Micrel, Inc.

MIC2199.

January 2010

7

M9999-011310

Current Limit

The MIC2199 output current is detected by the voltage drop

2.). The current limit threshold is 75mV±20mV. The current-

sense resistor must be sized using the minimum current

limit threshold. The external components must be designed

to withstand the maximum current limit. The current-sense

resistor value is calculated by the equation below:

R

55mV

I

CS

OUT(max)

=

The maximum output current is:

I

95mV

R

OUT(max)

CS

=

noise sensitive due to the low signal level and high input im-

pedance.ThePCBtracesshouldbeshortandroutedclose

to each other. A small (1nF to 0.1µF) capacitor across the

pins will attenuate high frequency switching noise.

When the peak inductor current exceeds the current limit

threshold, the current limit comparator, in Figure 2, turns off

thehigh-sideMOSFETfortheremainderofthecycle.The

output voltage drops as additional load current is pulled from

theconverter.Whentheoutputvoltagereachesapproximately

0.4V, the circuit enters frequency-foldback mode and the

oscillator frequency will drop to 75kHz while maintaining the

peak inductor current equal to the nominal 75mV across the

externalcurrent-senseresistor.Thislimitsthemaximumoutput

power delivered to the load under a short circuit condition.

Reference, Enable and UVLO Circuits

The output drivers are enabled when the following conditions

aresatisfied:

voltage threshold (typically 4.25V).

• Thevoltageontheenablepinisgreaterthanthe

enable UVLO threshold (typically 2.5V).

The internal bias circuit generates a 0.8V bandgap reference

forthegatedrivecircuit.TheMIC2199usesFB(pin3)for

output voltage sensing.

The enable pin (pin 2) has two threshold levels, allowing

the MIC2199 to shut down in a low current mode, or turn off

output switching in UVLO mode. An enable pin voltage lower

than the shutdown threshold turns off all the internal circuitry

and reduces the input current to typically 0.1µA.

If the enable pin voltage is between the shutdown and UVLO

turned on. The output drivers are inhibited from switching and

remain in a low state. Raising the enable voltage above the

UVLO threshold of 2.5V enables the output drivers.

EitheroftwoUVLOconditionswilldisabletheMIC2199from

switching.

• Whentheenablepindropsbelowthe2.5Vthreshold

MOSFET Gate Drive

The MIC2199 high-side drive circuit is designed to switch

anN-ChannelMOSFET.Referringtotheblockdiagramin

chargedwhilethelow-sideMOSFETisonandthevoltageon

turntheMOSFETon.AstheMOSFETturnson,thevoltage

keepthehigh-sideMOSFETon.Whenthelow-sideswitch

supply. The nominal low-side gate drive voltage is 5V and

the nominal high-side gate drive voltage is approximately

4.5VduethevoltagedropacrossD2.Afixed80nsdelay

between the high- and low-side driver transitions is used

topreventcurrentfromsimultaneouslyflowingunimpeded

throughbothMOSFETs.

Oscillator

The internal oscillator is free running and requires no external

components. The nominal oscillator frequency is 500kHz. If

the output voltage is below approximately 0.4V, the oscillator

operates in a frequency-foldback mode and the switching

frequency is reduced to 75kHz.

TIME

Figure 4. Startup Waveform

Above 0.4V, the switching frequency increases to 500kHz

causing the output voltage to rise a greater rate. The rise

time of the output is dependent on the output capacitance,

output voltage, and load current. The oscilloscope photo in

Figure 4 show the output voltage at startup.