Chapter 17 Gate Drive Unit (GDUV4)
MC9S12ZVM Family Reference Manual Rev. 1.3
Freescale Semiconductor
660
17.5
Application Information
17.5.1
FET Pre-Driver Details
The basic concept of the high-side driver is shown in
. If the FET pre-driver is switched on
the transistor T2 is driving the output HG. For on resistance R
gduon
of transistor T2 refer to GDU
electricals. The output current is limited to I
OUT
which is derived from the reference current I
REF
. The
current source is controlled by the slew rate control bits GSRCHS[2:0]. If the FET pre-driver is switched
off transistors T3 and T4 are switched on. For on resistance R
gduoffn
and R
gduoffp
of transistors T3 and T4
refer to GDU electricals.
The reference current I
REF
is controlled by the slew rate conrol bits GSRCHS[2:0] :
•
I
REF
= 10uA + GSRCHS 10uA, [10uA, 20uA . . . 80uA]
Assuming an ideal op-amp the voltage across R
1
is equal voltage across R
2
and I
OUT2
is given by:
•
V
1
= V
2
= I
REF
R
1
= I
OUT2
R
2
•
I
OUT2
= I
REF
(R
1
/R
2
)
With the ratio of the transistor sizes of T
1
and T
2
k=450, and the ratio of the resistors R
1
/R
2
=36, and
neglect the current through R
HSpul
the output current I
OUT
is:
•
I
OUT1
= k I
OUT2
•
I
OUT
= I
OUT1
+ I
OUT2
= I
REF
(R
1
/R
2
) (1+k)
•
I
OUT
~ I
REF
(R
1
/R
2
) k
Figure 17-25. FET Pre-Driver Concept for High-Side Driver
+
_
R
gduoffp
R
gduoffn
GSRCHS[2:0]
VBS
HG
HS
R
1
R
2
Driver Off
Driver On
R
HSpul
I
REF
I
out2
I
out1
I
out
T
1
T
2
T
3
T
4
V
1
V
2
R
gduon
C
G
