October 2002

7

MIC2194

MIC2194

Micrel

The MIC2194 controller is broken down into several func-

tions.

• Control loop

• PWM operation

• Current mode control

• Current limit

• Reference, enable and UVLO

• MOSFET gate drive

• Oscillator

Control Loop

PWM Control Loop

The MIC2194 uses current mode control to regulate the

output voltage. This dual control loop method (illustrated in

Figure 2) senses the output voltage (outer loop) and the

inductor current (inner loop). It uses inductor current and

output voltage to determine the duty cycle of the buck

converter. Sampling the inductor current effectively removes

the inductor from the control loop, which simplifies compen-

sation.

Switching

Converter

Voltage

Divider

Switch

Driver

Figure 2. Current Mode Control Example

As shown in Figure 1, the inductor current is sensed by

measuring the voltage across the resistor, R

added to the amplified current sense signal to provide slope

compensation, which is required to prevent unstable opera-

tion at duty cycles greater than 50%.

A transconductance amplifier is used for the error amplifier,

which compares an attenuated sample of the output voltage

with a reference voltage. The output of the error amplifier is

the compensation pin (COMP), which is compared to the

current sense waveform in the PWM block. When the current

signal becomes greater than the error signal, the comparator

turns off the high side drive. The COMP pin provides access

to the output of the error amplifier and allows the use of

external components to stabilize the voltage loop.

Current Limit

The output current is detected by the voltage drop across the

external current sense resistor (R

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

MIN CURRENT

SENSE THRESHOLD

I

SENSE

OUT

MAX

=

__

_

_

The maximum output current is:

I

MAX

CURRENT

SENSE THRESHOLD

R

OUT MAX

SENSE

_

__

_

=

The current sense pins VIN (pin 8) and CSL (pin 4) are noise

sensitive due to the low signal level, high input impedance

and input ripple voltage. The PCB traces should be short and

routed close to each other. A 0.1

µF capacitor across the pins

will attenuate high frequency switching noise.

When the peak inductor current exceeds the current limit

threshold, the overcurrent comparator turns off the high-side

MOSFET for the remainder of the switching cycle, effectively

decreasing the duty cycle. The output voltage drops as

additional load current is pulled from the converter. When the

voltage at the feedback pin (FB) reaches approximately 0.3V,

the circuit enters frequency foldback mode and the oscillator

frequency will drop to 1/4 of the switching frequency. 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

undervoltage threshold.

• The voltage on the enable pin (pin 3) is greater

than the enable UVLO threshold.

The enable pin (pin 3) has two threshold levels, allowing the

MIC2194 to shut down in a low current mode, or turn off output

switching in standby mode. An enable pin voltage lower than

the shutdown threshold turns off all the internal circuitry and

places the MIC2194 in a micropower shutdown mode.

If the enable pin voltage is between the shutdown and

standby thresholds, the internal bias, V

voltages are turned on. The output drivers are inhibited from

switching. The OUTP pin is in a high state. Raising the enable

voltage above the standby threshold enables the output

driver. The standby threshold is specified in the electrical

characteristics. A resistor divider can be used with the enable

pin to prevent the power supply from turning on until a

specified input voltage is reached. The circuit in Figure 3

shows how to connect the resistors.