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PRODUCT SPECIFICATION
RC4153
12
R60
Q35
Q36
-V
S
+7.3V
REF
V
A
B
Switched
Current
Source
Output
From
One Shot
4153-13
Gnd
Trigger
C
D1
Q2
D2
Q4
Q5
Q3
-V
S
R-S Latch
Comparator
Ramp Generator
I
1.0 mA
I
Q7 Q8
Q11 Q12
D3
Q6
(B)
Q10
V
TH
I
OUT
Q13 Q14
D
Q1
C
O
T
4153-12
I
The switched current source is shown as a block diagram in
Figure 10 and detailed in Figure 11. The summing node
(+ input of the op amp) is held at 0V by the amplifier feed-
back, causing V
REF
to be applied across R60. This current
(V
REF
/R60), minus the small amplifier bias current, flows
through Q35. Q35 develops a V
BE
dependent on that current.
This V
BE
is developed across Q36. Since Q35 and Q36 are
equal in area, the currents are equal. The mirrored current is
switched by the one-shot output.
The detail schematic shows the amplifier and load (Q21
through Q34), the mirror transistors (Q35, Q36) and the dif-
ferential switching transistors (Q7, Q8). The amplifier uses a
complementary paraphase input composed of Q21 through
Q26 with a current mirror formed by Q27 through Q30,
which converts from differential to single-ended output.
Level-shift diodes Q32 and Q34 and emitter follower Q31
bootstrap the emitters of the mirror devices Q29 and Q30 to
increase gain and lower input offsets, which would otherwise
be caused by unbalanced collector voltages on Q23 and Q26.
Figure 9. One-Shot (Detail)
Figure 10. Switched Current Source Block Diagram