ISL24211_14 Datasheet, PDF (7/12 Pages) Intersil Corporation – Programmable VCOM Calibrator with EEPROM and Output Buffer

ISL24211
The maximum value of I_{DVR_OUT} can be calculated by
substituting the maximum register value of 255 into Equation 2,
resulting in Equation 3:
I_DVROUT(MAX)
=
----^A----^V---^D----^D-----
20 R_{S E T}
(EQ. 3)
Equation 2 can also be used to calculate the unit sink current
step size per Register Code, resulting in Equation 4:
I_STEP
=
----------------^A----^V----^D----^D-----------------
( 256 ) ( 20 ) ( R_{S E T} )
(EQ. 4)
Determination of R_SET
The ultimate goal for the ISL24211 is to generate an adjustable
voltage between two endpoints, V_{COM_MIN} and V_{COM_MAX}, with
a fixed power supply voltage, AV_DD. This is accomplished by
choosing the correct values for R_SET, R₁ and R₂. The exact value
of R_SET is not critical. Values from 1k to more than 100k will
work under most conditions. The following expression calculates
the minimum R_SET value:
⎛
⎜
-^A----^V---^D----^D--
⎞
⎟
R_SET(MIN)
=
⎜
⎜
⎜
⎝
⎝⎛^---V----O^---U^----T^---(^--M^-----I^1--N^--6-)^{----–----A--------}₂^-V-------₀^{D--------D-----}⎠⎞^-⎠⎟⎟⎟
(k
Ω)
(EQ. 5)
Note that this is the absolute minimum value for R_SET. Larger
R_SET values reduce quiescent power, since R₁ and R₂ are
proportional to R_SET. The ISL24211 is tested with a 5kΩ R_SET.
Determination of R₁ and R₂
With AV_DD, V_COM(MIN) and V_COM(MAX) known and R_SET chosen
per the above requirements, R₁ and R₂ can be determined using
Equations 6 and 7:
R₁
=
5120
⋅
R_SE
T
⎛
⎜
⎝
2^----5^----6V^----C-⋅^--O-V^---MC---_O^-(--M-_M^----A-₍^--_M^-X---)-_A^--–-_X^----₎^{V---–C----O}V^---M-_C^---_O^-(--M-_M^----I-₍^-N-_M^--)--_I^--_N^----_{)⎠⎟⎞}
(EQ. 6)
R₂
=
5120
⋅
R_SE
T
⎛
⎜
⎝
-₂---₅----₅-----_â‹---_A----_V---_D---^V-_D---^C--+-^O----_V^M---_C--⁽--^MO----_MA----^X₍--_M-⁾---–-_I--_N--^V-₎--^C--–--^O--₂--^M--₅--⁽-₆-^M----_â‹-^I--^N_V----⁾_C---_O----_M-----₍--_M----_A----_X---_{)⎠⎟⎞}
(EQ. 7)
Final Transfer Function
The voltage at DVR_OUT can be calculated from Equation 8:
V_DVROUT
=
AV_DD
⎛
⎜
⎝
--_R----₁-^R--+--²--_R-----₂--_{⎠⎟⎞}
⎛
⎜1
⎝
–
R_-----e_---g_----i_--s_---t_---e_---r_---V_---a_----l_--u_----e_----+-----1_--
256
⎛
⎜
⎝
2^----0^---R-R^---1-_S^----_E^---_T^-_{⎠⎟⎞⎠⎟⎞}
(EQ. 8)
With amplifier A2 in the unity-gain configuration (V_{COM_OUT} tied
to IN_N as shown in Figure 5), V_DVROUT = V_{COM_OUT} = V_COM.
Example
As an example, suppose the A_VDD supply is 15V, the desired
V_{COM_MIN}= 6.5V and the desired V_{COM_MAX} = 8.5V. R_SET is
arbitrarily chosen to be 7.5kΩ.
First, verify that our chosen R_SET meets the minimum
requirement described in Equation 5:
( 7.5 k Ω )
>
⎛
⎜
⎜
⎜
R_SE
T
(MIN
)
⎝
=
⎛
⎜
⎜
------------^{11--------56----------------}_{⎟⎟⎞}
⎜
⎝
⎝⎛ ^{6.5 V}
–
12_----50_{--⎠⎞}
⎟
⎠
=
⎞
⎟
0.163 k Ω⎟
⎟
⎠
(EQ. 9)
Using Equations 6 and 7, calculate the values of R₁ and R₂:
R₁
=
5120
⋅
7500
⋅
⎛
⎝
-₂---₅----₆-⁸----_â‹^.-⁵--₈--–-_.-₅-⁶----^.-–⁵----₆----_.-₅--_⎠⎞
=
35.4 k Ω
(EQ. 10)
R₂
=
5120
⋅
7500
⋅
⎛
⎝
2^----5^----5^-----⋅^---1^---5^{---8--+--.-5--}6^---–-.^-5^{---6--–-.--5--}2^---5^----6^-----⋅^---8^---.^-5^{-- ⎠⎞}
=
46.4 k Ω
(EQ. 11)
Table 1 shows the resulting V_COM voltage as a function of register
value for these conditions.
TABLE 1. EXAMPLE V_{DVR_OUT} vs REGISTER VALUE
REGISTER VALUE
0
V_{DVR_OUT} (V)
8.49
20
8.34
40
8.18
60
8.02
80
7.87
100
7.71
120
7.55
127
7.50
140
7.40
160
7.24
180
7.09
200
6.93
220
6.77
240
6.62
255
6.50
Output Voltage Span Calculation
It is also possible to calculate V_COM(MIN) and V_COM(MAX) from the
existing resistor values.
V_{COM_MIN} occurs when the greatest current, I_DVR(MAX), is drawn
from the middle node of the R1/R2 divider. Substituting
RegisterValue = 255 into Equation 8 gives the following:
V_COM(MIN)
=
A
⎛
V_{D D ⎝⎜}
--_R----₁-^R--+--²--_R-----₂--_{⎠⎟⎞}
⎛
⎜
⎝
1
–
⎛
⎜
⎝
2^----0^---R-R^---1-_S^----_E^---_T^-_{⎠⎟⎞}
⎞
⎟
⎠
(EQ. 12)
Similarly, RegisterValue = 0 for V_COM(MAX):
V_COM(MAX)
=
A
V_D
⎛
D _⎝⎜
--_R----₁-^R--+--²--_R-----₂--_{⎠⎟⎞}
⎛
⎜
⎝
1
–
----¹-----
256
⎛
⎜
⎝
2^----0^---R-R^---1-_S^----_E^---_T^-_{⎠⎟⎞}
⎞
⎟
⎠
(EQ. 13)
7
FN7585.0
February 23, 2011