MIC2128 datasheet(24/32 Pages) MICROCHIP

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MIC2128

DS20005620A-page 24

 2016 Microchip Technology Inc.

FIGURE 5-7:

Invisible Ripple at FB.

The injected ripple at the FB pin in this case is given by

the Equation 5-28:.

EQUATION 5-28:

In the above Equation 5-28, it is assumed that the time

constant associated with the CFF meets the criteria

shown in Equation 5-29.

EQUATION 5-29:

The process of sizing the ripple injection resistor and

capacitors is:

Select CINJ in the range of 47 nF to 100 nF,

which can be considered as short for a wide

range of the frequencies.

Select CFF in the range of 0.47 nF to 10 nF, if R1

and R2 are in k range.

Select RINJ according to Equation 5-30.

EQUATION 5-30:

Once all the ripple injection component values are cal-

culated, ensure that the criteria shown in the

Equation 5-29 is met.

5.9

Power Dissipation in MIC2128

The MIC2128 features two Low Dropout Regulators

(LDOs) to supply power at the PVDD pin from either VIN

or EXTVDD depending on the voltage at the EXTVDD

pin. PVDD powers MOSFET drivers and VDD pin, which

is recommended to connect to PVDD through a low

pass filter, powers the internal circuitry. In the applica-

tions where the output voltage is 5V and above (up to

14V), it is recommended to connect EXTVDD to the

output to reduce the power dissipation in the MIC2128,

to reduce the MIC2128 junction temperature and to

improve the system efficiency.

The power dissipation in the MIC2128 depends on the

internal LDO being in use, gate charge of the external

MOSFETs and switching frequency. The power dissi-

pation and the junction temperature of the MIC2128

can

estimated

using

the

Equation 5-31,

Equation 5-32 and Equation 5-33.

Power dissipation in the MIC2128 when EXTVDD is not

used:

EQUATION 5-31:

Power dissipation in the MIC2128 when EXTVDD is

used:

EQUATION 5-32:

The junction temperature of the MIC2128 can be esti-

mated using Equation 5-33.

MIC2128

COUT

ESR

CFF

RINJ

CINJ

V

FB PP



OUT

–





INJ



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

 T



 C

1 R2 RINJ







INJ

OUT

–





V

FB PP





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

Where:

VOUT

= Output voltage

= Duty cycle

fSW

= Switching frequency

VFB(pp) = Feedback Ripple





EXTVDD





G_HS

G_LS

Where:

ISW

= Switching current into the VIN pin

= Quiescent current (1.4 mA typ)

= Total gate charge of the external MOS-

FETs which is sum of the gate charge of

high-side MOSFET (QG_HS) and the

low-side MOSFET (QG_LS) at 5V gate to

source voltage. Gate charge information

can be obtained from the MOSFETs

datasheet.

VEXTVDD = Voltage at the EXTVDD pin

(4.6 ≤ VEXTVDD ≤ 14 V typ.)

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