Rainbow Electronics DS4426 - Manual (Page 9)

Ratiometric Tracking
The DS4426 can maintain a defined ratio between a
slave voltage and the master voltage where:
K SM = V SLAVE /V MASTER .
In Figure 5, this ratio is given by the following:
K SM = [R B[3:1] /(R A[3:1] + R B[3:1] )]/[R D[3:1] /(R C[3:1]
+ R
D[3:1]
)].
Nonratiometric tracking is the special case where K SM = 1.
Power-Supply Tracking Loop Gain Stability
Slave DC-DC output tracking is controlled by the
DS4426 sourcing current into the slave DC-DC's feed-
back loop. This changes the stability of the loop during
tracking. The amount of gain used can be adjusted by
changing the ratio of R L /R G . If oscillations occur,
increasing R G reduces gain and increases the system’s
phase margin. If the slave DC-DC has a compensation
pin, the RC network connected to this pin can also be
adjusted to improve phase margin. This pin is often
labeled COMP or ITH. A larger compensation time con-
stant (increased R and/or increased C) often increases
the stability of the system during tracking; however, this
also modifies the DC-DC's transient response. In order
to prevent modification of the slave DC-DC’s transient
response after power-supply tracking is complete, R
G
should first be modified before adjusting the compen-
sation network. The higher the gain, the less the gain
error. Reducing the gain increases the gain error during
tracking. See Figure 4 for more information.
DS4426
Quad-Channel, I
2
C-Margining IDACs with
Three Channels of Power-Supply Tracking
_______________________________________________________________________________________ 9
DS4426
R
FS0
R FS[3:1]
R C[3:1]
R G[3:1]
10k Ω 10k Ω
R B[3:1]
R THR[3:1]
R A[3:1]
10k Ω
R E0
R F0
R D[3:1]
0.1 μF
SDA
V CC
SCL
A0
A1
FS0
OUT0
I 2 C CONTROL
INTERFACE
V CC 5.0V
5.0V
I 2 C
V REF
REF
1.24V
INP[3:1]
FS[3:1]
OUT[3:1]
INN[3:1]
THR[3:1]
GAIN[3:1]
V REF /2
10k Ω
VOUT
FB
MASTER
DC-DC
CONVERTER
R
E[3:1]
R F[3:1]
VOUT
FB
SLAVE
DC-DC
CONVERTER
COMP
x3
Figure 5. Typical DC-DC Power-Supply Tracking Application