MIC24066 datasheet(22/38 Pages) MICROCHIP

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MIC24066/7

DS20006730A-page 22

 2022 Microchip Technology Inc. and its subsidiaries

EQUATION 4-4:

It is assumed that the time constant associated with

CFF must be much greater than the switching period:

EQUATION 4-5:

If the voltage divider resistors R1 and R2 are in the kΩ

range, a CFF of 1 nF to 22 nF can easily satisfy the

large time constant requirements. Also, an 100 nF

injection capacitor CINJ is used in order to be

considered as short for a wide range of the

frequencies.

The process of sizing the ripple injection resistor and

capacitors is:

Select CFF to feed all output ripples into the

feedback pin and make sure the large time con-

stant assumption is satisfied. Typical choice of

CFF is 1 nF to 22 nF if R1 and R2 are in kΩ

range.

Select RINJ according to the expected feedback

voltage ripple using Equation 4-6:

EQUATION 4-6:

Select CINJ as 100 nF, which could be

considered as short for a wide range of the

frequencies.

4.5

Detailed Device Description

The MIC24066 always operates in CCM and soft start

is adjustable with external capacitor. MIC24067 has

MODE selector for CCM/DCM and has fix internal sift

start.

4.5.1

HYPERLIGHT LOAD (HLL) MODE

(MIC24067)

In continuous mode, the inductor current can go

negative at light loads. However, at light loads the

MIC24067 is able to force the inductor current to

operate in discontinuous mode when MODE is set to

HLL Mode. In HLL mode, the efficiency is optimized by

shutting down all the non-essential circuits and

minimizing the supply current. The MIC24067 wakes

up and turns on the high-side MOSFET when the

feedback voltage VFB drops below 0.6V.

The MIC24067 has a zero crossing comparator (ZC

Detection) that monitors the inductor current by

sensing the voltage drop across the low-side MOSFET

during its ON-time. If the VFB >0.6V and the inductor

current goes slightly negative, then the MIC24067

automatically power down most of the IC circuitry and

goes into a low-power mode.

Once the MIC24067 goes into discontinuous mode,

both the high-side and low-side MOSFETs are kept in

off state. The load current is supplied by the output

capacitors and VOUT drops. If the drop of VOUT causes

VFB to go below VREF, then all the circuits will wake up

into normal continuous mode. Figure 4-10 shows the

control loop timing in discontinuous mode.

FIGURE 4-10:

MIC24066/7 Control Loop

Timing (HLL Mode).

During discontinuous mode, the bias current of most

circuits are reduced. As a result, the total power supply

current during discontinuous mode is only about

300 μA, allowing the MIC24066/7 to achieve high

efficiency in light load applications.

V

FB PP



VIN KDIV



–





fSW 



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



KDIV

R1 R2



RINJ R1 R2



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

Where:

VIN

= Power stage input voltage

= Duty Cycle

fSW

= Switching Frequency

= (R1//R2//RINJ) × CFF

fSW 



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



--- 1

KDIV

V

FB PP



VIN

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

fSW 



D1 D

–





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



Then the value or RINJ is obtained as:

RINJ

R1 R2





KDIV

------------ 1

–







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