isppac-powr1208p1 Lattice Semiconductor Corp., isppac-powr1208p1 Datasheet - Page 20

• Manufacturer Part Number: isppac-powr1208p1
• Description: In-system Programmable Power Supply Sequencing Controller And Precision Monitor
• Manufacturer: Lattice Semiconductor Corp.

Lattice Semiconductor
Predicting MOSFET Turn-on Time
Because the ispPAC-POWR1208P1’s MOSFET output drivers source a precise and well-defined output current, it
becomes possible to predict MOSFET gate rise times if one knows the value of the load capacitance presented by
the MOSFET being driven. The other method is by relating the total gate charge to the gate-to-source voltage.
Figure 3-9. MOSFET Gate Charge vs. Gate-Source Voltage
Using this method, it becomes straightforward to estimate the gate rise time for a given charging current. As an
example a MOSFET’s source voltage (V
(V
and down the plot of Figure 3-9, a V
equal to the product of current (I) and time (t
expressed as:
For this example, let us assume a charging current of 10.9µA. Gate charging time is given by:
Validation of this result can be seen in the scope plot shown in Figure 3-10. The top set of traces shows gate rise
times for various (5.5µA to 50.3µA) gate drive currents. The trace labeled 10.9µA shows gate voltage V
from 0V to 10V in slightly over 3 milliseconds, which agrees to within 25% of our predicted value, well within the lim-
its of device-to-device variation.
G
) will be 10V in this condition. The device’s gate-to-source voltage (V
t
CHARGE-TIME
GS
6.7
0
9
6
3
S
0
of 6.7V corresponds to ~40 nC of gate charge (Q
) will be 3.3V when the device is fully switched on, while the gate voltage
CHARGE-TIME
t
Q
CHARGE-TIME
G
=
, Total Gate Charge (nC)
20
10.9 x 10
i
40 x 10
=
3-20
dQ
dt
) when current is constant, gate charging time can be
40
-9
=
-6
C
A
Q
I
G
= 3.7 x 10
60
ispPAC-POWR1208P1 Data Sheet
GS
) will therefore be 6.7V. Reading across
-3
s
G
). Because charge is
G
rising
(1)
(2)
(3)