ML4828
V+
I
SWITCH
I
1
SS
7
C
SS
TERMINATE
PWM CYCLE
I
LIM
20
+
–
R1
C1
1V
S
R
SENSE
V+
Q
I
RST
R
RST
CLOCK
12
INHIBIT
OUTPUT
R
C
RST
RST
+
–
2.5V
1.25V
UNDER-VOLTAGE
LOCKOUT
Figure 5. Over-Current, Soft-Start, and Integrating Fault Detect Circuits.
SOFT START TIME CONSTANT
During start up, the output voltage is much lower than the
steady state value. Without soft start circuitry, the error
amplifier output (EAO) would swing all the way to the
upper limit and the phase modulator would issue pulses
with full duty cycle, possibly causing output overshoot. To
ensure smooth start up, EAO (pin 8) is pulled low and
then gradually released through the charging of an
external soft start capacitor connected to SS (pin 7). The
soft start charging current is internally set at 25µA. Hence,
EAO will rise with a time constant of:
source I . This current charges the reset capacitor C
.
RST
RST
For proper design, R
should be very large (in the order
RST
of 100kΩ). This will cause nearly all of the I
current
RST
(approximately 500µA) to go into charging C
at a rate of:
RST
500µA
dv
dt
=
(11)
C
RST
in volts per second. I
will be turned off at the beginning
RST
of the next clock cycle. If the current limit condition is
removed, RST will be gradually discharged to ground,
and normal operation resumes as shown in Figure 6.
25µA
dv
dt
=
(9)
C
SS
For example, with C = 25µF, the soft start rate of change
SS
will be:
1V
V(PIN 20)
25µA
dt 25µF
dv
V
s
=
= 1
(10)
FAULT TIME CONSTANT AND RESTART DELAY
2.5V
Figure 5 shows the internal circuitry and external
components involved in fault detection. During normal
operation, RST (pin 12) is discharged to ground through
V(PIN 12)
the external resistor R . The I
comparator has a
RST
LIM
threshold of 1V. R
is selected so that the voltage
SENSE
across it will be equal to the I
threshold at the
LIM
maximum desired I
current. When the voltage
SWITCH
exceeds 1V, the I
Figure 6. I
and Resulting RC
RST
LIM
across R
comparator trips,
SENSE
LIM
Waveforms During Load Surge.
terminating the present power cycle, and at the same time
activating the fault logic to turn on the 500µA current
7
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
