April 2005

13

M9999-040805

MIC9130

Micrel, Inc.

until the error amplifier takes control of the duty cycle. The

soft start capacitor is discharged by an internal MOSFET in

the MIC9130.

The soft start circuit is activated by the following events:

1. Line undervoltage pin less than the 1.21V threshold

.................................................................off threshold.

3. The current limit comparator threshold is exceeded.

This can be disabled with a low level on the

CPWR

pin.

4. A low level on the enable pin.

Calculating the soft capacitor depends on many parameters

such as the current limit of the circuit input voltage, output

power and output loading.Astarting value of capacitor should

be chosen and the value can be adjusted later in the design.

Recommended starting values of soft start capacitance is

typically 10nF to 100nF. Values below 1nF may be ineffective

in slowing the output voltage turn on time.

CPWR Current Limit Selection

This pin controls whether the soft start circuit is reset if the

voltage on the Isns pin exceeds the overcurrent threshold.

When the CPWR pin is high, an overcurrent condition at the

ISNS pin will terminate the on-time of the gate drive pulse

and discharge the soft start capacitor to zero volts. This delay

in start up contributes to a reduction in the average output

current during an overcurrent or short circuit condition. A

smaller MOSFET may be used since the power dissipation

in the MOSFET is minimized under short circuit or overcur-

rent conditions.

If the CPWR pin is low an overcurrent or short circuit condi-

tions will not trip the soft start circuit. The pulse-by-pulse

current limit, inherent in current mode control, provides a

“brick wall” or constant current limit. With the power supply

operating in this mode, a smaller soft start capacitor can be

used to increase the turn on speed of the supply.

If the CPWR in is held low during the initial turn on at power

up and then raised high, the power supply can maximize

the turn-on time at start up and still provide a high level of

overcurrent and short circuit protection. The circuit shown

in Figure 7 performs this function.

CPWR

R1

D1

C1

AGND

MIC9130

Figure 7

MOSFET Gate Drive Output

The MIC9130 has the capability to directly drive the gate of

a MOSFET. The output driver consists of a complimentary

P-channel and N-channel pair. The typical switching time

of the output is dependent on the IC supply voltage and the

gate charge required to turn the MOSFET on and off.

A resistor placed in series with the gate drive output attenu-

ates ringing in the etch connection between the MIC9130

and the MOSFET. Figure 8 shows a single resistor in series

between the driver output and the gate of the MOSFET. The

zener value should be greater than the gate drive voltage

to prevent excessive power dissipation, but less than the

maximum gate to source voltage rating.

Gate Drive

Output

GND

Figure 8

Thecircuitryshowninfigure9allowdifferentriseandfalltimes.

R1 and the input capacitance of the MOSFET determine the

rise-time of the gate voltage and therefore the turn-on time of

theMOSFET.Thediode,D1isreversedbiased,whichremoves

R2 from the circuit. At turn-off, D1 is forward biased and the

parallel combination of R1 and R2 controls the turn-off time

of the MOSFET. The turn on-time is slower, which reduces

switching noise and ringing during turn-on. The turn-off time

is faster, which minimizes switching losses during turn-off and

improves efficiency. If the turn-on time is to be faster than

the turn-off time, the diode should be reversed.

Gate Drive

Output

GND

R2

D1

R1

Figure 9

A gate drive transformer is used where an increase in drive

voltage, isolation and/or voltage level shifting are required.

Gate drive transformers can have multiple windings and drive

multiple MOSFETs, including MOSFETs that require a drive

signal 180 degrees out of phase with the ICs drive signal.

Figure 10 shows a gate drive transformer circuit. The ca-

pacitor, C1 removes DC from the drive circuit and prevents

transformer saturation. R1 provides damping to eliminate

ringing in the circuit. R1 is usually in the 5 to 20Ω range,

depending on the amount of damping necessary. D1 and

D2 form a clamp circuit, which prevents the voltage from

the diodes may be removed R2 is used to allow the trans-

former to reset properly.