MIC23451 datasheet(14/28 Pages) MICROCHIP

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MIC23451

DS20006662A-page 14



2022 Microchip Technology Inc. and its subsidiaries

In HLL mode, the inductor is charged with a fixed tON

pulse on the high-side switch (HSD). After this, the LSD

is switched on and current falls at a rate of VOUT/L. The

controller remains in HLL mode while the inductor

falling current is detected to cross approximately

–50 mA. When the LSD (or tOFF) time reaches its

minimum and the inductor falling current is no longer

able to reach this –50 mA threshold, the part is in CCM

mode and switching at a virtually constant frequency.

Once in CCM mode, the tOFF time does not vary.

Therefore, it is important to note that if L is large

enough, the HLL transition level will not be triggered.

That inductor is:

EQUATION 5-2:

5.4

Compensation

The MIC23451 is designed to be stable with a 0.47 µH

to 2.2 µH inductor with a 4.7 µF ceramic (X5R) output

capacitor.

5.5

Duty Cycle

The typical maximum duty cycle of the MIC23451 is

80%.

5.6

Efficiency Considerations

Efficiency is defined as the amount of useful output

power, divided by the amount of power supplied.

EQUATION 5-3:

Maintaining high efficiency serves two purposes. It

reduces power dissipation in the power supply,

reducing the need for heat sinks and thermal design

considerations, and it reduces current consumption for

battery-powered applications. Reduced current draw

from a battery increases the device’s operating time

and is critical in hand-held devices.

There are two types of losses in switching converters:

DC losses and switching losses. DC losses are the

power dissipation of I2R. Power is dissipated in the

high-side switch during the on cycle. Power loss is

equal to the high-side MOSFET RDS(ON) multiplied by

the switch current squared. During the off cycle, the

low-side

N-channel

MOSFET conducts,

also

dissipating power. Device operating current also

reduces efficiency. The product of the quiescent

(operating) current and the supply voltage represents

another DC loss. The current required to drive the

gates on and off at a constant 4 MHz frequency, and

the switching transitions, make up the switching losses.

FIGURE 5-2:

Efficiency Under Load.

Figure 5-2 shows an efficiency curve. From no load to

100 mA, efficiency losses are dominated by quiescent

current losses, gate drive, and transition losses. By

using the HyperLight Load mode, the MIC23451 can

maintain high efficiency at low output currents.

Over 100 mA, efficiency loss is dominated by MOSFET

RDS(ON) and inductor losses. Higher input supply

voltages will increase the gate-to-source voltage on the

internal MOSFETs, thereby reducing the internal

RDS(ON). This improves efficiency by reducing DC

losses in the device. All but the inductor losses are

inherent to the device. Because of this, inductor

selection becomes increasingly critical in efficiency

calculations. As the inductors are reduced in size, the

DC resistance (DCR) can become very significant. The

DCR losses can be calculated as shown in

Equation 5-4.

EQUATION 5-4:

From that, the loss in efficiency caused by inductor

resistance can be calculated as shown in Equation 5-5.

EQUATION 5-5:

LMAX

VOUT 135ns



2 50mA



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

Efficiency %

VOUT IOUT



VIN IIN



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







 100



PDCR

IOUT

DCR



Efficiency Loss

VOUT IOUT



VOUT IOUT



PDCR

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









–

100



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