ML4828
If the current limit condition persists, then I
will be
For example, with C
= 25µF and R
= 240kΩ:
RST
RST
RST
reactivated, thus charging C
to a higher level as shown
RST
in Figure 7. Eventually, the voltage at RST will exceed
2.5V, and the soft start comparator will trip, shutting down
all power drivers and inhibiting any further delivery of
500µA
25µF
dv
dt
V
s
=
= 20
(14)
(15)
power. At the same time, the soft start capacitor C is
and
SS
discharged to prepare for the next start up cycle.
t
= (240kΩ × 25µF) ×1.39 = 8.3s
D(RST)
1V
Since the threshold for shutdown is 2.5V, the controller
will shut down after approximately 125ms. After the
converter recovers form the current limit condition, the
controller will reactivate after 8.3s.
V(PIN 20)
UNDERVOLTAGE LOCKOUT
2.5V
During start-up, the ML4828 draws very little current
V(PIN 12)
(typically 150µA) and V
is disabled. When V rises
REF
CC
above 6.0V, the internal circuitry and V
are enabled,
REF
and will stay enabled until V falls below the 4.5V UV
CC
lockout threshold.
Figure 7. I
Waveforms During Short Circuit.
and Resulting RC
RST
LIM
SHUTDOWN FUNCTION
The ML4828 can be externally shut down by bringing
SDN (pin 19) low. The shutdown threshold (V ) is given
SD
by
During the I
shutdown, I
is turned off, and C
RST
is
LIM
RST
discharged through R
with a time constant of:
RST
(16)
For example, if V = 5V, then V = 1.67V. As long as
V
= 0.33 × V
SD
CC
(12)
t
= R
× C
RST
RST RST
CC
SD
2.4V < V < 6.0V, the SDN pin will be TTL compatible.
CC
When the condition causing the current limit is removed,
will discharge C with a time constant of t
R
RST
RST
RST.
When the voltage at RST (pin 12) drops to 1.25V, the soft
start comparator and the converter will undergo a start up
cycle. The restart delay (t
) is given by:
D(RST)
(13)
t
= t
×1.39
D(RST)
RST
8
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
