December 10, 2001

9

MIC2184

MIC2184

Micrel

The MIC2184 controller is broken down into 7 functions.

• Control loop

• PWM operation

• Current mode control

• Current limit

• Reference, enable and UVLO

• FreqOut

• MOSFET gate drive

• Oscillator and Sync

• Soft-start

Control Loop

PWM Control Loop

The MIC2184 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 CSH (pin 9) and CSL (pin 8) are noise

sensitive due to the low signal level and high input imped-

ance. The PCB traces should be short and routed close to

each other. A small (1nF) 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 7) is greater

than the enable UVLO threshold.

The enable pin (pin 7) has two threshold levels, allowing the

MIC2184 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 MIC2184 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 soft start pin is forced low by an

internal discharge MOSFET. The output drivers are inhibited

from switching. The OUTP pin is in a high state and the OUTN

pin remains in a low state. Raising the enable voltage above

the standby threshold allows the soft start capacitor to charge

and enables the output drivers. 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.