MIC9131 Datasheet, PDF (13/20 Pages) Micrel Semiconductor – High-Voltage, High-Speed Telecom DC-to-DC Controller

MIC9131
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.
D1 R1
MIC9131
V_REF
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 of
the output is dependent on the IC supply voltage and the gate
charge required to turn the MOSFET on and off.
Micrel
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.
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 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 20Ω 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.
July 2001
13
MIC9131