SG3526B MICROSEMI [Microsemi Corporation], SG3526B Datasheet - Page 7

• Manufacturer Part Number: SG3526B
• Description: REGULATING PULSE WIDTH MODULATOR
• Manufacturer: MICROSEMI [Microsemi Corporation]

4/90 Rev 1.1 2/94
Copyright
APPLICATION INFORMATION
DIGITAL CONTROL PORTS
The three digital control ports of the SG1526B are bi-
directional. Each pin can drive TTL and 5 volt CMOS logic
directly, up to a fan-out of 10 low-power Schottky gates.
Each pin can also be directly driven by open-collector TTL,
open-drain CMOS, and open-collector voltage comparators,
fan-in is equivalent to 1 low-power Schottky gate. Each port
is normally HIGH; the pin is pulled LOW to activate the
particular function. Driving SYNC LOW initiates a discharge
cycle in the oscillator. Pulling SHUTDOWN LOW immedi-
ately inhibits all PWM output pulses. Holding RESET LOW
discharges the soft-start capacitor. The logic threshold is
+1.1 volts at +25
expense of fan-out with an external 2K pull-up resistor to +5
volts.
OSCILLATOR
The oscillator is programmed for frequency and dead time
with three components: R
generated: a sawtooth waveform at pin 10 for pulse width
modulation, and a logic clock at pin 12. The following
procedure is recommended for choosing timing values:
The SG1526B can be synchronized to an external logic clock
by programming the oscillator to free-run at a frequency 10%
slower than the sync frequency. A periodic LOW logic pulse
approximately 0.5 Sec wide at the SYNC pin will then lock
the oscillator to the external frequency.
ERROR AMPLIFIER
The error amplifier is a transconductance design, with an
output impedance of 2 megohms. Since all voltage gain
takes place at the output pin, the open-loop gain/frequency
characteristics can be controlled with shunt reactance to
ground. When compensated for unity-gain stability with 100
pF, the amplifier has an open-loop pole at 400 Hz.
The input connections to the error amplifier and determined
by the polarity of the switching supply output voltage. For
positive supplies, the common-mode voltage is +5.0 volts
and the feedback connections in Figure 25A are used. With
negative supplies, the common-mode voltage is ground and
the feedback divider is connected between the negative
output and the +5.0 volt reference voltage, as shown in
Figure 25B.
1994
1. With R
2. If more dead time is required, select a larger value of
3. Increasing the dead time will cause the oscillator
for R
oscillator period. Remember that the frequency at
each driver output is half the oscillator frequency, and
the frequency at the +V
oscillator frequency.
R
increases by 300 ns/ .
frequency to decrease slightly. Go back and de-
crease the value of R
back to the nominal design value.
D
using Figure 14 as a guide. At 40 KHz dead time
T
D
and C
= 0 (pin 11 shorted to ground) select values
o
C. Noise immunity can be gained at the
T
from Figure 19 to give the desired
T
C
T
T
slightly to bring the frequency
, and R
C
terminal is the same as the
(continued)
D
. Two waveforms are
7
FIGURE 23
DIGITAL CONTROL PORT SCHEMATIC
Multiple devices can be synchronized together by program-
ming one master unit for the desired frequency, and then
sharing its sawtooth and clock waveforms with the slave
units. All C
master, and all SYNC terminals are likewise connected to
the SYNC pin of the master. Slave R
be left open; at least 50K should be connected from each pin
to ground. Slave R
grounded.
FIGURE 24.
OSCILLATOR CONNECTIONS ANDD WAVEFORMS
FIGURE 25.
ERROR AMPLIFIER CONNECTIONS
SG1526B/SG2526B/SG3526B
(A)
T
terminals are connected to the C
D
terminals may be either left open or
11861 Western Avenue
L
T
INFINITY Microelectronics Inc.
[phone]
terminals should not
(B)
Garden Grove, CA 92841
T
pin of the
FAX: [phone]