MIC24045 datasheet(33/46 Pages) MICROCHIP

33 / 46 page

 2016 Microchip Technology Inc.

DS20005568A-page 33

MIC24045

For polarized output capacitor, compensator pole

placement at the ESR zero frequency is achieved, as

shown in Equation 7-20 below:

EQUATION 7-20:

For ceramic output capacitor, compensator pole place-

ment at N × fXO (N  5, N × fXO < fS) is achieved, as

detailed in Equation 7-21:

EQUATION 7-21:

7.8

Output Voltage Soft-Start Rate

The MIC24045 features internal, I2C programmable

soft-start, such that the output voltage can be smoothly

increased to the target regulation voltage. The soft-start

rate given in the Electrical Characteristics refers to the

error amplifier reference, and therefore the effective

soft-start rate value seen at the output of the module

has to be scaled according to the internal feedback

divider attenuation values listed in Table 7-2. To calcu-

late the effective output voltage soft-start slew rate

SS_SROUT, based on the particular output voltage set-

ting and the reference soft-start slew rate SS_SRx

(x = 0, 1, 2, 3 depending on selection), use the follow-

ing formula:

EQUATION 7-22:

7.9

Minimum TON and Minimum TOFF

Limitations

The valley current-mode control method utilized in the

MIC24045 allows very small minimum controllable ON

time (around 26 ns), so that it is possible to convert

from 19V down to very low voltages at high frequency.

Note that the high-side current limit circuit may not be

able to detect an overcurrent event if the ON time is

below the high side switch current limit leading edge

blanking time (LEB, see Electrical Characteristics).

Conversely, some minimum OFF time is needed for

valley

current-mode

modulator

operation.

This

TOFF(MIN) specification (see Electrical Characteristics)

may dictate a limit on the maximum attainable output

voltage for a given VIN voltage. The maximum

attainable output voltage (at no load) is calculated as

follows:

EQUATION 7-23:

It is advisable to use a safe headroom margin against

the calculated value of VOUT,max for DC load and good

dynamic performance.

OUT

ESR



-------------------------------

----------

–

------------------------------------------------

 R



----------

–



----------------------------------------------------------------

SS_SR

OUT

SS_SRx



where:

A = amplification (see Table 7-2 for A values.)

OUT,max

OFF(MIN)



–





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