Rainbow Electronics DS4426 - Manual (Page 13)

Applications Information
Example Calculations for an Adjustable
Power Supply
In this example, the circuit shown in Figure 8 is used to
margin a +2.0V supply by ±20%. The margined power
supply has a DC-DC converter output voltage, V
OUT , of
+2.0V and a DC-DC converter feedback voltage, V FB ,
of +0.8V. To determine the relationship of R
0A
and R
0B
,
start with the equation:
Substituting V FB = +0.8V and V OUT = +2.0V, the rela-
tionship between R 0A and R 0B is determined to be:
R 0A = 1.5 x R 0B
I OUT0 is chosen to be 100µA (midrange source/sink
current for the DS4426). Summing the currents into the
feedback node, we have the following:
I
OUT0
= I
R0B
- I
R0A
where:
and
To create a ±20% margin in the supply voltage, the
value of V
OUT is set to +2.4V. With these values in
place, R 0B is calculated to be 2.67k
Ω, and R
0A is
I
V V
R
R A
OUT FB
A
0
0
=
−
I
V
R
R B
FB
B
0
0
=
V
R
R R
V
FB
B
A B
OUT
=
+
× 0
0 0
DS4426
Quad-Channel, I
2
C-Margining IDACs with
Three Channels of Power-Supply Tracking
______________________________________________________________________________________ 13
DC-DC
CONVERTER
FB
OUT
SDA
SCL
OUT0
GND
R FS0 = 80k Ω
4.7k Ω 4.7k Ω V CC
V
CC
V
OUT
= 2.0V*
FS0
R
0B
= 2.67k Ω
R 0A = 4k Ω
V FB = 0.8V*
I R0A
I R0B
I OUT0
DS4426
*V OUT AND V FB VALUES ARE DETERMINED BY THE DC-DC CONVERTER AND SHOULD NOT BE CONFUSED WITH V OUT AND V REF OF THE DS4426.
Figure 8. Example Typical Application Circuit