FLF3215T-R47N datasheet(14/19 Pages) MICREL

14 / 19 page

Micrel, Inc.

MIC23163/4

July 29, 2013

Revision 2.0

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 VOUT/L. The

controller remains in HLL mode while the inductor falling

current is detected to cross approximately

−50mA. When

the LSD (or TOFF) time reaches its minimum and the

inductor falling current is no longer able to reach this

−50mA threshold, the part is in CCM mode and switching

at a virtually constant frequency.

Once in CCM mode, the TOFF time will not vary.

Compensation

The MIC23163/4 is designed to be stable with a 0.47µH

inductor with a 10µF ceramic (X5R) output capacitor. A

feed-forward capacitor in the range of 15pF to 68pF is

essential across the top feedback resistor.

Duty Cycle

The maximum duty cycle of the MIC23163/4 is 100%,

allowing operation in dropout to extend battery life.

Efficiency Considerations

Efficiency is defined as the amount of useful output

power, divided by the amount of power supplied, as

shown in Equation 3:

100

Efficiency

OUT













Eq. 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 consumption of current for

battery-powered applications. Reduced current draw from

a battery increases the device’s operating time and is

critical in handheld devices.

There are two types of losses in switching converters; DC

losses and switching losses. DC losses are simply the

power dissipation of I

2R. Power is dissipated in the high

side switch during the on cycle. Power loss is equal to the

high side MOSFET RDSON 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 driving

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

the switching transitions make up the switching losses.

Figure 3. Efficiency under Load

Figure 3 shows an efficiency curve. From no load to

100mA, efficiency losses are dominated by quiescent

current losses, gate drive and transition losses. By using

the HLL mode, the MIC23163/4 is able to maintain high

efficiency at low output currents.

Over 100mA, efficiency loss is dominated by MOSFET

RDSON and inductor losses. Higher input supply voltages

will increase the gate-to-source threshold on the internal

MOSFETs, thereby reducing the internal RDSON. This

improves efficiency by reducing DC losses in the device.

All but the inductor losses are inherent to the device. In

which case, inductor selection becomes increasingly

critical in efficiency calculations. As the inductors are

reduced in size, the DC resistance (DCR) can become

quite significant. The DCR losses can be calculated as in

Equation 4:

DCR

OUT

DCR

Eq. 4

From that, the loss in efficiency due to inductor resistance

can be calculated as in Equation 5:

100

Loss

Efficiency

DCR

OUT

























−

Eq. 5

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.

100

1000

10000

OUTPUT CURRENT (mA)

Efficiency vs. Output Current

OUT = 1.8V @ 25°C

IN = 3V

IN = 3.6V

IN = 5V

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