MIC2128 datasheet(20/32 Pages) MICROCHIP

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MIC2128

DS20005620A-page 20

 2016 Microchip Technology Inc.

5.3

Setting the Soft-Start Time

The output Soft-Start time can be set by connecting a

capacitor from SS to AGND from 2 ms to 100 ms as

shown in Figure 5-4.

FIGURE 5-4:

Setting the Soft-Start Time.

The value of the capacitor can be calculated using

Equation 5-4.

EQUATION 5-4:

5.4

MOSFET Selection

Important parameters for MOSFET selection are:

• Voltage rating

• On-resistance

• Total gate charge

The voltage rating for the high-side and low-side

MOSFETs are essentially equal to the power stage

input voltage VIN. A safety factor of 30% should be

added to the VIN(MAX) while selecting the voltage rating

of the MOSFETs to account for voltage spikes due to

circuit parasitic elements.

5.4.1

HIGH-SIDE MOSFET POWER

LOSSES

The total power loss in the high-side MOSFET

(PHSFET) is the sum of the power losses because of

conduction (PCONDUCTION), switching (PSW), reverse

recovery charge of low-side MOSFET body diode

(PQrr) and MOSFET's output capacitance discharge as

calculated in the Equation 5-5.

EQUATION 5-5:

EQUATION 5-6:

ILOAD is the load current and D is the operating duty

cycle, given by Equation 5-7.

EQUATION 5-7:

MIC2128

0.6V

VREF

Comparator

COMPENSATION

CSS

ISS

1.3 µA



REF

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

Where:

CSS

= Capacitor from SS pin to AGND

ISS

= Internal Soft-Start current (1.3 µA typical)

tSS

= Output Soft-Start time

VREF

=0.6V

HSFET

CONDUCTION HS



SW HS



Qrr

COSS

CONDUCTION HS



RMS HS



DS ON_HS





SW HS



0.5



LOAD







Qrr



COSS

---C

OSS HS



OSS HS





Where:

RDS(ON_HS) = On-resistance of the high-side MOSFET

VIN

= Operating input voltage

ILOAD

= Load current

fSW

= Operating switching frequency

Qrr

= Reverse recovery charge of low-side

MOSFET body diode or of external

diode across low-side MOSFET

COSS(HS)

= Effective high-side MOSFET output

capacitance

COSS(LS)

= Effective low-side MOSFET

output

capacitance

IRMS(HS)

= RMS current of the high-side MOSFET

which

can

calculated

using

Equation 5-6.

tR, tF

= The high-side MOSFET turn-on and

turn-off transition times which can be

approximated by Equation 5-8 and

Equation 5-9

RMS HS



LOAD



OUT

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

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