Datasheet

+V
IN
75 Am
UC1903
G.P.OP-AMP
OUTPUTSTAGE
D1
TOOP-AMP
INPUTSTAGE
Q6
Q5
Q4
Q1
Q2
Q3
R1
10kW
R2
15 W
R3
150 W
OUTPUT
150 Am
R4
500 W
16
Sensing a Negative Voltage Level
Note: A similar scheme w/the G.P. op-amp will allow a sec-
ond negative supply to be monitored.
2.5V
R1
R2
V
S
UC1903
SENSE4INPUT
SENSE4
INVERTER
GROUND
V (NOM)=2.5V
S
NEGATIVE
SUPPLY (-V )
S
UC1903
UC2903
UC3903
SLUS233A – OCTOBER 1999 – REVISED SEPTEMBER 2008 ........................................................................................................................................ www.ti.com
Figure 4 demonstrates one of many auxiliary functions that the uncommitted op-amp on the UC1903 can be used
for. Alternatively, this op-amp can be used to buffer high impedance points, perform logic functions, or for
sensing and amplification. For example, the G.P. op-amp, combined with the 2.5-V reference, can be used to
produce and buffer an optically coupled feedback signal in isolated supplies with primary side control. The output
stage of this op-amp is detailed in Figure 5 . The NPN emitter follower provides high source current capability.
≥ 20 mA while the substrate device, Q3, provides good transient sinking capability.
A. The G.P. op-amp on the UC1903 has a high source current (20 mA) capability and enhanced transient sinking
capability through substrate device Q3.
Figure 5. The G.P. Op-Amp on the UC1903
The UC1903 has a dedicated inverter coupled to the sense 4 input. With this inverter, a negative voltage level
can be sensed as shown in Figure 6 . The output of the inverter is an unbiased emitter follower. By tying the
inverting input, Pin 5, high the output emitter follower will be reverse biased, leaving the sense 4 input in a high
impedance state. In this manner, the sense 4 input can be used, as the remaining sense inputs would be, for
sensing positive voltage levels.
Figure 6. Inverting the Sense 4 Input for Monitoring a Negative Supply, Accommodated with the
Dedicated Inverter
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Product Folder Link(s): UC1903 UC2903 UC3903