MIC33153 datasheet(15/28 Pages) MICROCHIP

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DS20006223B-page 15

MIC33153

The DCR losses can be calculated by using

Equation 5-2:

EQUATION 5-2:

From that, the loss in efficiency due to inductor

resistance can be calculated by using Equation 5-3:

EQUATION 5-3:

Efficiency loss due to DCR is minimal at light loads and

gains significance as the load is increased. Inductor

selection becomes a trade-off between efficiency and

size in this case.

The effect of MOSFET voltage drops and DCR losses

in conjunction with the maximum duty cycle combine to

limit maximum output voltage for a given input voltage.

The following graph shows this relationship based on

the typical resistive losses in the MIC33153:

FIGURE 5-2:

VOUT(MAX) vs. VIN.

5.6

HyperLight Load® Mode

The MIC33153 uses a minimum on and off time

proprietary control loop. When the output voltage falls

below the regulation threshold, the error comparator

begins a switching cycle that turns the PMOS on and

keeps it on for the duration of the minimum on-time.

When the output voltage is over the regulation

threshold, the error comparator turns the PMOS off for

a minimum off-time. The NMOS acts as an ideal

rectifier that conducts when the PMOS is off. Using a

NMOS switch instead of a diode allows for lower

voltage drop across the switching device when it is on.

The asynchronous switching combination between the

PMOS and the NMOS allows the control loop to work

in discontinuous mode for light load operations. In

discontinuous mode, MIC33153 works in pulse

frequency modulation (PFM) to regulate the output. As

the output current increases, the switching frequency

increases. This improves the efficiency of the

MIC33153 during light load currents. As the load

current increases, the MIC33153 goes into continuous

conduction mode (CCM) at a constant frequency of

4 MHz. The equation to calculate the load when the

MIC33153 goes into continuous conduction mode may

be approximated by the following Equation 5-4:

EQUATION 5-4:

As shown in the above equation, the load at which

MIC33153 transitions from HyperLight Load® mode to

PWM mode is a function of the input voltage (VIN),

output voltage (VOUT), duty cycle (D), inductance (L)

and frequency (f). For example, if VIN = 3.6V,

VOUT = 1.8V, D = 0.5, f = 4 MHz and the internal

inductance of MIC33153 is 0.47 µH, then the device

will enter HyperLight Load® mode or PWM mode at

approximately 200 mA.

5.7

Power Dissipation Considerations

As with all power devices, the ultimate current rating of

the output is limited by the thermal properties of the

package and the PCB it is mounted on. There is a

simple Ohm’s law type relationship between thermal

resistance, power dissipation, and temperature which

is analogous to an electrical circuit:

PDCR IOUT

2 DCR



EfficiencyLoss

VOUT IOUT



VOUT IOUT PDCR



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









–

100



0.5

1.5

2.5

3.5

4.5

2.5

3.5

4.5

5.5

INPUT VOLTAGE (V)

1.2A

800mA

400mA

100mA

ILOAD

VIN VOUT

–







2L f



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









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