MIC9131BM Просмотр технического описания (PDF) - Micrel

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MIC9131BM

High-Voltage, High-Speed Telecom DC-to-DC Controller

Micrel 

MIC9131

cycle of the gate drive output increases until the error ampli-

fier 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. VCC 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.

MIC9131

VREF

D1 R1

CPWR

C1

AGND

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

Micrel, Inc.

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

maximum 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 de-

termine 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.

R2

D1

Gate Drive

R1

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 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

exceeding the VGMAX level. If the gate drive is well damped,

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

former to reset properly.

August 2006

13

M9999-080206

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