April 2004

7

M9999-042704

MIC2193

Micrel

The MIC2193 controller is broken down into five functions.

• Control loop

- PWM operation

- Current mode control

• Current limit

• Reference and V

DD

• MOSFET gate drive

• Oscillator

Control Loop

PWM Control Loop

The MIC2193 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 1) and CSL (pin 4) are noise

sensitive due to the low signal level and high input impedance

and switching noise on the VIN pin. The PCB traces should

be short and routed close to each other. A 10nF 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 approximately 1/4 of the switching

frequency. This limits the maximum output power delivered to

the load under a short circuit condition.

Reference and V

The output drivers are enabled when the V

is greater than its undervoltage threshold.

The internal bias circuit generates an internal 1.245V band-

gap reference voltage for the voltage error amplifier and a 3V

V

must be decoupled with a 1

µF ceramic capacitor. The capaci-

tor must be placed close to the VDD pin. The other end of the

capacitor must be connected directly to the ground plane.

MOSFET Gate Drive

The MIC2193 is designed to drive a high-side, P-Channel

MOSFET and a low side, N-Channel MOSFET. The source

pin of the P-channel MOSFET is connected to the input of the

power supply. It is turned on when OUTP pulls the gate of the

MOSFET low. The advantage of using a P-channel MOSFET

is that it does not required a bootstrap circuit to boost the gate

voltage higher than the input, as would be required for an N-

channel MOSFET.

The VIN pin (pin 1) supplies the drive voltage to both gate

drive pins, OUTN and OUTP. The VIN pin must be well

decoupled to prevent noise from affecting the current sense

circuit, which uses VIN as one of the sense pins.

A non-overlap time is built into the MOSFET driver circuitry.

This dead time prevents the high-side and low-side MOSFET

drivers from being on at the same time. Either an external

diode or the low-side MOSFET internal parasitic diode con-

ducts the inductor current during the dead time.