MIC9131 datasheet(13/20 Pages) MICREL

13 / 20 page

July 2001

MIC9131

Micrel

the duty cycle of the gate drive output increases until the error

amplifier takes control of the duty cycle. The soft start

capacitor is discharged by an internal MOSFET in the

MIC9131.

The soft start circuit is activated by the following events:

1. Line undervoltage pin less than the 1.21V threshold

2. V

CC becomes less than the pre-regulator voltage turn

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. A starting 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 0V. 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 overcurrent 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.

VREF

CPWR

AGND

MIC9131

Figure 7

MOSFET Gate Drive Output

The MIC9131 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 MIC9131 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 maxi-

mum gate to source voltage rating.

Gate Drive

Output

GND

Figure 8

The circuitry shown in figure 9 allow different rise and fall

times. R1 and the input capacitance of the MOSFET deter-

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

on time of the MOSFET. The diode, D1 is reversed biased,

which removes 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

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

°C out of phase with the ICs drive signal.

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

tor, 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 20

Ω range,

depending on the amount of damping necessary. D1 and D2

form a clamp circuit, which prevents the voltage from exceed-

ing the V

GMAX level. If the gate drive is well damped, the

diodes may be removed R2 is used to allow the transformer

to reset properly.

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