MIC23303 datasheet(13/21 Pages) MICREL

13 / 21 page

Micrel, Inc.

MIC23303

September 6, 2013

090613-2.0

Figure 2. HyperLight Load (HLL) and Continuous

Conduction Mode (CCM) Switching Diagram

Figure 2 shows the signals for high-side switch drive

(HSD) for Ton control, the inductor current and the low-

side switch drive (LSD) for Toff control.

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 300mA. When

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

inductor falling current is no longer able to reach this

300mA threshold, the part is in CCM mode and switching

at a virtually constant frequency.

Compensation

The MIC23303 is designed to be stable with a 0.33µH to

1.0µH inductor with a minimum 10µF ceramic (X5R) output

capacitor. The total feedback resistance should be kept

around 500kΩ to reduce the I2R losses through the

feedback resistor network, improving efficiency. A feed-

forward capacitor (CFF) of 33pF is recommended across

the top feedback resistor to reduce the effects of parasitic

capacitance and improve transient performance.

Duty Cycle

The typical maximum duty cycle of the MIC23303 is 85%.

Efficiency Considerations

Efficiency is defined as the amount of useful output power,

divided by the amount of power supplied.

100

Efficiency

OUT













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

battery

increases the devices 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 to drive the gates on and off at a

constant 4MHz frequency and the switching transitions

make up the switching losses.

Efficiency vs. Load

1.8 VOUT

100

0.0001

0.001

0.01

0.1

LOAD CURRENT(A)

VIN = 3.6V

VIN = 5V

L = 0.33µH

COUT = 44µF

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

HyperLight Load mode, the MIC23303 is able to maintain

high efficiency at low output currents.

Over 300mA, efficiency loss is dominated by MOSFET

RDSON and inductor losses. Higher input supply voltages

will increase the Gate-to-Source voltage 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.

When dealing with inductor losses, inductor selection

becomes increasingly critical in efficiency calculations.

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