LMZ12003EXT datasheet(15/29 Pages) TI

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LMZ12003EXT

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SNVS663H – JUNE 2010 – REVISED AUGUST 2015

8.2.2.5 CIN Selection

The LMZ12003EXT module contains an internal 0.47-µF input ceramic capacitor. Additional input capacitance is

required external to the module to handle the input ripple current of the application. 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. Worst case input ripple current rating is dictated by

the equation:

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

where

•

≊ VO / VIN

(5)

(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-µF X7R ceramic with a voltage rating at least 25% higher than

the maximum applied input voltage for the application. It is also recommended that attention be paid to the

voltage and temperature deratings 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 rating.

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

ΔVIN be maintained then the

following equation may be used.

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

(6)

ΔVIN is 1% of VIN for a 20V input to 3.3-V output application this equals 200 mV and fSW = 400 kHz.

CIN ≥ 3 A × 3.3 V / 20 V × (1 – 3.3 V / 20 V) / (400000 × 0.200 V)

≥ 5.2 μF

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.

8.2.2.6 RON Resistor selection

Many designs will begin with a desired switching frequency in mind. For that purpose the following equation can

be used.

fSW(CCM) ≊ VO / (1.3 × 10

-10 × R

ON)

(7)

This can be rearranged as

RON ≊ VO / (1.3 × 10

-10 × f

SW(CCM))

(8)

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

control section.

The ON-time of the LMZ12003EXT timer is determined by the resistor RON and the input voltage VIN. It is

calculated as follows:

tON = (1.3 × 10

-10 × R

ON) / VIN

(9)

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

equation:

fSW(MAX) = VO / (VIN(MAX) × 150 ns)

(10)

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 ns / (1.3 × 10

-10)

(11)

If RON calculated in Equation 8 is less than the minimum value determined in Equation 11 a lower frequency

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

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