MIC2194

Micrel

MIC2194

8

October 2002

1.5V

Typical

140mV

Hysteresis

(typical)

EN/UVLO

(3)

MIC2194

R1

R2

Bias

Circuitry

Figure 3. UVLO Circuitry

The line voltage turn on trip point is:

VV

R

RR

INPUT ENABLE

THRESHOLD

_

=×

+

2

12

where:

V

comparator reference, typically 1.5V.

The input voltage hysteresis is equal to:

VV

RR

R

INPUT HYST

HYST

_

=×

+

12

2

where:

V

typically 140mV.

V

The MIC2194 will be disabled when the input voltage drops

back down to:

V

V

(V

+

R

RR

2

12

Either of 2 UVLO conditions will pull the soft start capacitor

low:

• When the V

undervoltage lockout level.

• When the enable pin drops below the its enable

threshold

The internal bias circuit generates an internal 1.245V band-

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

VDD voltage for the internal control circuitry. The VDD pin

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 MIC2194 is designed to drive a high-side P-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 advan-

tage of using a P-channel MOSFET is that it does not require

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 8) supplies the drive voltage to the gate drive pin,

OUTP.

MOSFET Selection

The P-channel MOSFET must have a V

equal to or lower than the input voltage when used in a buck

converter topology. There is a limit to the maximum gate

charge the MIC2194 will drive. MOSFETs with high gate

charge will have slower turn-on and turn-off times. Slower

transition times will cause higher power dissipation in the

MOSFET due to higher switching transition losses.

The MOSFET gate charge is also limited by power dissipation

in the MIC2194. The power dissipated by the gate drive

circuitry is calculated below:

P

where: Q

channel MOSFETs.

f

V

The graph in Figure 4 shows the total gate charge that can be

driven by the MIC2194 over the input voltage range, for

different values of switching frequency.

0

50

100

150

200

250

0

5

10

15

INPUT VOLTAGE (V)

Max Gate Charge

Figure 4. MIC2194 V

Oscillator

The internal oscillator is free running and requires no external

components. The maximum duty cycle for both frequencies

is 100%. This is another advantage of using a P-channel

MOSFET for the high-side drive; it can be continuously turned

on.

A frequency foldback mode is enabled if the voltage on the

feedback pin (pin 2) is less than 0.3V. In frequency foldback,

the oscillator frequency is reduced by approximately a factor

of 4. Frequency foldback is used to limit the energy delivered

to the output during a short circuit fault condition.

Voltage Setting Components

The MIC2194 requires two resistors to set the output voltage

as shown in Figure 5.