December 10, 2001

11

MIC2183

MIC2183

Micrel

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.

The SYNC input (pin 11) lets the MIC2183 synchronize with

an external clock signal. The rising edge of the sync signal

generates a reset signal in the oscillator, which turns off the

low side gate drive output. The high side drive then turns on,

restarting the switching cycle. The sync signal is inhibited

when the controller operates in frequency foldback. The sync

signal frequency must be greater than the maximum speci-

fied free running frequency of the MIC2183. If the synchroniz-

ing frequency is lower, double pulsing of the gate drive

outputs will occur. When not used, the sync pin must be

connected to ground.

The maximum recommended output switching frequency is

600kHz. Synchronizing to higher frequencies may be pos-

sible, however, higher power dissipation in the internal gate

drive circuits will occur. The MOSFET gates require charge

to turn on the device. The average current required by the

MOSFET gate increases with switching frequency.

Soft Start

Soft start reduces the power supply input surge current at

start up by controlling the output voltage risetime. The input

surge appears while the output capacitance is charged up. A

slower output risetime will draw a lower input surge current.

Soft start may also be used for power supply sequencing.

The soft start voltage is applied directly to the PWM compara-

tor. A 5

µA internal current source is used to charge up the soft

start capacitor. The capacitor is discharged when either the

enable pin voltage drops below the standby threshold or the

V

The part switches at a low duty cycle when the soft start pin

voltage is zero. As the soft start voltage rises from 0V to 0.7V,

the duty cycle increases from the minimum duty cycle to the

operating duty cycle. The oscillator runs at the foldback

frequency (1/4 of the switching frequency) until the feedback

voltage rises above 0.3V. The risetime of the output is

dependent of the soft start capacitor output capacitance,

input and output voltage and load current.

Voltage Setting Components

The MIC2183 requires two resistors to set the output voltage

as shown in Figure 5.

1.245V

Voltage

Amplifier

Pin 6

MIC2183

R1

R2

Figure 5

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

• The V

required to switch the external MOSFETs

• Core losses in the output inductor

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

transition faster than those with higher gate charge

requirements.

• For the same size inductor, a lower value will have

fewer turns and therefore, lower winding resistance.

However, using too small of a value will require more

output capacitors to filter the output ripple, which will

force a smaller bandwidth, slower transient response

and possible instability under certain conditions.

• Lowering the current sense resistor value will de

crease the power dissipated in the resistor. However,

it will also increase the overcurrent limit and will

require larger MOSFETs and inductor components.

• Use low ESR input capacitors to minimize the power

dissipated in the capacitors ESR.