Datasheet

OPERATION AND APPLICATION INFORMATION
Setting a Fault Window
ADJ
V
2.5 V ±
4
(1)
( )
S ADJ
V ± 10 × V %
(2)
Hysteresis
Fault
Window
Fault
Window
NoFault
Fault
25
20
15
10
5
0
-5
-10
-15
-20
-25
SupplyFaultWindow-%
0
0.5
1
1.5 2
2.5
Window AdjustVoltage(V )atPin4
ADJ
3.125
3
2.875
2.750
2.625
2.5
2.25
2.125
2
2.375
1.875
FaultWindowatSenseInputs-V
UC1903
UC2903
UC3903
SLUS233A – OCTOBER 1999 – REVISED SEPTEMBER 2008 ........................................................................................................................................ www.ti.com
The fault thresholds on the UC1903 are generated by creating positive and negative offsets, equal in magnitude,
that are referenced to the chip ’ s 2.5-V reference. The resulting fault window is centered around 2.5 V and has a
magnitude equal to that of the applied offsets. Simplified schematics of the fault window and reference circuits
are shown in Figure 1 along with the Typical Characteristics diagrams. The magnitude of the offsets is
determined by the voltage applied at the window adjust pin, Pin 4. A bias cancellation circuit keeps the input
current required at Pin 4 low, allowing the use of a simple resistive divider off the reference to set the adjust pin
voltage.
The adjust voltage at Pin 4 is internally applied across R4, and an 8-k Ω resistor. The resulting current is mirrored
four times to generate current sources I
OA
, I
OB
, I
OC
, and I
OD
, all equal in magnitude. When all four of the sense
inputs are inside the fault window, a no-fault condition, Q4 and Q5 are turned on. In combination with D1 and D2
this prevents L
OB
and L
OD
from affecting the fault thresholds. In this case, the OV and UV thresholds are equal to
V
REF
+ I
OA
(R5 + R6) and V
REF
– I
OC
(R7 + R8) respectively. The fault window can be expressed as:
In terms of a sensed nominal voltage level, V
S
, the window as a percent variation is:
When a sense input moves outside the fault window given in Equation 1 , the appropriate hysteresis control signal
turns off Q4 or Q5. For the under-voltage case, Q5 is disabled and current source I
OB
flows through D2. The net
current through R7 becomes zero as I
OB
cancels I
OC
, giving an 8% reduction in the UV threshold offset. The
overvoltage case is the same, with Q4 turning off, allowing I
OD
to cancel the current flow, I
OA
, through R6. The
result is a hysteresis at the sense inputs which is always 8% of the window magnitude. This is shown graphically
in Figure 2 .
Figure 2. Fault Window and Threshold Hysteresis Scale as a Function of the Voltage Applied at Pin 4
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Product Folder Link(s): UC1903 UC2903 UC3903