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UC1914 数据表(PDF) 10 Page - Texas Instruments

部件名 UC1914
功能描述  5V to 35V Hot Swap Power Manager
Download  14 Pages
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制造商  TI [Texas Instruments]
网页  http://www.ti.com
标志 TI - Texas Instruments

UC1914 数据表(HTML) 10 Page - Texas Instruments

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10
UC1914
UC2914
UC3914
Overload Comparator
The linear amplifier in the UC3914 ensures that the ex-
ternal NMOS does not source more than the current
IMAX, defined above as:
I
VCC
V
R
MAX
IMAX
SENSE
=
In the event that output current exceeds the programmed
IMAX by more than 200mV/RSENSE, the output of the lin-
ear amplifier will immediately be pulled low (with respect
to VOUTS) providing no gate drive to the NMOS, and
preventing current from being delivered to the load. This
situation could occur if the external NMOS is not re-
sponding to a command from the IC or output load condi-
tions change quickly to cause an overload condition
before the linear amplifier can respond. For example, if
the NMOS is sourcing current into a load and the load
suddenly becomes short circuited, an overload condition
may occur. The short circuit will cause the VGS of the
NMOS to immediately increase, resulting in increased
load current and voltage drop across RSENSE. If this drop
exceeds the overload comparator threshold, the amplifier
output will be quickly pulled low. It will also cause the CT
pin to begin charging with I3, a 3mA current source (refer
to Fig. 2) and continue to charge until approximately one
volt below VCC, where it is clamped. This allows a con-
stant fault to show up on FAULT and since the voltage on
CT will only charge past 2.5V in an overload fault condi-
tion, it can be used for detection of output NMOS failure
or to build redundancy into the system.
Estimating Minimum Timing Capacitance
The startup time of the IC may not exceed the fault time
for the application. Since the timing capacitor, CT, deter-
mines the fault time, its minimum value can be deter-
mined by calculating the startup time of the IC. The
startup time is dependent upon several external compo-
nents. A load capacitor, CLOAD, should be tied between
VOUTS and GND. Its value should be greater than that
of CPUMP, the reservoir capacitor tied from VPUMP to
VOUTS (see Fig. 4). Given values of CLOAD, Load,
RSENSE, VCC and the resistors determining the voltage
on IMAX, the user can calculate the approximate startup
time of the node VOUT. This time must be less than the
time it takes for CT to charge to 2.5V. Assuming the user
has determined the fault current, RSENSE can be calcu-
lated by:
R
mV
I
SENSE
FAULT
=
50
IMAX is the maximum current the UC3914 will allow
through the transistor M1. During startup with an output
capacitor, M1 can be modeled as a constant current
source of value IMAX where:
I
VCC
V
R
MAX
IMAX
SENSE
=
Given this information, calculation of startup time is now
possible via the following:
Current Source Load:
T
CVCC
II
START
LOAD
MAX
LOAD
=
Resistive Load:
TR
C
n
VCC
IR
START
LOAD
LOAD
MAX
LOAD
=−


l 1
The only remaining external component which may af-
fect the minimum timing capacitor is the optional power
limiting resistor, RPL. If the addition of RPL is desirable,
its value can be determined from the “Fault Timing” sec-
tion above. The minimum timing capacitor values are
now given by
Current Source Load:
CT
T
R
VCC
R
START
PL
PL
min
=•
•+


10
2
2
4
APPLICATION INFORMATION (cont.)
Figure 4. Estimating minimum timing capacitor.
UDG-97056


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