MIC2193

Micrel

M9999-042704

8

April 2004

MOSFET Selection

The P-channel MOSFET must have a V

equal to or lower than the input voltage when used in a buck

converter topology. There is a limit to the maximum gate

charge the MIC2193 will drive. MOSFETs with higher gate

charge will have slower turn-on and turn-off times. Slower

transition times will cause higher power dissipation in the

MOSFETs due to higher switching transition losses. The

MOSFETs must be able to completely turn on and off within

the driver non-overlap time If both MOSFETs are conducting

at the same time, shoot-through will occur, which greatly

increases power dissipation in the MOSFETs and reduces

converter efficiency.

The MOSFET gate charge is also limited by power dissipation

in the MIC2193. The power dissipated by the gate drive

circuitry is calculated below:

P

where:

Q

channel MOSFETs.

f

V

The graph in Figure 3 shows the total gate charge that can be

driven by the MIC2193 over the input voltage range, for

different values of switching frequency.

0

10

20

30

40

50

60

70

80

90

100

02468

10 12 14

INPUT VOLTAGE (V)

Max. Gate Charge

Figure 3. MIC2193 Frequency vs Max. Gate Charge

Oscillator

The internal oscillator is free running and requires no external

components. The maximum duty cycle is 100%. This is

another advantage of using a P-channel MOSFET for the

high-side drive: it can continuously turned on.

A frequency foldback mode is enabled if the voltage on the

feedback pin (pin 3) is less than 0.3V. In frequency foldback,

the oscillator frequency is reduced by approximately a factor

of 4. Frequency foldback is used to limit the energy delivered

to the output during a short circuit fault condition.

Voltage Setting Components

The MIC2193 requires two resistors to set the output voltage

as shown in Figure 4.

1.245V

Voltage

Amplifier

Pin 3

MIC2193

R1

R2

Figure 4

The output voltage is determined by the equation below.

VV

R

R

OUT

REF

=× +

1

1

2

Where: V

Lower values of R1 are preferred to prevent noise from

appearing on the FB pin. A typically recommended value is

10k

Ω. If R1 is too small in value it will decrease the efficiency

of the power supply, especially at low output loads.

Once R1 is selected, R2 can be calculated with the following

formula.

R

VR

VV

REF

OUT

REF

2

1

=

×

–

Efficiency Considerations

Efficiency is the ratio of output power to input power. The

difference is dissipated as heat in the buck converter. Under

light output load, the significant contributors are:

• The V

To maximize efficiency at light loads:

• Use a low gate charge MOSFET or use the smallest

MOSFET, which is still adequate for maximum output

current.

• Use a ferrite material for the inductor core, which has

less core loss than an MPP or iron power core.

Under heavy output loads the significant contributors to

power loss are (in approximate order of magnitude):

• Resistive on time losses in the MOSFETs

• Switching transition losses in the high side MOSFET

• Inductor resistive losses

• Current sense resistor losses

• Input capacitor resistive losses (due to the capacitors

ESR)

To minimize power loss under heavy loads:

• Use low on resistance MOSFETs. Use low threshold

logic level MOSFETs when the input voltage is below

5V. Multiplying the gate charge by the on resistance

gives a figure of merit, providing a good balance

between low load and high load efficiency.

• Slow transition times and oscillations on the voltage

and current waveforms dissipate more power during

the turn on and turn off of the MOSFETs. A clean

layout will minimize parasitic inductance and capaci-

tance in the gate drive and high current paths. This

will allow the fastest transition times and waveforms

without oscillations. Low gate charge MOSFETs will