PRODUCT DATABOOK 1996/1997
SG1844/SG1845 Series
C
URRENT
-M
ODE
PWM C
ONTROLLER
P
R O D U C T I O N
D
A T A
S
H E E T
T Y P I C A L A P P L I C AT I O N C I R C U I T S
Pin numbers referenced are for 8-pin package and pin numbers in parenthesis are for 14-pin package.
V
CC
V
IN
V
CC
V
IN
7 (12)
7 (12)
7 (11)
6 (10)
5 (8)
R
3 (5)
C
Q1
SG1844/45
I
PK
7 (11)
R
1
6 (10)
Q1
SG1844/45
R
S
I
PK(MAX)
= 1.0V
R
S
3 (5)
5 (8)
R
S
FIGURE 14.
— CURRENT SENSE SPIKE SUPPRESSION
The RC low-pass filter will eliminate the leading edge current spike
caused by parasitics of Power MOSFET.
FIGURE 15.
— MOSFET PARASITIC OSCILLATIONS
A resistor (R
1
) in series with the MOSFET gate reduce overshoot and
ringing caused by the MOSFET input capacitance and any inductance
in series with the gate drive. (Note: It is very important to have a
low inductance ground path to insure correct operation of the I.C.
This can be done by making the ground paths as short and as wide
as possible.)
V
CC
V
IN
Isolation
Boundary
Q1
I
B
V
C
+
»
R
2
V
C1
»
R
1
| R
2
_
|
V
C
V
C1
7 (11)
R
2
SG1844/45
6 (10)
C
1
R
1
Q1
SG1844/45
5 (8)
V
IN
7 (12)
7 (11)
6 (10)
V
GS
Waveforms
+
0
_
50% DC
R
+
0
_
25% DC
3 (5)
5 (8)
C
R
S
N
S
N
P
3 (5)
R
S
I
PK
=
V (PIN 1) - 1.4
3R
S
N
P
N
S
FIGURE 16.
— BIPOLAR TRANSISTOR DRIVE
The 1844/45 output stage can provide negative base current to
remove base charge of power transistor (Q
1
) for faster turn off. This
is accomplished by adding a capacitor (C
1
) in parallel with a resistor
(R
1
). The resistor (R
1
) is to limit the base current during turn on.
FIGURE 17.
— ISOLATED MOSFET DRIVE
Current transformers can be used where isolation is required
between PWM and Primary ground. A drive transformer is then
necessary to interface the PWM output with the MOSFET.
Copyright © 2000
Rev. 1.4 4/00
11
MICROSEMI [ MICROSEMI CORPORATION ]
