Datasheet

LTC2970/LTC2970-1
30
29701fc
APPLICATIO S I FOR ATIO
WUU
U
7-Step Procedure for Calculating Tracking Application
Circuit Resistor Values, Counter Delay Values, and
Terminal IDAC Codes
The following 7-step procedure should be used to calculate
the resistor values, tracking counter delays, and terminal
IDAC codes for the Tracking Application Circuit shown in
Figure 3.
1. Assume a value for R20 and solve for R21.
V
DCn,NOM
is the output voltage of the DC/DC converter when
the LTC2970’s V
OUTn
pin is in a high impedance state.
RR
V
V
DC NOM
DC NOM
21 20
1
0
=•
,
,
(12)
2. Solve for R10 and R11.
R
Rn
V
V
DC NOM
FB
1
2
1
n
n
n
=
−
⎛
⎝
⎜
⎞
⎠
⎟
,
(13)
3. Solve for R40 and R41.
For simplicity, this procedure assumes that R40 = R41.
V
DCn,MAX
and V
DCn,MIN
are the maximum and minimum
converter output margin voltages, respectively.
The value of R40 = R41 is constrained by:
RR
V
VV
VV
FB
DC NOM DC MIN
DC MAX D
40 41=≥
•
−
()
−
n
nn
n
,,
,
CC NOM
mV
A
n,
()
+
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
+110
236μ
(14)
Due to the forward drop of diodes D10 and D11 (0.8V
max), the minimum value for R40 = R41 from expression
(14) may result in small or even negative values of R30
and R31 in Step 4. If this is the case, assume a minimum
allowable value for R3nB, and use the following expression
to calculate the minimum value R40 = R41:
RR
V
RB
R
RB
R
V
FB
40 41
1
3
1
3
2
08 1
=≥
•+ +
⎛
⎝
⎜
⎞
⎠
⎟
++
n
n
n
n
n
.
00
236
mV
Aμ
(15)
Note: Use the channel whose parameters yield the maxi-
mum value for R40 = R41.
4. Solve for R30B and R31B.
Solve for the upper limits of R30B and R31B and then
determine which resistor value constrains the maximum
value of the other resistor using Equation 17.
RB
RAVVmV
V
RR
FB
FB
3
4 236 0 8 10
1
1
1
n
n
n
n
n
≤
•−−−
()
•+
μ .
22n
⎛
⎝
⎜
⎞
⎠
⎟
(16)
RB
R
RB
R
30
20
31
21
=
(17)
5. Solve for R30A and R31A.
R30A and R31A are constrained by:
RA
R
R
R
VV
V
DC MAX DC NOM
DC
3
2
1
2
1
n
n
n
n
nn
≤
+
⎛
⎝
⎜
⎞
⎠
⎟
•
−
,,
nn
n
,NOM
RB
⎛
⎝
⎜
⎞
⎠
⎟
− 3
(18)