MIC2199

Micrel

MIC2199

8

November 2004

Current Limit

The MIC2199 output current is detected by the voltage drop

across the external current-sense resistor (R

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

=

The current-sense pins CSH (pin 4) and V

noise sensitive due to the low signal level and high input

impedance. The PCB traces should be short and routed close

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

the high-side MOSFET for the remainder of the cycle. The

output voltage drops as additional load current is pulled from

the converter. When the output voltage reaches approxi-

mately 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 external current-sense resistor. This limits the maximum

output power delivered to the load under a short circuit

condition.

Reference, Enable and UVLO Circuits

The output drivers are enabled when the following conditions

are satisfied:

• The V

voltage threshold (typically 4.25V).

• The voltage on the enable pin is greater than the

enable UVLO threshold (typically 2.5V).

The internal bias circuit generates a 0.8V bandgap reference

voltage for the voltage error amplifier and a 5V V

for the gate drive circuit. The MIC2199 uses FB (pin 3) 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

thresholds, the internal bias, V

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.

Either of two UVLO conditions will disable the MIC2199 from

switching.

• When the V

• When the enable pin drops below the 2.5V threshold

MOSFET Gate Drive

The MIC2199 high-side drive circuit is designed to switch an

N-Channel MOSFET. Referring to the block diagram in

Figure 2, a bootstrap circuit, consisting of D2 and C

supplies energy to the high-side drive circuit. Capacitor C

BST

is charged while the low-side MOSFET is on and the voltage

on the V

side MOSFET driver is turned on, energy from C

to turn the MOSFET on. As the MOSFET turns on, the voltage

on the V

reversed biased and C

keep the high-side MOSFET on. When the low-side switch is

turned back on, C

The drive voltage is derived from the internal 5V V

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

nominal high-side gate drive voltage is approximately 4.5V

due the voltage drop across D2. A fixed 80ns delay between

the high- and low-side driver transitions is used to prevent

current from simultaneously flowing unimpeded through both

MOSFETs.

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.