊ VO / VIN

(12)

(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 10-uF X7R 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 deratings 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

rating.

If the system design requires a certain maximum value of input ripple voltage

ΔVIN to be maintained then

Equation 13 may be used.

CIN ≥ IO × D × (1–D) / fSW-CCM × ΔVIN

(13)

ΔVIN is 1% of VIN for a 24-V input to 12V output application this equals 240 mV and fSW = 400 kHz.

CIN≥ 3 A × 12 V/24 V × (1– 12 V/24 V) / (400000 × 0.240 V)

(14)

CIN≥ 7.8 μF

(15)

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.

8.2.2.1.6

ON-Time, RON, Resistor Selection

Many designs will begin with a desired switching frequency in mind. As seen in the Typical Characteristics

section, the best efficiency is achieved in the 300kHz-400kHz switching frequency range. Equation 16 can be

used to calculate the RON value.

fSW(CCM) ≊ VO / (1.3 × 10

-10 × R

ON)

(16)

This can be rearranged as

RON ≊ VO / (1.3 × 10

-10 × f

SW(CCM)

(17)

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 LMZ14203H timer is determined by the resistor RON and the input voltage VIN. It is calculated

as follows:

tON = (1.3 × 10

-10 x R

ON) / VIN

(18)

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 19.

fSW(MAX) = VO / (VIN(MAX) × 150 ns)

(19)

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) × 150 nsec / (1.3 × 10

-10)

(20)

If RON calculated in Equation 17 is less than the minimum value determined in Equation 20, 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 (typical) limits the maximum duty ratio. Larger RON (lower FSW)

should be selected in any application requiring large duty ratio.

8.2.2.1.6.1

Discontinuous Conduction and Continuous Conduction Mode Selection

Operating frequency in DCM can be calculated as follows:

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

20 × I

O / (VIN–VO) × RON

(21)

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