UCC3912
SLUS241A - NOVEMBER 1999
FEATURES
Integrated 0.15 Power MOSFET
3V to 8V Operation
Digital Programmable Current Limit
from 0 to 3A
Electronic Circuit Breaker Function
1A I
CC
when Disabled
Programmable on Time
Programmable Start Delay
Fixed 3% Duty Cycle
Uni-Directional Switch
Thermal Shutdown
Fault Output Indicator
Maximum Output Current can be set
to 1A above the Programmed Fault
Level or to a full 4A
Power SOIC, Low Thermal
Resistance Packaging
DESCRIPTION
The UCC3912 Hot Swap Power Manager provides complete power man-
agement, hot swap capability, and circuit breaker functions. The only com-
ponent required to operate the device, other than supply bypassing, is the
fault timing capacitor, C
T
. All control and housekeeping functions are inte-
grated, and externally programmable. These include the fault current level,
maximum output sourcing current, maximum fault time,and start-up delay.
In the event of a constant fault, the Internal fixed 3% duty cycle ratio limits
average output power.
The internal 4 bit DAC allows programming of the fault level current from 0
to 3A with 0.25A resolution. The IMAX control pin sets the maximum sour-
cing current to 1A above the fault level when driven low, and to a full 4A
when driven high for applications which require fast output capacitor charg-
ing.
When the output current is below the fault level, the output MOSFET is
switched on with a nominal on resistance of 0.15 . When the output cur-
rent exceeds the fault level, but is less than the maximum sourcing level,
the output remains switched on, but the fault timer starts charging C
T
.
Once C
T
charges to a preset threshold, the switch is turned off, and re-
mains off for 30 times the programmed fault time. When the output current
reaches the maximum sourcing level, the MOSFET transitions from a
switch to a constant current source.
(continued)
6
7
8
9
1A
ABOVE
FAULT
OVER CURRENT
COMPARATOR
CURRENT FAULT
LEVEL 03 AMPS
10
H = 4A
MAX
CURRENT
LEVEL
4A
ON TIME
CONTROL
3% DUTY
CYCLE
03A
0.25
RES
11
13
12
4
5
B3
B2
B1
B0
4 BIT DAC
GND
HEAT SINK
GND PINS
CT
CHARGE
PUMP
+
16
LINEAR CURRENT
AMPLIFIER
POWER
FET
2
3
V
OUT
REVERSE VOLTAGE
COMPARATOR
30mV
FAULT
THERMAL
SHUTDOWN
INTERNAL
BIAS
+
1
15
14
VOUT
VIN
CURRENT SENSE
H = OPEN
1.5V
SHTDWN
IMAX
BLOCK DIAGRAM
Programmable Hot Swap Power Manager
UDG-99146
application
INFO
available
2
UCC3912
VIN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +8 V
FAULT Sink Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50mA
FAULT Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.3 to V
IN
Output Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . Self Limiting
Input Voltage
(B0, B1, B2, B3, IMAX, SHTDWN) . . . . . . . . . . . 0.3 to V
IN
Storage Temperature Range . . . . . . . . . . . . . 65
C to +150C
Operating Junction Temperature Range . . . . 55
C to +150C
Lead Temperature (Soldering, 10 sec.) . . . . . . . . . . . . . +300
C
Currents are positive into, negative out of the specified termi-
nal. Consult Packaging Section of Databook for thermal limita-
tions and considerations of packages.
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS:
Unless otherwise stated, these specifications apply for T
J
= 0
C to 70C, VIN = 5V,
IMAX = 0.4V, SHTDWN = 2.4V.
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNITS
Supply Section
Voltage Input Range
3.0
8.0
V
Supply Current
1.0
2.0
mA
Sleep Mode Current
SHTDWN = 0.2V
0.5
5.0
A
Output Section
Voltage Drop
I
OUT
= 1A
0.15
0.22
V
I
OUT
= 2A
0.3
0.45
V
I
OUT
= 3A
0.45
0.68
V
I
OUT
= 1A, VIN = 3V
0.17
0.27
V
I
OUT
= 2A, VIN = 3V
0.35
0.56
V
I
OUT
= 3A, VIN = 3V
0.5
0.8
V
CONNECTION DIAGRAMS
*Pin 5 serves as lowest impedance to the electrical ground;
Pins 4, 12, and 13 serve as heat sink/ground. These pins
should be connected to large etch areas to help dissipate
heat. For N package, pins 4, 12, and 13 are N/C.
The UCC3912 is designed for unidirectional current flow,
emulating an ideal diode in series with the power switch.
This feature is particularly attractive in applications
where many devices are powering a common bus, such
as with SCSI Termpwr.
The UCC3912 can be put into sleep mode drawing only
1
A of supply current. The SHTDWN pin has a preset
threshold hysteresis which allows the user the ability to
set a time delay upon start-up to achieve sequencing of
power. Other features include an open drain FAULT out-
put indicator, Thermal Shutdown, Under Voltage Lock-
out, and a low thermal resistance Small outline package.
DESCRIPTION (cont.)
B3
N/C
GND*
GND*
FAULT
VIN
EGND*
GND*
GND*
VIN
N/C
GND*
SHTDWN
GND*
B2
GND*
GND*
CT
12
11
10
9
8
7
6
5
4
3
2
1
13
14
15
16
17
18
19
20
21
22
23
24
IMAX
VOUT
VOUT
B1
B0
GND*
TSSOP-24 (Top View)
PWP Package
DIL-16, SOIC-16 (Top View)
N, DP Package
*Pin 9 serves as lowest impedance to the electrical ground;
other GND pins serve as heat sink/ground. These pins
should be connected to large etch areas to help dissipate
heat.
3
UCC3912
ELECTRICAL CHARACTERISTICS:
Unless otherwise stated, these specifications apply for T
J
= 0
C to 70C, VIN = 5V,
IMAX = 0.4V, SHTDWN = 2.4V.
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNITS
Output Section (cont.)
Reverse Leakage Current
V
IN
< V
OUT ,
SHTDWN = 0.2V,
V
OUT
= 5V
5
20
A
Initial Start-up Time
(Note 2)
100
s
Short Circuit Response
(Note 2)
100
ns
Thermal Shutdown
(Note 2)
170
C
Thermal Hysteresis
(Note 2)
10
C
DAC Section
Output Leakage
Code = 0000-0011
0
20
A
Trip Current
Code = 0100
0.1
0.25
0.45
A
Code = 0101
0.25
0.50
0.75
A
Code = 0110
0.5
0.75
1.0
A
Code = 0111
0.75
1.00
1.25
A
Code = 1000
1.0
1.25
1.5
A
Code = 1001
1.25
1.50
1.75
A
Code = 1010
1.5
1.75
2.0
A
Code = 1011
1.7
2.00
2.3
A
Code = 1100
1.9
2.25
2.58
A
Code = 1101
2.1
2.50
2.9
A
Code = 1110
2.3
2.75
3.2
A
Code = 1111
2.5
3.0
3.5
A
Max Output Current
Code = 0000 to 0011
0.02
mA
Max Output Current Over Trip (Current Source
Mode)
Code = 0100 to 1111, IMAX = 0V
0.5
1.0
1.8
A
Max Output Current (Current Source Mode)
Code = 0100 to 1111, I
MAX
= 2.4V
3.0
4.0
5.2
A
Timer Section
CT Charge Current
V
CT
= 1.0V
-45.0 -36.0 -22.0
A
CT Discharge Current
V
CT
= 1.0V
0.72
1.2
1.5
A
Output Duty Cycle
V
OUT
= 0V
2.0
3.0
6.0
%
CT Fault Threshold
1.3
1.5
1.7
V
CT Reset Threshold
0.4
0.5
0.6
V
Shutdown Section
Shutdown Threshold
1.1
1.5
1.9
V
Shutdown Hysteresis
100
mV
Input Current
SHTDWN = 1V
100
500
nA
Fault Output Section
Output Leakage Current
500
nA
Low Level Output Voltage
I
OUT
= 10mA
0.4
0.8
V
TTL Input DC Characteristics Section
TTL Input Voltage High
(can be connected to V
IN
)
2.0
V
TTL Input Voltage Low
0.8
V
TTL Input High Current
V
IH
= 2.4V
3
10
A
TTL Input Low Current
V
IL
= 0.4V
1
A
Note 1: All voltages are with respect to Ground. Current is positive into and negative out of the specified terminal.
Note 2: Guaranteed by design. Not 100% tested in production.
4
UCC3912
PIN DESCRIPTIONS
B0 - B3: These pins provide digital input to the DAC
which sets the fault current threshold. They can be used
to provide a digital soft-start, adaptive current limiting.
CT: A capacitor connected to ground sets the maximum
fault time. The maximum fault time must be more than
the time to charge the external capacitance in one cycle.
The maximum fault time is defined as FAULT = 27.8
10
3
CT
.
Once the fault time is reached the output will shut-
down for a time given by: T
SD
= 833
10
3
CT, this
equates to a 3% duty cycle.
FAULT: Open drain output which pulls low upon any con-
dition which causes the output to open: Fault, Thermal
Shutdown, or Shutdown.
IMAX: When this pin is set to logic low the maximum
sourcing current will always be 1A above the pro-
grammed fault level. When set to logic high, the maxi-
mum sourcing current will be a constant 4A for
applications which require fast charging of load capaci-
tance.
SHTDWN: When this pin is brought to a logic low, the IC
is put into a sleep mode drawing typically less than 1
A
of I
CC
. The input threshold is hysteretic, allowing the user
to program a start-up delay with an external RC circuit.
VIN: Input voltage to the UCC3912. The recommended
voltage range is 3 to 8 volts. Both VIN pins should be
connected together and to the power source.
VOUT: Output voltage from the UCC3912. When
switched the output voltage will be approximately V
IN
-
(0.15
I
OUT
). Both VOUT pins should be connected to-
gether and to the load.
6
7
8
9
11
5
B3
B2
B1
B0
IMAX
16
3
2
4
12
13
5
14
15
1
SHTDWN
VOUT
VIN
FAULT
CT
GND
HEAT SINK
GND PINS
UCC3912
R
L
C
OUT
V
OUT
C
SD
R
SD
V
IN
C
IN
R1
LED
D1
C
T
S1
S2
S3
S4
S5
DIP
SWITCH
S6
V
IN
V
IN
APPLICATION INFORMATION
Figure 1. Evaluation circuit.
UDG-99171
Note: For demonstration board schematic see Design Note DN-58.
Protecting The UCC3912 From Voltage Transients
The parasitic inductance associated with the power distri-
bution can cause a voltage spike at V
IN
if the load current
is suddenly interrupted by the UCC3912. It is important to
limit the peak of this spike to less than 8V to prevent
damage to the UCC3912. This voltage spike can be mini-
mized by:
Reducing the power distribution inductance (e.g., twist
the "+" and "" leads of the power supply feeding V
IN
,
locate the power supply close to the UCC3912, use a
PCB ground plane,...etc.).
Decoupling V
IN
with a capacitor, C
IN
(refer to Fig. 1),
located close to pins 2 and 3. This capacitor is typically
less than 1
F to limit the inrush current.
Clamping the voltage at V
IN
below 8V with a Zener
diode, D1(refer to Fig. 1), located close to pins 2 and 3.
5
UCC3912
APPLICATION INFORMATION (cont.)
Figure 2. Load current, timing capacitor voltage, and output voltage of the UCC3912 under Fault conditions.
Estimating Maximum Load Capacitance
For hot swap applications, the rate at which the total
output capacitance can be charged depends on the
maximum output current available and the nature of the
load. For a constant-current current-limited controller,
the output will come up if the load asks for less than the
maximum available short-circuit current.
To
guarantee
recovery
of
a
duty-cycle
from
a
short-circuited load condition, there is a maximum total
output capacitance which can be charged for a given
unit ON time (Fault time). The design value of ON or
Fault time can be adjusted by changing the timing ca-
pacitor C
T
.
For worst-case constant-current load of value just less
than the trip limit; C
OUT(max)
can be estimated from:
(
)
C
I
I
CT
V
OUT
MAX
LOAD
OUT
(max)
-
28 10
3
where V
OUT
is the output voltage.
For a resistive load of value RL, the value of C
OUT(max)
can
be estimated from:
(
)
C
CT
RL
n
V
I
RL
OUT
OUT
MAX
max
-
28 10
1
1
3
l
UDG-93019-4
6
UCC3912
Figure 3. UCC3912 on time control circuitry.
The overcurrent comparator senses both the DAC output
and a representation of the output current. When the out-
put current exceeds the programmed level the timing ca-
pacitor C
T
charges with 36
A of current. If the fault
occurs for the time it takes for C
T
to charge up to 1.5V,
the fault latch is set and the output switch is opened. The
output remains opened until C
T
discharges to 0.5V with a
1.2
A current source. Once the 0.5V is reached the out-
put is enabled and will either appear as a switch, if the
fault is removed, or a current source if the fault remains.
If the over current condition is still present then C
T
will
begin charging, starting the cycle over, resulting in ap-
proximately a 3% on time.
APPLICATION INFORMATION (cont.)
UNITRODE CORPORATION
7 CONTINENTAL BLVD. MERRIMACK, NH 03054
TEL. (603) 424-2410 FAX (603) 424-3460
UDG-94019-1
Figure 4. RDS(on) vs temperature at 2A load current.
UDG-94101
SAFETY RECOMMENDATIONS
Although the UCC3912 is designed to provide system
protection for all fault conditions, all integrated circuits
can ultimately fail short. For this reason, if the UCC3912
is intended for use in safety critical applications where
UL or some other safety rating is required, a redundant
safety device such as a fuse should be placed in series
with the device. The UCC3912 will prevent the fuse from
blowing virtually all fault conditions, increasing system
reliability and reducing maintainence cost, in addition to
providing the hot swap benefits of the device.
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