鈥?/div>
Fixed 3% Duty Cycle
DESCRIPTION
The UCC3918 Low on Resistance Hot Swap Power Manager provides complete
power management, hot swap capability, and circuit breaker functions. The only
components needed to operate the device, other than supply bypassing, are a tim-
ing capacitor, and 2 programming resistors. All control and housekeeping functions
are integrated, and externally programmable. These include the fault current level,
maximum output sourcing current, maximum fault time, and startup delay. In the
event of a constant fault, the internal fixed 3% duty cycle ratio limits the average
output power. The IFAULT pin allows linear programming of the fault level current
from 0A to 4A.
Fast overload protection is accomplished by an additional overload comparator. Its
threshold is internally set above the maximum sourcing current limit setting. In the
event of a short circuit or extreme current condition, this comparator is tripped,
shutting down the output. This function is needed since the maximum sourcing cur-
rent limit loop has a finite bandwidth.
When the output current is below the fault level, the output MOSFET is switched on
with a nominal resistance of 0.06鈩? When the output current exceeds the fault level
or the maximum sourcing level, the output remains on, but the fault timer starts
charging CT. Once CT charges to a preset threshold, the switch is turned off, and
remains off for 30 times the programmed fault time. When the output current
reaches the maximum sourcing level, the MOSFET transitions form a switch to a
constant current source.
(continued)
BLOCK DIAGRAM
CHARGE
PUMP
OVERLOAD
COMPARATOR
REVERSE
COMPARATOR
+
20mV
1
VIN
VIN
VIN
2
3
V
OUT
+
CURRENT SENSE
H = OPEN
IMAX
9
MAXIMUM
CURRENT
LEVEL
CURRENT
FAULT LEVEL
IFAULT
OVERCURRENT
COMPARATOR
ON TIME
CONTROL
3% DUTY
CYCLE
14
VOUT
15
VOUT
8
THERMAL
SHUTDOWN
1.5V
16
VOUT
INTERNAL
BIAS
+
鈥?/div>
7
SHTDWN
5
GND
4
13
12
10
CT
6
FAULT
UDG-99153
HEAT SINK GND
PINS
SLUS384A - NOVEMBER 1999
1
2
3
4
5
6
7
