NCP4330
http://onsemi.com
14
2. High−Side Turn Off:
The high−side MOSFET turns in a very soft way (no
current, no voltage). The turn off drive is in fact designed to
face the high dV/dt that occurs when the MOSFET Q1
abruptly turns on. The current capability has been set to
750 mA so that a 30 V variation in 10 ns cannot parasitically
switch on a high−side MOSFET exhibiting a 250 pF Crss.
4.5010 M 4.5015 M
4.5020 M
4.5025 M 4.5030 M
Figure 31. High−Side MOSFET Turn Off
Time in Secs
Vin
lind
i(l1)
vin1
ihs
vdshs
hs_drv
I
Q
V
Q
−40.0
−20.0
0
20.0
40.0
ls_drv in volts
0
4.00
8.00
12.0
16.0
irl in amps
−320
−240
−160
−80.0
0
i(l1) in amps
0
20.0
40.0
60.0
80.0
vin1 in volts
−8.00
−4.00
0
4.00
8.00
v(vsn) in volts
HS_DRV
3. Input Voltage Limitation:
Traditional high−side drivers turn off the MOSFET they
control, by forcing nearly 0 V between gate and source. The
NCP4330 high−side stage is not referenced to the MOSFET
source but to ground, and the HS MOSFET is forced off by
grounding its gate. This technique that saves the pin that is
traditionally connected to the MOSFET source, allows a
robust turn off of the power switch. In effect, the low−side
MOSFET that is ON when the high−side is off, forces the
High−side MOSFET source to approximately 0 V.
However the high−side MOSFET turn on is preceded by
a 100 ns phase during which both the low−side and
high−side power switches are off. During this 100 ns phase,
the drain source voltage of the low−side MOSFET may get
high, given that in light load operation, the L2 coil current
may get negative and flow from the load toward the input
through the HS MOSFET body diode. In this case, the gate
source voltage of the high−side MOSFET becomes negative
and substantially equal to the input voltage amplitude of the
post−regulator in absolute value.
Therefore, the maximum value of the pulsed input
voltage should be chosen lower than the maximum
source−gate voltage the HS MOSFET can sustain.
Traditional MOSFET Vgs maximum ratings are generally
+/−20 V. Therefore, with this kind of MOSFETs, the input
voltage must keep below 20 V (possible spikes being
included).
Undervoltage Lockout
An undervoltage lockout comparator is incorporated to
guarantee that the device is fully functional before enabling
the output stages. The NCP4330 starts to operate when the
power supply V
DD
exceeds 5.8 V. A 600 mV hysteresis
avoids that some noise on the V
DD
might produce some
erratic turns on and off of the device. When the NCP4330
detects an undervoltage lockout condition, it keeps both the
high−side and low−side drivers in low state.
The undervoltage lockout has a 4.9 V minimum threshold
(falling). As a consequence, around 3.4 V are available on
the driver outputs to force on the MOSFET. Such a level
allows to properly drive most MOSFETs.
