LMZ14202HTZXSLASHNOPB datasheet(16/28 Pages) TI

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LMZ14202H

SNVS691D – JANUARY 2011 – REVISED FEBRUARY 2013

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VO = 0.8V x (1 + RFBT / RFBB)

(2)

Rearranging terms; the ratio of the feedback resistors for a desired output voltage is:

RFBT / RFBB = (VO / 0.8V) - 1

(3)

These resistors should be chosen from values in the range of 1 k

Ω to 50 kΩ.

A feed-forward capacitor is placed in parallel with RFBT to improve load step transient response. Its value is

usually determined experimentally by load stepping between DCM and CCM conduction modes and adjusting for

best transient response and minimum output ripple.

A table of values for RFBT , RFBB , and RON is included in the simplified applications schematic.

SOFT-START CAPACITOR, CSS, SELECTION

Programmable soft-start permits the regulator to slowly ramp to its steady state operating point after being

enabled, thereby reducing current inrush from the input supply and slowing the output voltage rise-time to

prevent overshoot.

Upon turn-on, after all UVLO conditions have been passed, an internal 8uA current source begins charging the

external soft-start capacitor. The soft-start time duration to reach steady state operation is given by the formula:

tSS = VREF x CSS / Iss = 0.8V x CSS / 8uA

(4)

This equation can be rearranged as follows:

CSS = tSS x 8 μA / 0.8V

(5)

Use of a 4700pF capacitor results in 0.5ms soft-start duration. This is a recommended value. Note that high

values of CSS capacitance will cause more output voltage droop when a load transient goes across the DCM-

CCM boundary. Use Equation 18 below to find the DCM-CCM boundary load current for the specific operating

condition. If a fast load transient response is desired for steps between DCM and CCM mode the softstart

capacitor value should be less than 0.018µF.

As the soft-start input exceeds 0.8V the output of the power stage will be in regulation. Note that the following

conditions will reset the soft-start capacitor by discharging the SS input to ground with an internal 200

μA current

sink:

•

The enable input being “pulled low”

•

Thermal shutdown condition

•

Over-current fault

•

Internal VINUVLO

OUTPUT CAPACITOR, CO, SELECTION

None of the required output capacitance is contained within the module. At a minimum, the output capacitor must

meet the worst case RMS current rating of 0.5 x ILR P-P, as calculated in Equation 19. Beyond that, additional

capacitance will reduce output ripple so long as the ESR is low enough to permit it. A minimum value of 10

μF is

generally required. Experimentation will be required if attempting to operate with a minimum value. Low ESR

capacitors, such as ceramic and polymer electrolytic capacitors are recommended.

CAPACITANCE:

Equation 6 provides a good first pass approximation of CO for load transient requirements:

CO≥ISTEP x VFB x L x VIN/ (4 x VO x (VIN — VO) x VOUT-TRAN)

(6)

As an example, for 2A load step, VIN = 24V, VOUT = 12V, VOUT-TRAN = 50mV:

CO≥ 2A x 0.8V x 15μH x 24V / (4 x 12V x ( 24V — 12V) x 50mV)

CO≥ 20μF

ESR:

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