Datasheet

OV and UV Comparators Maintain Accurate Thresholds
6.4V
Q1
Q2
Q3
Q4 Q5
Q6
Q7 Q8 Q9
Q10
Q11 Q12 Q13 Q14
Q15 Q16
Q17 Q18
OVCOMPARATOR
UVCOMPARATOR
SENSE1
2kW
R1
Z1
5.7V
D1
SENSE2
SENSE3
SENSE4
100 Am
6.4V
50 Am
TOUV
THRESHOLD
VOLTAGE
OV
HYSTERESIS
CONTROL
6.4V
60 Am
Q20
Q19
1.8V
TOOV
THRESHOLD
VOLTAGE
UV
HYSTERESIS
CONTROL
6.4V
60 Am
1.8V
EXT.
OVDELAY
CAPACITOR
OVFAULT
INDICATION
TOOUTPUT
LOGIC
V
EXT.
UVDELAY
CAPACITOR
REF
UVFAULT
INDICATION
TOOUTPUT
LOGIC
Start Latch and Supply Under-Voltage Sense Allow Predictable Power-Up
UC1903
UC2903
UC3903
SLUS233A – OCTOBER 1999 – REVISED SEPTEMBER 2008 ........................................................................................................................................ www.ti.com
The structure of the OV and UV comparators, shown in Figure 8 results in accurate fault thresholds even in the
case where multiple sense inputs cross a fault threshold simultaneously. Unused sense inputs can be tied either
to the 2.5-V reference, or to another, utilized, sense input. The four under- and over-voltage sense inputs on the
UC1903 are clamped as detailed on the Sense 1 input in Figure 8 . The series 2-k Ω resistor, R1, and zener diode
Z1, prevent extreme under- and over-voltage conditions from inverting the outputs of the fault comparators. A
parasitic diode, D1, is present at the inputs as well. Under normal operation it is advisable to insure that voltage
levels at all of the sense inputs stay above – 0.3 V. The same type of input protection exists at the line sense
input, Pin 15, except a 5-k Ω series resistor is used.
The fault delay circuitry on the UC1903 is also shown in Figure 8 . In the case of an over-voltage condition at one
of the sense inputs Q20 is turned off, allowing the internal 60-mA current source to charge the user-selected
delay capacitor. When the capacitor voltage reaches 1.8 V, the OV and POWER OK outputs become active low.
When the fault condition goes away Q20 is turned back on, rapidly discharging the delay capacitor. Operation of
the under- voltage delay is, with appropriate substitutions, the same.
A. The OV and UV comparators on the UC1903 trigger respective fault delay circuits when one or more of the sense
inputs move outside the fault window. Input clamps insure proper operation under extreme fault conditions.
Terminating the UV delay capacitor to VREF assures correct logic at power up.
Figure 8. OV and UV Comparators on the UC1903
At power-Up, while the +V
IN
input supply is below 3 V, all open collector outputs are off. With +V
IN
greater than 3
V the POWER OK output will be driven low and the UV OV FAULT outputs are disabled. Once +V
IN
rises above
the V
SUV
threshold of 7 V the fault outputs will be enabled. As would be expected, the SENSE 1-4 voltages at
power up may be below the UVFAULT window and the UVFAULT output may be driven low.
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