Datasheet

LTC2970/LTC2970-1
32
29701fc
APPLICATIO S I FOR ATIO
WUU
U
VV
R
R
VmV
DC MAX DC NOM FB00 0
20
30
10
,,
>+•−
()
→>+•
−
()
=
VV
k
k
VmV
DC MAX0
2 631
10
768
08 10 36
,
.
.
..
Ω
Ω
660V
From Equation 7, the margining resolution will be less
than:
V
R
R
RA
k
k
k
RES
<
••
=
••
20
30
40 276
256
10
768
665
μ
Ω
Ω
Ω
.
.
2276
256
933
μA
= . mV/LSB
Margining DC/DC Converter with TRIM Pin Design
Example
The output voltage of the DC/DC converter in Figure 2 needs
to be margined ±10% about its nominal value. Assume
that R
TRIM
= 10.22kΩ and V
REF
= 1.225V.
1. Solve for R30 using Equation 10:
RR
k
TRIM
DOWN
DOWN
30
50
10 22
50
≤
−
⎛
⎝
⎜
⎞
⎠
⎟
=
−
•
%
%
.•
Δ
Δ
Ω
110
10
40 880
⎛
⎝
⎜
⎞
⎠
⎟
= , Ω
Let R30 = 39.2kΩ.
2. Solve for R40 using Equations 11:
R
V
A
UP
DOWN
REF
40 1
236
1
10
10
≥+
Δ
Δ
⎛
⎝
⎜
⎞
⎠
⎟
=+
⎛
⎝
⎜
⎞
%
%
•
μ
⎠⎠
⎟
=•
.
,
1 225
236
10 381
V
Aμ
Ω
Let R40 = 10.5kΩ.
Tracking Application Circuit Design Example
Consider the LTC2970-1 application circuit shown in Figure
3. Channel 0 is a 1.8V DC/DC converter while channel 1
is a 2.5V switching power supply. Both converters have
a feedback node voltage of 0.8V and need to track on and
off coincidentally. In addition, a margin range of +5% and
–10% is required for each supply.
1. Assume a value for R20 and solve for R21.
Let R20 = 5,970Ω. From Equation 12:
RR
V
V
V
V
DC NOM
DC NOM
21 20 5 970
25
18
8
1
0
=• = • =
,
,
,
.
.
,Ω 2292Ω
Let R21 = 8,250Ω (the nearest E192 Series resistor
value).
2. Solve for R10 and R11.
From Equation 13:
R
R
V
V
V
V
DC NOM
FB
10
20
1
5 970
18
08
0
0
=
−
⎛
⎝
⎜
⎞
⎠
⎟
=
−
,
,
.
.
Ω
11
4 776
⎛
⎝
⎜
⎞
⎠
⎟
= , Ω
R
R
V
V
V
V
DC NOM
FB
11
21
1
8 250
25
08
1
1
=
−
⎛
⎝
⎜
⎞
⎠
⎟
=
−
,
,
.
.
Ω
11
3 882
⎛
⎝
⎜
⎞
⎠
⎟
= , Ω
Let R10 = 4,750Ω and R11 = 3,880Ω.
3. Solve for R40 and R41.
Assume that R40 = R41.
RR
V
VV
VV
FB
DC NOM DC MIN
DC MAX D
40 41=≥
•
−
()
−
n
nn
n
,,
,
CC NOM
mV
A
V
n,
.
(.)
(
()
+
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
+
=
•
−
110
236
08
109
μ
1105 1
110
236
10 212
.)
,
−
+
⎛
⎝
⎜
⎞
⎠
⎟
+
=
mV
Aμ
Ω
Let R40 = R41 = 10.5kΩ